Lexicon Power Supply 20 20 AD User Manual

User Guide  
20/20 AD  
20 Bit A/D Converter  
 
SAFETY SUMMARY  
The following general safety precautions must be observed during all phases of operation, service and  
repair of this instrument. Failure to comply with these precautions, or with specific warnings elsewhere  
intheseinstructionsviolatessafetystandardsofdesignmanufactureandintendeduseoftheinstrument.  
Lexicon assumes no liability for the customer's failure to comply with these requirements.  
General definitions of safety symbols used on equipment or  
in manuals.  
GROUND THE INSTRUMENT  
To minimize shock hazard the instrument chassis and cabi-  
net must be connected to an electrical ground. The instru-  
ment is equipped with a three-conductor AC power cable.  
The power cable must either be plugged into an approved  
three-contact electrical outlet or used with a three-contact to  
two-contact adapter with the grounding wire (green) firmly  
connected to an electrical ground (safety ground) at the  
power outlet. The power jack and mating plug of the power  
cable meet International Electrotechnical Commission (IEC)  
safety standards.  
Instruction manual symbol: the product  
will be marked with this symbol when it is  
necessary for the user to refer to the  
instruction manual in order to protect  
against damage to the instrument.  
Indicates dangerous voltage. (Terminals  
fed from the interior by voltage exceed-  
ing 1000 volts must be so marked.)  
DO NOT OPERATE IN AN EXPLOSIVE  
ATMOSPHERE  
Do not operate the instrument in the presence of flammable  
gases or fumes. Operation of any electrical instrument in  
such an environment constitutes a definite safety hazard.  
The WARNING sign denotes a hazard. It  
calls attention to a procedure, practice,  
condition or the like which, if not correctly  
performed or adhered to, could result in  
injury or death to personnel.  
WARNING  
CAUTION  
KEEP AWAY FROM LIVE CIRCUITS  
The CAUTION sign denotes a hazard. It  
callsattentiontoanoperatingprocedure,  
practice, condition or the like which, if not  
correctly performed or adhered to, could  
result in damage to or destruction of part  
or all of the product.  
Operating personnel must not remove instrument covers.  
Component replacement and internal adjustments must be  
made by qualified maintenance personnel. Do not replace  
components with power cable connected. Under certain  
conditions, dangerous voltages may exist even with the  
power cable removed. To avoid injuries, always disconnect  
power and discharge circuits before touching them.  
The NOTE sign denotes important infor-  
mation. It calls attention to procedure,  
practice, condition or the like which is  
essential to highlight.  
DO NOT SERVICE OR ADJUST ALONE  
Do not attempt internal service or adjustment unless another  
person, capable of rendering first aid and resuscitation, is  
present.  
NOTE:  
DO NOT SUBSTITUTE PARTS OR MODIFY  
INSTRUMENT  
Because of the danger of introducing additional hazards, do  
not install substitute parts or perform any unauthorized  
modification to the instrument.  
CAUTION  
Electrostatic Discharge (ESD) Precautions  
The following practices minimize possible damage to ICs  
resulting from electrostatic discharge or improper inser-  
tion.  
DANGEROUS PROCEDURE WARNINGS  
Warnings, such as the example below, precede potentially  
dangerous procedures throughout this manual. Instructions  
contained in the warnings must be followed.  
Keep parts in original containers until ready for use.  
Avoid having plastic, vinyl or styrofoam in the work  
area.  
WARNING  
•. Wear an anti-static wrist-strap.  
Dangerousvoltages, capableofcausingdeath, arepres-  
ent in this instrument. Use extreme caution when han-  
dling, testing and adjusting.  
Discharge personal static before handling devices.  
Remove and insert boards with care.  
Whenremovingboards,handleonlybynon-conductive  
surfaces and never touch open-edge connectors ex-  
cept at a static-free workstation.*  
SAFETY SYMBOLS  
Minimize handling of ICs.  
•. Handle each IC by its body.  
Do not slide ICs or boards over any surface.  
Insert ICs with the proper orientation, and watch for  
bent pins on ICs.  
Use anti-static containers for handling and transport.  
*To make a plastic-laminated workbench anti-static, wash with a solution of Lux liquid  
detergent, and allow to dry without rinsing.  
 
Contents  
Introduction  
Controls and Connectors ...................................................................... 1  
Unpacking • Power • Mounting .......................................................... 1  
Front Panel Controls.......................................................................... 2  
The Converter • The Level Meters • The Compressor  
Synchronization and Output Formatting • Alternate Modes  
Rear Panel Connectors ..................................................................... 5  
Audio Connections ............................................................................ 6  
Connectors • Cables  
Using the 20/20 AD................................................................................. 9  
Analog Inputs..................................................................................... 9  
Termination and Grounding via Internal Jumpers ............................. 9  
Balanced and Unbalanced Input Signals......................................... 12  
Front Panel Adjustments ................................................................. 13  
Synchronization and Sample Rate .................................................. 14  
AES Sync • Word Clock • Internal and External Modes  
About Dither..................................................................................... 16  
Psychoacoustically Optimized Noise Shaping (PONS)  
Automatic Offset Removal ...............................................................18  
Compressor Modes ......................................................................... 18  
Dynamic Range Compression • Non-Linear Transfer  
Functions • Overload Protection • Digitizing Analog Tape  
Link • Display  
Digital Audio Outputs ....................................................................... 23  
Realigning Two DAT Recordings .................................................... 24  
Digital Sinewave Generator ............................................................. 24  
Specifications.......................................................................................25  
Definition of Terms .............................................................................. 27  
 
20/20 AD User Guide  
Introduction  
The 20/20 AD is an analog-to-digital converter which can be configured for  
two20-bitchannels, orfour18-bitchannels. Twinon-boardsignalprocessors  
provide 20-bit to 16-bit compression in the digital domain, as well as DC  
removal and a choice of four dither types.  
The compressor allows leeway in setting levels and provides insurance  
against clipping in the digital domain. In addition, it provides 195 soft  
compression curves and 13 time constants to allow for more creative  
functions, such as emulation of tape saturation or analog compression.  
This manual provides some basic information on conversion, but assumes  
the user will be familiar with the principles of analog-to-digital conversion, as  
well as with the digital audio interface formats output by the 20/20 AD.  
Information on specific data which may be required by any target device  
should be obtained from the manufacturer.  
Lexicon has attempted to provide correct information and functional capability  
corresponding to digital audio standards known and available to us at the time of  
publication of this document and manufacture of this instrument. As standards  
evolve, Lexicon may make updates to documentation and/or system software  
available for purchase.  
Lexicon's warranty on this product excludes consequential damages resulting from  
the use of this product. Local jurisdiction may extend the user additional rights.  
AES/EBU Interface (Professional)  
The AES/EBU interface conforms to both the AES3-1992 (ANSI S4.40-1992) specification and the EBU  
document, Tech 3250-E. Both inputs and outputs are balanced, transformer-coupled designs, with a female  
XLR input and a male XLR output, conforming to the standard convention of IEC 268, Part 12. Input and output  
levels comply with CCITT V.11. On this device, the professional form of the digital audio and auxiliary data has  
been labeled "AES" in accordance with general usage.  
S/PDIF – Sony/Phillips Digital Interface Format (Consumer)  
The S/PDIF interface conforms to the Channel Status Type II format, as specified in the EIAJ CP-340 Digtial  
Audio Interface Standard, dated September, 1987, and to IEC 958, First Edition 1989-03. As of November 20,  
1991, amendments to IEC 958, which include specifications for the implementation of auxiliary data for the  
consumer format, were under review. Until new specifications are published, this device must conform to  
currently approved specifications. On this device, the consumer form of the digital audio and auxiliary data has  
been labeled "SPDIF" in accordance with general usage.  
 
Controlsand
Connectors  
20/20 AD User Guide  
After unpacking the 20/20 AD, save all packing materials in case you ever need  
toshiptheunit. Thoroughlyinspectthe20/20ADandpackingmaterialsforsigns  
of damage. Report any shipment damage to the carrier at once. The following  
accessories are included with the 20/20 AD:  
Unpacking  
1. Power Cable  
2. User Guide  
3. Quick Reference Guide  
The 20/20 AD is equipped with a 3-pin IEC connector and detachable line cord.  
Connect the cable end of the 20/20 AD line cord to the 20/20 AD power  
connector. Then plug the line cord into an AC wall socket providing voltage  
corresponding to the data plate on the unit.  
Power  
The 20/20 AD measures 19"W x 1.75"H x 13.9"D (483 x 45 x 353 mm). Make  
sure that the 20/20 AD is securely screwed into the rack, and that support is  
provided for the rear of the chassis during transport to avoid possible damage  
from severe mechanical shock.  
Mounting  
Themaximumambientoperatingtemperatureis95°F(35°C). Provideadequate  
ventilation if the 20/20 AD is mounted in a closed rack with heat-producing  
equipment.  
1
 
Controls and Connectors  
Lexicon  
20/20 AD  
Front Panel  
Controls  
The 20/20 AD front panel controls are divided into three functional groups for  
conversion, compression, and synchronization/output formatting.  
Synchronization and  
Output Formatting  
Converter  
Compressor  
UNBAL THRESH  
UNBAL THRESH  
UNBAL THRESH  
UNBAL THRESH  
OVL  
0
OVL  
0
POWER  
DIGITAL COMP/LIMITER  
6
1dB SCALE  
ALT  
DISPLAY  
LINK  
DITHER  
WIDE  
HF  
20BIT  
PL DN  
44.1  
100  
mS  
RMT  
VID  
3
3
-18  
-14  
4
CHAN  
SAMPLE RATE  
EXT SYNC  
6
6
4
0
8
12  
18  
24  
30  
36  
42  
60  
12  
18  
24  
30  
36  
42  
60  
-22  
-10  
48  
~
PK HOLD  
ATTEN  
FORMAT  
20/20 AD  
EMPH  
2
10  
10  
1S  
AES  
WC  
SPDIF  
AES  
20 BIT A/D CONVERTER  
-26  
-6  
12  
M
C
P
+
0
ON  
+
M
-36  
0
P
C
AES / WC:  
PRES  
LOCK  
MUTE  
1
2
3
4
GAIN  
THRESHOLD  
RELEASE  
The Converter  
Gain Controls: 1, 2, 3 and 4  
Provide continuously vari-  
able gain control over a 30dB  
range.  
THRESH  
UNBAL  
When the compressor is on,  
indicates that the input level  
has risen above the selected  
compression threshold.  
Lights when there is a mis-  
match between the input sig-  
nal and the setting of the  
rear-panel Balance switch.  
Note: In 2-channel mode,  
only 1 and 2 are active.  
4 CHAN  
Push on-push off selection of  
2-channel or 4-channel  
mode; lit when in 4-channel  
mode.  
UNBAL THRESH  
UNBAL THRESH  
UNBAL THRESH  
UNBAL THRESH  
OVL  
0
OVL  
0
1dB SCALE  
ALT  
3
3
4
CHAN  
6
6
12  
18  
24  
30  
36  
42  
60  
12  
18  
24  
30  
36  
42  
60  
~
PK HOLD  
EMPH  
M
C
P
+
ATTEN  
MUTE  
EMPH  
1
2
3
4
Turns pre-emphasis on and  
off.  
PK HOLD  
ATTEN  
1dB SCALE  
Press to cause the level Press to attenuate balanced  
meter LEDs to hold at the input signals by 6dB. Press  
highest level received. Press again to turn attenuation off.  
Press to select 1dB Scale  
display on level meters.  
Also used to select alternate  
modes.  
again to deactivate.  
Do not use this control on  
unbalanced signals.  
2
 
20/20 AD User Guide  
Controls and Connectors  
The Level Meters  
Whenthecompressoris off, thebehaviorofthelevelmetersisdeterminedbythesettings  
of the 1dB SCALE, and PK HOLD switches.  
When the compressor is on, pressing DISPLAY will cause gain reduction (in dB) to be  
displayed on the two right hand meters (2-channel mode) or on all four meters (4-channel  
mode).  
The following charts show all of the possible states of the level meter and the value in  
decibels for each state.  
GAIN  
REDUCTION  
Switches ON  
Meters 1 & 2  
Meters 3 & 4  
NORMAL  
1dB SCALE  
OVL  
0
- red -  
- red -  
OVL  
0
OVL  
0
1
2
none  
Normal  
1 dB Scale  
Normal  
1 dB Scale  
4-Channel  
Mode  
1 dB SCALE  
DISPLAY  
3
- yellow -  
- yellow -  
- green -  
- green -  
- green -  
- green -  
- green -  
- green -  
- green -  
-3  
-1  
-2  
-3  
-4  
-5  
-6  
-7  
-8  
-9  
3
Gain Reduction  
Gain Reduction  
6
-6  
4
none  
Normal  
Normal  
Normal  
(Blank)  
12  
18  
24  
30  
36  
42  
60  
-12  
-18  
-24  
-30  
-36  
-42  
-60  
5
2-Channel  
Mode  
6
1 dB SCALE  
DISPLAY  
1 dB Scale  
7
Gain Reduction  
8
9
10  
11  
The Compressor  
DISPLAY  
Press to cause gain reduc-  
tion to be displayed on the  
level meters — In 2-channel  
mode, only the right two level  
meters are activated; in 4-  
channel mode, all four level  
meters display compression.  
Press again to deactivate.  
DIGITAL COMP/LIMITER  
DISPLAY  
100  
mS  
RMT  
VID  
6
C
-14  
-18  
4
0
8
-22  
-26  
-10  
-6  
LINK  
ON  
2
10  
1S  
10  
12  
0
M
P
-36  
0
+
GAIN  
THRESHOLD  
RELEASE  
THRESHOLD  
GAIN  
RELEASE  
LINK  
Adjusts the gain of low level  
signals (below the compres-  
Sets the level at which com-  
pression starts.  
This control allows adjust-  
ment of the release time of  
the compressor. Release  
times from 0mS to are  
available. The attack time of  
the compressor is automati-  
cally set as a function of the  
signal rise time.  
In 2-channel mode, links the  
leftandrightcompressors;in sion threshold) between the  
4-channel mode, where 1,2 converter and the compres-  
and 3,4 are already linked, sor.  
this control links the two  
pairs.  
ON  
Turns the compressor on  
and off.  
3
 
Controls and Connectors  
Lexicon  
Synchronization  
and  
Output Formatting  
DITHER  
SAMPLE RATE  
Selects the type of dither:  
wide bandwidth, 20 bit, or  
High Frequency .  
Selects the sample rate of  
the A/D conversion. This  
switch should be set to the  
same sample rate as your  
sync source. When Word  
Clockisselectedasanexter-  
nal sync source, the AES  
ChannelStatusSampleRate  
bit will be set to the sample  
rate value you have selected  
here.  
DITHER  
WIDE  
HF  
20 BIT  
PL DN  
44.1  
SAMPLE RATE  
EXT SYNC  
48  
FORMAT  
AES  
WC  
SPDIF  
AES  
FORMAT  
Selects the format to be  
available at the output con-  
nectors, AES or S/PDIF.  
AES / WC:  
PRES  
LOCK  
Note: The format selected  
here will be sent to all out-  
puts.  
AES/WC  
In internal sync mode, PRES LOCK lights in external sync  
indicates that either WC or mode to indicate the unit has  
AES is available at the inputs successfully locked to the in-  
(The PL DN setting of this  
switch is reserved for future  
enhancements.)  
as a sync source.  
coming signal (WC or AES).  
EXT SYNC  
Inexternalsyncmode,PRES  
indicates that the selected  
sync source is present at the  
input connectors.  
This switch allows selection  
of: Internal mode (unlit),  
Word Clock source, or AES  
source.  
Alternate Modes  
To activate  
the alternate modes...  
press and hold  
...then press one of these.  
1dB SCALE  
ALT  
DISPLAY  
LINK  
DITHER  
ATTEN  
RMT  
MUTE  
4 CHAN  
~
ON  
VID  
Repeat to exit the alternate mode.  
ALT - ATTEN  
ALT - DISPLAY  
ALT - LINK  
ALT - DITHER  
Mutes the outputs. Note that Activates remote mode/front Only active in 4-channel  
the level meters still work. To panel lockout. When acti- mode. Activates a short tone  
unmute, hold ALT and press vated, the 1dB SCALE and burst. See Realigning Two  
Activates psychoacousti-  
cally optimized noise shap-  
ing. When activated, WIDE,  
20BIT and HF will be lighted.  
ATTEN again.  
DISPLAY LEDs flash and DAT Recordings.  
front panel controls are  
Note that this type of dither  
can only be used in 2-chan-  
nel mode; activating will dis-  
able 4-channel mode and  
compressor functions.  
ALT - 4 CHAN  
locked into their current set- ALT - ON  
Activates output of a digital tings. To unlock, hold ALT Reserved for future en-  
sinewave at the AES ports. and press DISPLAY again. hancements.  
4
 
20/20 AD User Guide  
Controls and Connectors  
20/20 AD  
Rear Panel  
Connectors  
BAL/UNBAL  
Push on/push off switch. Set  
button to correspond to input  
signal type. A mismatch be-  
tween the input signal and  
the setting of this switch, will  
be indicated on the front  
panel by the BAL LED.  
Analog Input  
AC Power  
Standard 3-pin IEC power  
connector.  
3-pin XLR connectors. In 2-  
channel mode, only inputs 1  
and 2 are active.  
3
+
DO  
4
1
+
DO  
2
4
IN  
3
2
IN  
1
WORD CLOCK  
IN OUT  
PUSH  
PUSH  
PUSH  
PUSH  
PUSH  
UNBAL  
BAL  
UNBAL  
BAL  
AES  
SYNC  
IN  
V ~  
Sync Input  
Digital Output  
3-pinXLRconnector,professionalformatAES/EBUbalanced  
digital input.  
Two 3-pin XLR connectors, professional format AES bal-  
anced digital outputs.  
75, unbalanced BNC connectors for word clock in/out.  
Two consumer format optical (fiber-optic) outputs.  
Two consumer format unbalanced coaxial RCA outputs.  
Note: In 2-channel mode 3 & 4 and 1 & 2 are identical.  
The input and output connectors conform to published stan-  
dards for professional and consumer formats. The 20/20 AD,  
however,providestheuser-selectedformatatallconnectors,  
e.g. the AES format, if selected, will be available at the  
consumer-type outputs, as well as at the XLR outputs.  
5
 
Controls and Connectors  
Lexicon  
Audio Connections  
Connectors  
Signal  
Mating Connector  
Description  
AES/EBU  
XLR A3M  
Balanced RS-422  
Digital Input  
AES/EBU  
Digital Output  
XLR A3F  
RCA  
Balanced RS-422  
S/PDIF  
Consumer Digital  
Output  
Unbalanced 75Ω  
S/PDIF  
Consumer Digital  
Consumer Digital  
Audio Optical  
Output  
Word Clock  
Analog Input  
BNC  
XLR  
Unbalanced 75Ω  
pin 2 high  
XLR Pinout  
2 = high  
Male  
3 = low  
1 = ground  
Female  
1 = ground  
3 = low  
2 = high  
XLR Pinout pin 2 high by convention  
6
 
20/20 AD User Guide  
Controls and Connectors  
Cables  
This interface requires balanced connections using high-quality, low-capaci- AES/EBU  
tance, controlled-impedance, data communication, twisted-shielded pair cable. Digital Output  
Microphone cable may introduce a significant amount of jitter into the  
signal, causing distortion.  
S/PDIF  
Use commercially-available, consumer audio optical cable assemblies.  
Consumer Digital Audio  
Output  
Word Clock I/O  
This interface is unbalanced but, because it carries digital signals, it requires the  
use of 75RG-59 coaxial cable.  
Below are recommended cable and cable assemblies. Similar cables from  
sources other than those listed here may be appropriate.  
AES/EBU  
Belden 9271 (foil shielded twisted pair, 124, 12.2 pF/ft)  
S/PDIF Consumer Digital Audio  
Belden 9259 (RG-59/U: 22 AWG conductor, .242 O.D., 75, 17.3 pF/ft)  
Belden 8218 (27 AWG conductor, .150 O.D., 75, 20.5 pF/ft)  
Maximum recommended length: 32 ft. (10M)  
S/PDIF Consumer Digital Audio Optical  
Toshiba TOCP174y  
Sony POC-15  
Maximum recommended length: 16 ft. (5M)  
Word Clock  
Belden 9259 (RG-59/U: 22 AWG conductor, .242 O.D., 75, 17.3 pF/ft)  
Belden 8218 (27 AWG conductor, .150 O.D., 75, 20.5 pF/ft)  
Maximum recommended length: 32 ft. (10M)  
7
 
20/20 AD User Guide  
Using the 20-/20 AD  
The 20/20 AD has four analog inputs: CH1, CH2, CH3 and CH4. In two-channel  
mode, only CH1 and CH2 are used, and the dynamic range of the unit is  
extended. If you have two stereo pairs, one of which is used more often than the  
other, the most often used pair should be assigned to CH1 and CH2, and the  
other pair to CH3 and CH4. This allows you to take advantage of the extended  
dynamic range of two-channel mode when using only a single stereo pair.  
Analog Inputs  
When digitizing stereo pairs, the left channel of each pair should be sent to CH1  
and CH3; the right to CH2 and CH4.  
The 20/20 AD accepts balanced or unbalanced signals with the following peak  
input levels:  
Signal Type  
Attenuator  
dBu  
dBV  
balanced  
balanced  
unbalanced  
OFF  
ON  
OFF*  
+2 to +22  
+8 to +28  
-4 to +16  
0 to +20  
+6 to +26  
-6 to +14  
* Unbalanced signals cannot be correctly attenuated with the front panel ATTEN button. Input levels  
of unbalanced signals must be lowered before entering the 20/20 AD.  
The analog XLR connectors should be wired as follows:  
Signal Type  
Pin 1  
Pin 2  
Pin3  
balanced  
screen  
(shield)  
positive  
polarity  
return  
unbalanced  
screen and  
return  
positive  
polarity  
open or  
return  
In a typical studio environment, each piece of equipment is connected to a “star  
ground” through the safety ground on the AC connector, or through a ground  
strap tied to the chassis. In these cases, connecting the shield of the audio  
cable(s)topin1atbothendsofthecablewillcausethe20/20ADtobegrounded  
to the audio source through two paths: the shield of the audio cable and the  
safety ground or ground strap. This will result in a “ground loop” which manifests  
itself as hum at the power line frequency.  
Termination and  
Grounding via  
Internal Jumpers  
A common solution to this problem is to connect the cable shields only at the  
source end, and to leave pin 1 of the 20/20 AD end floating. If this is not practical,  
you can open the ground jumpers (W1, W9, W11 and W13) on the 20/20 AD’s  
analog inputs.  
9
 
Using the 20/20 AD  
Lexicon  
CAUTION  
These instructions are for use by qualified personnel only.  
Before opening the 20/20 AD, REMOVE THE POWER CORD. You may be  
exposed to hazardous voltages even if power to the unit is turned off.  
WARNING  
Turn off power and REMOVE THE POWER CORD  
before opening the unit.  
Take a moment to study the figure below. This diagram shows the terminating  
and grounding of input signals for the unit, and explains the function of most of  
the jumpers on the 20/20 AD Main pc board. The configuration shown illustrates  
the jumper settings when the 20/20 AD is shipped from the factory.  
CHASSIS GROUND  
XLR  
CONNECTOR  
ANALOG IN  
W1  
W9  
W11  
W13  
1
2
3
+
-
XLR  
CONNECTOR  
W2  
600 OHM  
W10  
W12  
W14  
AES OUT  
1
2
3
W26  
W27  
SHIELDED  
TWISTED PAIR  
AES SYNC IN  
SHIELDED  
TWISTED PAIR  
W18  
1
2
3
RCA  
CONNECTOR  
W17 110 OHM  
S/PDIF OUT  
XLR  
CONNECTOR  
75 OHM  
COAXIAL  
BNC  
CONNECTOR  
WORD CLOCK IN  
J13  
75 OHM  
COAXIAL  
W16  
75 OHM  
CHASSIS CONNECTED TO  
EARTH GROUND  
THROUGH POWER CABLE  
OR GROUND STRAP  
Jumper settings  
(as set at the factory)  
10  
 
20/20 AD User Guide  
Using the 20/20 AD  
Thefollowingfigureindicatesthelocationsandfactorysettingsofthesejumpers  
on the board. In their closed positions, the jumpers have their right two (or lower  
two) pins shunted together. To open a jumper, move the shunting block to the  
left(orupper)twopins.(Notethatthejumpersin J13haveonlytwopins;toopen,  
simply remove the shunting block.)  
W1  
W2  
W9  
W10  
W11  
W12 W13  
W14  
Analog Jumpers  
W17  
W27  
U30 U31  
W18  
W16  
W26  
U48  
U42  
U49  
J13  
Digital Jumpers  
Jumper locations  
(shown with factory settings)  
Note that the 20/20 ADs analog inputs are not transformer isolated. It is,  
therefore, necessary to have one ground connection between the 20/20 AD and  
the audio source.  
In addition to the ground lift jumpers, each analog input has a 600termination  
jumper. If the analog source expects to see a 600load, these jumpers should  
be closed.  
11  
 
Using the 20/20 AD  
Lexicon  
Make sure the BALANCE switches located on the rear panel are set to match  
the type of input. (OUT=balanced , IN=unbalanced). The red UNBAL lights on  
the front panel will indicate an error if there is a mismatch between the switch  
setting and the signal type.  
Balanced and  
Unbalanced  
Input Signals  
Note that when an UNBAL light goes on, there is a good chance that the input  
signal has been audibly clipped, even if the level meter does not indicate an  
overload.  
If it is necessary to mismatch the switch settings to the signals (for example,  
when switching quickly between balanced and unbalanced inputs) you can  
avoid clipping by setting the peak input levels about 3dB below full scale.  
Undersomeconditions,theUNBALlightmayremainon,eventhoughthesource  
and the 20/20 AD are both set for unbalanced operation.  
1. The source has a pin-3-high unbalanced signal. Try swapping pins 2 and 3.  
2. The source is floating (transformer-isolated). Tie pin 3 to pin 1 at either end  
of the cable.  
The UNBAL light may also flash intermittently if the source is balanced and  
transformer-isolated, and the 20/20 AD shares the source with another con-  
verter or an amplifier. The other box may be unequally loading the positive and  
negative source signals, causing them to become unbalanced. Some solutions  
to this problem are:  
1. Drive the 20/20 AD by itself. This will ensure a balanced signal and minimize  
common mode noise.  
2. If the source's isolation transformer has a center-tapped output, ground the  
center tap.  
3. Reduce the signal level into the 20/20 AD and make it up with the front panel  
gain control(s).  
4. Remove the power cord, open the 20/20 AD, and short out R4, R28, R54 and  
R95. Theseare10Kresistors(brown, black, black, red)locatedneartheinput  
relays. Note that this change may degrade the 20/20 AD's CMRR.  
5. Equalize the load resistances of the other box with the following procedure:  
a) With the 20/20 AD and the other box connected, send a high level tone  
through the source so that the 20/20 AD's UNBAL light goes on.  
b) Connect a 1-Meg potentiometer (set at maximum resistance) between the  
pin 2 signal and ground.  
c) Gradually reduce the resistance until the UNBAL light goes out. (If the  
UNBAL light does not go out, reconnect the pot between pin 3 and ground,  
and repeat.)  
d) Once the UNBAL light goes out, continue reducing the resistance until the  
lightcomesonagain. Setthepotsothatitisinthemiddleoftherangewhere  
the UNBAL light is out.  
e) Remove the pot and measure the resistance. Solder a resistor of that value  
between the correct pin and ground.  
12  
 
20/20 AD User Guide  
Using the 20/20 AD  
Adjust the front panel Gain knobs until the peak input levels are balanced and  
come within a few dB of clipping. For now, make sure that the DISPLAY switch  
intheDIGITAL COMP/LIMITERsectionissettoOFF. Innormaloperation, each  
level meter will indicate the level at the output of the converter in dB referenced  
to digital full scale (dBfs). A lighted green or yellow LED indicates that the signal  
has exceeded the level printed next to it. The red 0dB LED lights when the signal  
exceeds -0.5dBfs. The red OVL LED indicates audible clipping, (clipping which  
lasts for several samples).  
Front panel  
Adjustments  
Normal Level Meter settings  
(1 dB SCALE and DISPLAY set to OFF)  
- red -  
- red -  
OVL  
0
OVL  
0dB  
I dB SCALE  
Turning on the 1dB SCALE switch increases the resolution of the top end of the  
scale shown on the level meters. In this mode, each LED corresponds to 1dB.  
- yellow -  
- yellow -  
- green -  
- green -  
- green -  
- green -  
- green -  
- green -  
- green -  
3
-3dB  
6
-6dB  
12  
18  
24  
30  
36  
42  
60  
-12dB  
-18dB  
-24dB  
-30dB  
-36dB  
-42dB  
-60dB  
Meter Ballistics  
Each meter consists of a peak meter (indicated by an isolated LED) and an  
average meter (indicated by a growing and shrinking bar). The peak meter's  
attack time constant is instantaneous, and its decay time constant is roughly 1  
second. The average meter's time constants approximate those of a VU meter,  
with attack and delay time constants of 10 mS.  
PK HOLD  
1 dB Level Meter settings  
(1 dB SCALE set to ON,  
DISPLAY set to OFF)  
When you press PK HOLD, the meters will indicate the highest level received  
after PK HOLD was pressed. To reset the peak, turn PK HOLD off and then on  
again. Remember that the level meters will display either the normal scale or the  
1dB scale, dependent on the setting of the1 dB SCALE switch.  
- red -  
- red -  
OVL  
0
OVL  
0dB  
- yellow -  
- yellow -  
- green -  
- green -  
- green -  
- green -  
- green -  
- green -  
- green -  
3
-1dB  
-2dB  
-3dB  
-4dB  
-5dB  
-6dB  
-7dB  
-8dB  
-9dB  
ATTEN  
6
12  
18  
24  
30  
36  
42  
60  
The ATTEN button should only be used on balanced input signals.  
If a Gain knob is turned completely counter clockwise and the level meter still  
indicates an overload, you must either press the ATTEN button (which reduces  
the input level by 6dB), or reduce the signal level at its source. Note that ATTEN  
should not be used unnecessarily as it causes a slight reduction in the Common  
Mode Rejection Ratio (CMRR) and thereby increases the potential for hum.  
4-CHAN  
Whenthe4CHANswitchison, the20/20ADconvertsfourchannelsfromanalog  
to digital, with a dynamic range of better than 16 bits. When the 4 CHAN switch  
is off, the 20/20 AD only converts CH1 and CH2, buts adds 12dB (2 bits) to the  
dynamic range. Your selection of mode depends primarily on how many  
channels you need to digitize, and on whether or not you need the extra dynamic  
range. If you plan to record to 16-bit media without using the on-board digital  
compressor, 2-channel mode will give you duplicate digital outputs.  
EMPH  
The EMPH switch adds CD-standard 15/50µsec pre-emphasis to each input  
signal before digitization.  
13  
 
Using the 20/20 AD  
Lexicon  
The 20/20 AD can synchronize to its internally derived sample clock at 44.1kHz  
or 48kHz, or to an external AES or Word Clock signal, in the range of 41-51kHz.  
The EXT SYNC switch allows you to select external Word Clock (WC), external  
AES (AES), or internal source (WC and AES off).  
Synchronization  
and Sample Rate  
Before switching between Internal and External modes, be sure to reduce  
monitor volume as some D/A converters do not mute when they lose lock.  
AES Sync  
The AES SYNC IN signal can be any AES3-compatible signal running at the  
correct sample rate It does not require a Digital Audio Reference Signal  
(DARS). The 20/20 AD is shipped with a 110 ohm termination resistor across  
pins 2 and 3 of the XLR connector, as required by the AES3 specification. If you  
intendtodaisychainseveral20/20ADsfromthesameAESsource,youwillhave  
to open jumper W17 on all units except the last one in the chain.  
Jumper W18 connects the cable shield to system ground. The 20/20 AD is  
shipped with W18 open, assuming that ground is supplied from the generator of  
the sync signal. Note that, unlike the analog inputs, the AES SYNC input is  
transformer isolated; it is not necessary to have a ground connection between  
the 20/20 AD and the sync source.  
All AES connections require twisted -shielded pair cable with a controlled  
impedance between 100 and 125, and less than 15 pF/ft of capacitance.  
Microphone cable will not work reliably. Refer to Page 7 for recommended  
cables.  
Word Clock  
The WORD CLOCK IN signal should be a logic-level (logic high=3V-5V) square  
wave. When shipped, the 20/20 AD terminates the signal with a 75resistor  
through the leftmost jumper in J13. If the clock source is not capable of driving  
high through 75(most TTL and CMOS drivers are not), it may be necessary  
to open this jumper. In this case, it is important that the cable be kept as short  
as possible. As with AES SYNC, if you intend to daisy chain several 20/20 ADs  
from the same Word Clock, you should open the leftmost jumper on J13 on all  
unitsexceptthelastoneinthechain.Ifanalternativegroundpathexistsbetween  
the word clock generator and the 20/20 AD,W16 can be opened to prevent a  
ground loop.  
Note that, unlike the analog and AES SYNC inputs, the Word Clock input is not  
transformer isolated, and requires a ground connection between the 20/20 AD  
and the signal source.  
The Word Clock input requires a 75RG-59/U coaxial cable. Use of ordinary  
audio-quality cable may distort the edges of the Word Clock. This will add jitter  
to the sample rate, which will, in turn, add noise and distortion to the analog/  
digital converter. Refer to Page 7 for recommended cables.  
The WORD CLOCK OUT signal is a logic-level square wave at the sample  
frequency.Itiscapableofdrivinga75load,andisalwaysavailable,evenwhen  
an internal sync source is selected. This output may be used to synchronize one  
ormoreslave20/20ADstoamaster. AswiththeWordClockinput, useonly75Ω  
RG-59/U or similar cable.  
14  
 
20/20 AD User Guide  
Using the 20/20 AD  
Internal and  
External modes  
The 20/20 AD is shipped configured for Internal mode. To operate in External  
mode, press EXT SYNC until the desired sync source (WC or AES) lights. If the  
selected sync source light flashes and the PRES light is not on, it indicates that  
there is no signal present as a sync source. If the selected sync source and  
PRESlightsareon,buttheLOCKlightisoff,theselectedsyncsourceispresent,  
but is not within the 41-51kHz lock range.  
External Mode  
EXT SYNC  
WC  
AES  
AES / WC:  
PRES LOCK  
With WC selected as the external sync source, the front panel SAMPLE RATE  
switch determines the setting of the AES and S/PDIF Channel Status sample  
rate bits, not the actual sample rate, which tracks the external sync source. It is  
important to make sure that the sample rate selected at the front panel agrees  
with the external sync source, as some digital tape recorders determine sample  
rate from the Channel Status bits rather than from the actual sample rate.  
External Mode Error  
States  
EXT SYNC  
WC  
AES  
AES / WC:  
AES / WC:  
PRES  
LOCK  
EXT SYNC  
WC  
AES  
LOCK  
PRES  
To operate in Internal mode, press EXT SYNC until the WC and AES lights are  
both off. Then press SAMPLE RATE until the desired sample rate lights. In  
Internal mode, the PRES light indicates that either WC or AES is available as an  
external sync source.  
Internal Mode  
EXT SYNC  
WC  
AES  
AES / WC:  
PRES  
LOCK  
15  
 
Using the 20/20 AD  
Lexicon  
Dither is a low-level pseudo-random signal which is added to digital audio to  
reduce quantization noise, in effect, by replacing it with anicer soundingnoise.  
About Dither  
If the recorder or workstation which follows the 20/20 AD is capable of recording  
or processing 20 bits, the DITHER switch should be set to 20 BIT. If, however,  
the equipment can only record or process a 16-bit signal, either WIDE or HF  
dither should be selected. This is true even if the compressor is on; the dither  
generator always sends the appropriate signal for a 16-bit recorder, regardless  
of the compressor settings.  
All three of these basic dither types have the same quantization noise reducing  
effect, but differ in the spectral distribution of the replacementnoise.  
WIDE dither produces white noise.  
HF (high frequency) dither produces noise whose amplitude is lower than WIDE  
at low frequencies, but higher at high frequencies. Theoretically, this is less  
audible than WIDE dither, but just as effective.  
20 BIT dither has the same spectrum as HF dither, but with a lower amplitude.  
This setting should be used when recording to 20-bit media.  
It is possible to filter the quantization noise in such a way that almost all of the  
noise within the frequency range where human hearing is most acute is shifted  
tohigherfrequencies.Thisiscalledpsychoacousticallyoptimizednoiseshaping  
(PONS) because it takes advantage of the variations in the ear's sensitivity to  
noise within the 20 Hz - 20 kHz range.  
Psychoacoustically  
Optimized Noise  
Shaping (PONS)  
To activate this dither, press and hold the ALT button, then press DITHER. The  
three dither lights will go on (WIDE, 20BIT and HF) to indicate that you are in  
PONS mode. Note that, due to limitations in processing power, the compressor,  
4-channel mode and the ALT-MUTE function are disabled while PONS mode is  
active. To deactivate PONS, simply press DITHER.  
16  
 
20/20 AD User Guide  
Using the 20/20 AD  
-60.00  
-70.00  
-80.00  
-90.00  
-100.0  
-110.0  
-120.0  
-130.0  
-140.0  
-150.0  
-160.0  
-60.00  
-70.00  
-80.00  
-90.00  
-100.0  
-110.0  
-120.0  
-130.0  
-140.0  
-150.0  
-160.0  
0.0  
2.00k  
4.00k  
6.00k  
8.00k  
10.0k  
12.0k  
14.0k  
16.0k  
18.0k  
20.0k  
0.0  
2.00k  
4.00k  
6.00k  
8.00k  
10.0k  
12.0k  
14.0k  
16.0k  
18.0k  
20.0k  
20/20 AD in 2-channel mode, truncated to 16 bits without  
dither. Note harmonic distortion (spikes at multiples of 1kHz)  
and elevated noise floor.  
Plot of noise + distortion vs. frequency for the 20/20 AD in 2-  
channel mode with a -60dBfs sinewave at 1kHz.  
-60.00  
-70.00  
-80.00  
-90.00  
-100.0  
-110.0  
-120.0  
-130.0  
-140.0  
-150.0  
-160.0  
-60.00  
-70.00  
-80.00  
-90.00  
-100.0  
-110.0  
-120.0  
-130.0  
-140.0  
-150.0  
-160.0  
0.0  
2.00k  
4.00k  
6.00k  
8.00k  
10.0k  
12.0k  
14.0k  
16.0k  
18.0k  
20.0k  
22.0k  
24.0k  
0.0  
2.00k  
4.00k  
6.00k  
8.00k  
10.0k  
12.0k  
14.0k  
16.0k  
18.0k  
20.0k  
HF dither is added before truncation. Noise floor in audio  
rangeisloweredby3dBattheexpenseofincreasednoise  
above 20kHz.  
WIDE dither is added before truncation. Noise floor is ele-  
vated further, but harmonic distortion is removed.  
-60.00  
-70.00  
-80.00  
-90.00  
-100.0  
-110.0  
-120.0  
-130.0  
-140.0  
-150.0  
-160.0  
-60.00  
-70.00  
-80.00  
-90.00  
-100.0  
-110.0  
-120.0  
-130.0  
-140.0  
-150.0  
-160.0  
0.0  
2.00k  
4.00k  
6.00k  
8.00k  
10.0k  
12.0k  
14.0k  
16.0k  
18.0k  
20.0k  
22.0k  
24.0k  
0.0  
2.00k  
4.00k  
6.00k  
8.00k  
10.0k  
12.0k  
14.0k  
16.0k  
18.0k  
20.0k  
PONS dither. Noise floor is down almost to the untruncated  
level in the critical band (2-4kHz), but significantly increased  
at high frequencies.  
20BIT dither is added before truncation. This is similar to HF  
dither, but at a lower level. The dither reduces spurious tones  
(low-level spikes visible in the first plot) at the expense of a  
slightly elevated noise floor.  
17  
 
Using the 20/20 AD  
Lexicon  
DC offsets are created by input amplifiers and A/D converters for a number of  
reasons. In the 20/20 AD, these typically include temperature changes,  
changes between 2-channel and 4-channel mode, change of sample rate or  
sync source, or large adjustments of input level controls. The DSP chips in the  
20/20 AD automatically remove these DC offsets by passing data from the  
converters through a digital high pass filter with a corner frequency of less than  
0.1 Hz.  
Automatic Offset  
Removal  
This high pass filter will reduce output offsets to less than -80dB within one  
minute of a change in input offsets. In order to minimize output offsets, wait at  
least one minute after any of the above changes before recording.  
DC offsets created outside the 20/20 AD are removed by blocking capacitors  
locatedbetweentheanaloginputconnectorsandthefirstamplifierstage. These  
capacitors have a corner frequency of less than 1 Hz, which guarantees less  
than 0.1dB of loss at 10 Hz (referenced to 1 kHz).  
It is possible to extend the low frequency response down to less than 1 Hz by  
shorting these blocking capacitors. This should only be done if the offset of the  
input signal is relatively constant, and less than 1% of full scale (approximately  
150 mV with a 22 dBu input). Transformer-isolated sources are generally safe,  
but non-isolated sources should be checked with a voltmeter. Offsets greater  
than 1% may take several minutes for the digital high pass filter to remove.  
To short out the blocking capacitors, remove C5, C6, C30, C31, C51, C52, C80  
and C81. These are upright tubular capacitors located near the analog input  
connectors. Solder short lengths of wire in place of each. Alternatively, you can  
leave the capacitors installed, and solder short lengths of wire across the pins  
of each on the solder side of the board.  
Until you become comfortable with the operation of the rest of the 20/20 AD, you  
should keep the compressor turned off. Make sure that the DISPLAY, LINK and  
ON buttons in the compressor block are all off (unlighted) and the compressor  
gain knob is set to 0dB.  
Compressor Modes  
Although the function of the digital compressor is similar to that of a standard  
analogcompressor, itsintendedusesaresomewhatdifferent.Threeofthemore  
common uses (Dynamic Range Compression, Non-Linear Transfer Functions  
and Overload Protection) are described below.  
Dynamic Range  
Compression  
This mode is primarily useful when recording program material with a wide  
dynamic range onto 16-bit media in 2-channel mode. (Although the compressor  
will work in 4-channel mode, the dynamic range of 4-channel mode is so close  
to the 16-bit level that any further compression will only raise the noise floor.)  
This use of the compressor requires a bit of artistry, because all settings must  
be optimized to the program material.  
With the compressor on, set compressor GAIN to 6dB. This will increase the  
gain of low level signals (below the THRESHOLD setting) by 6dB, effectively  
squeezing the dynamic range of the converter in 2-channel mode from 110dB  
to 104dB. The DITHER switch automatically adds just enough dither to squeeze  
the remaining dynamic range into the 96dB dynamic range of 16-bit media.  
18  
 
20/20 AD User Guide  
Using the 20/20 AD  
Note that the 6dB GAIN setting is given only as an example. Feel free to  
experiment with any level in this mode. (GAIN settings above 12dB are not  
available because at that point practically all of the dynamic range of the  
converter will fit onto 16-bit media. )  
Set THRESHOLD to -24dB. Now, when the signal level exceeds -24dB, the gain  
isgraduallyreduceduntil,at0dBsignallevel,thecompressiongainisalsoat0dB  
(unity).  
The graph below shows a plot of gain vs. input level with compressor GAIN at  
6dB and THRESHOLD set to -24dB. Note that, unlike some compressors, the  
gain curve is not linear, but changes smoothly. This reduces high-frequency  
artifacts which can result from aggressive compression.  
Threshold  
(-24dB)  
"0" Level  
Unity Gain  
-104dB  
-110dB  
"0" Level  
Input  
The settings of the GAIN and THRESHOLD knobs determine the compression  
ratio, according the formula:  
___| threshold |___  
ratio =  
| threshold| gain  
In our example, the compression ratio is 24÷(24 6) = 1.33:1. This is not a very  
intense compression by most standards but, remember, the point is to pack as  
much music onto the recording media as possible without introducing audible  
artifacts. As the following table of compression ratios shows, most of the ratios  
are below 2:1. In order to minimize the audibility of compression artifacts, use  
ratios from the unshaded portion of the table. Setting the threshold at any level  
higher than a 4:1 ratio will force the ratio to 4:1.  
19  
 
Using the 20/20 AD  
Lexicon  
GAIN  
0dB  
-3dB  
1
2
3
4
5
6
7
8
9
10  
11  
12  
1.50:1  
1.20:1  
1.14:1  
1.11:1  
1.09:1  
1.07:1  
1.06:1  
1.06:1  
1.05:1  
1.05:1  
1.04:1  
1.04:1  
1.03:1  
1.02:1  
3.00:1  
1.50:1  
1.33:1  
1.25:1  
1.20:1  
1.16:1  
1.14:1  
1.12:1  
1.11:1  
1.10:1  
1.09:1  
1.08:1  
1.07:1  
1.06:1  
4.00:1  
2.00:1  
1.66:1  
1.42:1  
1.33:1  
1.27:1  
1.23:1  
1.20:1  
1.17:1  
1.16:1  
1.14:1  
1.13:1  
1.11:1  
1.09:1  
4.00:1  
3.00:1  
2.00:1  
1.66:1  
1.50:1  
1.40:1  
1.33:1  
1.28:1  
1.25:1  
1.22:1  
1.20:1  
1.18:1  
1.15:1  
1.12:1  
4.00:1  
4.00:1  
2.66:1  
2.00:1  
1.71:1  
1.55:1  
1.45:1  
1.38:1  
1.33:1  
1.29:1  
1.26:1  
1.23:1  
1.20:1  
1.16:1  
4.00:1  
4.00:1  
4.00:1  
2.50:1  
2.00:1  
1.75:1  
1.60:1  
1.50:1  
1.42:1  
1.37:1  
1.33:1  
1.30:1  
1.25:1  
1.20:1  
4.00:1  
4.00:1  
4.00:1  
3.33:1  
2.40:1  
2.00:1  
1.77:1  
1.63:1  
1.53:1  
1.46:1  
1.41:1  
1.36:1  
1.30:1  
1.24:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
3.00:1  
2.33:1  
2.00:1  
1.80:1  
1.66:1  
1.57:1  
1.50:1  
1.44:1  
1.36:1  
1.28:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
2.81:1  
2.28:1  
2.00:1  
1.81:1  
1.69:1  
1.60:1  
1.52:1  
1.42:1  
1.33:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
3.50:1  
2.66:1  
2.25:1  
2.00:1  
1.83:1  
1.71:1  
1.62:1  
1.50:1  
1.38:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
3.20:1  
2.57:1  
2.22:1  
2.00:1  
1.84:1  
1.73:1  
1.57:1  
1.44:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
4.00:1  
3.00:1  
2.50:1  
2.20:1  
2.00:1  
1.85:1  
1.66:1  
1.50:1  
-6dB  
-8dB  
-10dB  
-12dB  
-14dB  
-16dB  
-18dB  
-20dB  
-22dB  
-24dB  
-26dB  
-30dB  
-36dB  
As both the formula and the table show, raising the threshold increases the ratio  
making the compression more audible above the THRESHOLD setting.  
However, because signals below the threshold are boosted linearly through the  
compressor, raising the threshold may decrease the amount of time the  
compressor is reducing gain. This may decrease the audible artifacts.  
Conversely, if you lower the threshold all the way to -36dB, you will pretty much  
guarantee that all but the quietest signals will experience some amount of gain  
reduction, but the ratio will be so small (1.2:1) that the total effect may be less  
noticeable than a higher threshold setting.  
Start with RELEASE time set to about 180 mS, then shorten the time for  
percussive program material and lengthen it for material such as voice.  
RELEASE times of 0mS to are available. Attack time of the compression is  
automatically set as a function of the signal rise time.  
Compressor Knob Values  
100  
180  
6
-16  
5
7
56  
-18  
-14  
-20  
-22  
-24  
-26  
-30  
-12  
-10  
-8  
-6  
32  
320  
560  
4
8
18  
10  
5
3
9
2
10  
11  
1S  
1.5  
1
0
0
12  
-3  
-36  
0
GAIN  
(in dB)  
THRESHOLD  
(in dB)  
RELEASE  
(in mS and S)  
Note that changing the settings of GAIN and THRESHOLD while recording may  
produce audible clicks. If, for artistic reasons, you wish to turn the compressor  
off during a recording, do so by increasing the release time to . (Using the  
compressor on/off switch may produce a click.)  
20  
 
20/20 AD User Guide  
Using the 20/20 AD  
When RELEASE is set to 0, the compressor acts like an amplifier with a non- Non-linear  
linear transfer function. For example, setting GAIN to 6dB and THRESHOLD to Transfer Functions  
-14dB produces a close approximation of a generic tape saturation curve. This  
mode is also useful for overload protection of some types of percussive  
instruments.  
Note: This function is not available in 4-channel mode.  
When recording live music onto 16-bit media, the recording engineer tries to Overload Protection  
maximize dynamic range by keeping the signal as close as possible to full scale  
without actually clipping. Peak levels are typically set a few dB below full scale,  
because losing a few dB of dynamic range is far preferable to clipping.  
The digital compressor eliminates this tradeoff. To use it, first set the input levels  
to give yourself a comfortable margin between peak level and clipping. For  
example, to give yourself 4dB of margin, push 1db SCALE and PK HOLD and  
adjust the input level until the peak reads -4dB (the LED labeled 18).  
Now, turn on the compressor . Set GAIN to 4dB. This increases the signal level  
by 4dB (restoring 0dB peak level ). Set THRESHOLD to 0dB to automatically  
select the maximum practical ratio.  
Whenaninputsignalexceedsthepeaklevelby4dBorless,thecompressorgain  
isautomaticallyreducedjustenoughtoassurethatthecompressoroutputnever  
exceeds 0dB. After the overload has passed, the compressor gradually brings  
the gain back up to 4dB. The RELEASE knob determines the speed of this  
recovery.  
Note: This automatic gain reduction can add considerable distortion to the  
leading edge of the overload signal. With THRESHOLD set at 0dB (or at any  
value less than or equal to the GAIN setting), gain reduction will not start until  
the signal is about to clip too late to prevent leading edge distortion. If this  
distortion is unacceptable, try lowering the THRESHOLD setting so that  
compression starts before the signal clips. If, for example, THRESHOLD is  
lowered to -8dB, compressor gain reduction will be gradual, instead of being  
reduced all at once at -4dB.  
It is important to remember that the lower you set THRESHOLD, the sooner  
compression will start, and the more likely you are to color signals which were  
not in any danger of clipping.  
21  
 
Using the 20/20 AD  
Lexicon  
To summarize:  
1. Set the input levels so that the peak level is a few dB below clipping.  
2. Turn on the compressor.  
3. Set compressor GAIN to the input level safety margin, and THRESHOLD to  
0dB.  
4. Adjust RELEASE to suit the program material. The faster the expected  
clipping source, the faster the release time should be.  
5. Adjust THRESHOLD to suit the program material. If the expected source  
material is percussive, the chance of clipping is relatively slim, and you dont  
want to color the rest of the music, leave THRESHOLD at 0dB. Otherwise,  
itmaybenecessarytolowertheTHRESHOLDsettinguntiltheclippedsignal  
sounds acceptable.  
Note: The compressor can only prevent clipping if the analog/digital converter  
doesnotoverload. Anysignalwhichoverloadstheconverterwillalsooverload  
the compressor.  
Although the compressor works in both 4-channel and 2-channel mode, the  
available dynamic range in 4-channel mode is only a few dB greater than the  
capacity of a 16-bit recorder, so nothing is gained by setting your margin greater  
than a few dB unless, of course, there is a specific audio effect you are trying  
to achieve.  
Digitizing Analog Tape Aslightlyunorthodoxuseofthecompressorallowsyoutoautomaticallyfinetune  
the level so that the highest peak will be set to exactly 0dBfs when transferring  
analog tape (or other repeatable source) to the digital domain.  
To do this:  
1. Do a dry run of the loudest section(s) of the tape through the 20/20 AD to  
check the level. Using 1dB SCALE and PK HOLD, adjust the input level until  
the peak comes within 6dB of clipping.  
2. Turn on the compressor. Set GAIN to 6dB, THRESHOLD to 0dB, and  
RELEASE to infinity. Do another dry run of the loudest section(s) of the tape.  
If you turn on DISPLAY, you will see the gain reduction meter grow down  
during the peaks and stay down.  
3. Rewind the tape, turn on the digital recorder, and play the tape all the way  
through. The compressor will add just enough fixed gain to boost the peak  
to 0dBfs. Because the gain doesn't change, there are no compression  
artifacts.  
If you are in 4-channel mode, remember that you have less dynamic range  
available, so you should try harder to set the input levels as close to 0dBfs as  
practical, only relying on the 20/20 AD for the last 2 or 3dB.  
22  
 
20/20 AD User Guide  
Using the 20/20 AD  
In 2-channel mode, when the LINK switch is off, each of the two channels is LINK  
compressed independently. When the LINK switch is on, the louder of the two  
channels sets the amount of compression. This serves the same function as  
stereolinkinanalogcompressors.Whenyouarerecordingastereosignaland  
want to maintain the stereo image, LINK should be on. However, if you have the  
compressor set up to protect against short overloads, or if you have the release  
time set to 0, keeping LINK off will probably sound better. When recording two  
mono signals, LINK should be off.  
In 4-channel mode, when the LINK switch is off, channels 1 and 2 and channels  
3 and 4 are linked in stereo pairs. When the link switch is on, all four channels  
are linked. (There is no way to compress four mono signals.)  
When the DISPLAY switch is on, the level meters indicate the amount of DISPLAY  
compressor gain reduction in each channel. This shows the amount that gain (in  
dB) which is being reduced as the input level exceeds the threshold.  
The 20/20 AD provides two sets of digital audio output connectors with three  
Digital Audio Outputs  
connectorsineachset.Infour-channelmode,thefirstsetcontainsdigitizedCH1  
and CH2, the second set contains CH3 and CH4. In two-channel mode, both  
sets contain identical copies of digitized CH1 and CH2. The unused set may be  
used to drive a backup recorder or to synchronize another 20/20 AD.  
Each set of digital outputs contains three connectors: an XLR connector which  
transmits AES3 digital audio, an RCA connector which transmits an S/PDIF-  
compatible digital audio signal, and a TOSLINK-compatible optical transmitter.  
All outputs are always active. The front panel FORMAT switch selects  
between AES and S/PDIF-compatible Channel Status bits, but it doesnt enable  
or disable the actual transmitters. Make sure that the FORMAT setting is set for  
AES when using the XLR connector and S/PDIF when using the RCA or optical  
connectors.  
Pin 1 of the AES outputs is connected to system ground through jumpers W26  
and W27. If the shield is connected to ground at the receiver end, these jumpers  
should be opened. Note that, because of the transformer or optical isolation, it  
isnotnecessarytoconnectthegroundbetweenthe20/20ADandtheequipment  
receiving the digital audio data, regardless of which output type you choose.  
Asmentionedabove,onlyappropriatehigh-frequencycablesshouldbeusedfor  
digital audio. Refer to Page 7 for recommended cables.  
23  
 
Using the 20/20 AD  
Lexicon  
When recording in 4-channel mode to two DAT recorders, it may be useful to  
record a "clapper" signal to both DATs to facilitate realignment during post-  
production. When you press ALT-LINK, the 20/20 AD will generate 8mS of  
silence, followed by an 8-cycle 1 kHz square wave burst at -12dBfs This signal  
is time-aligned on all four output channels. You can align the two DAT tapes on  
a disk audio production system by visually lining up the first edge of the burst,  
or you can align them audibly by subtracting channel 3 from channel 1 and  
adjusting the offset until the click disppears.  
Realigning Two DAT  
Recordings  
The 20/20 AD includes a digital sinewave generator which is accurate to 20 bits,  
with a dynamic range of more than 120dB. This generator is useful in testing the  
digital outputs. To use it:  
Digital Sinewave  
Generator  
1. Use the SAMPLE RATE, FORMAT and 4-CHAN buttons to select the output  
mode. (These controls are locked out in sinewave generator mode.)  
2. Press and hold down 1dB SCALE (ALT)and press the 4-CHAN button. 1dB  
SCALEand4-CHANwillflashtoindicatethattheunitwillnowgeneratedigital  
sinewaves. The compressor GAIN knob adjusts the frequency of the sine-  
wave and the THRESHOLD knob adjusts the amplitude, as shown below.  
3000  
2062.5  
1500  
1125  
937.5  
750  
5062.5  
6937.5  
9000  
-16  
-18  
-20  
-22  
-24  
-26  
-30  
-36  
-14  
-12  
-10  
-8  
11062.5  
14062.5  
17062.5  
20062.5  
-6  
562.5  
375  
-3  
187.5  
0
-60  
(GAIN)  
Frequency in Hz  
(THRESHOLD)  
Level in dBfs  
48 kHz Sample Rate  
2755  
1895  
1378  
1033.5  
861.5  
689  
4651  
6374  
8269  
10163.5  
12920  
15676  
18432.5  
517  
344.5  
172.5  
(GAIN)  
Frequency in Hz  
44.1 kHz Sample Rate  
3. To turn off the sinewave generator, press and hold down 1dB SCALE (ALT)  
and press 4-CHAN. The 20/20 AD will resume normal operation.  
Note that in sinewave generator mode, only the compressor GAIN and  
THRESHOLDknobshaveanyeffect.Pressinganydual-keycombinationsother  
than ALT4-CHAN may invoke undocumented diagnostics routines. If you  
accidentally get into diagnostics mode, the best way to exit is to turn the power  
off, then on again.  
24  
 
Definition of Terms  
AES : Audio Engineering Society.  
frequency : the number of vibrations per unit of time, ex-  
pressed in cycles per second (Hz).  
aliasing : a form of distortion which occurs when an A/D  
converter’s sample rate is less than twice the highest fre- harmonic distortion : the appearance of harmonics of the  
quency component of a signal. input signal at the output of a device.  
amplitude : the maximum departure of the value of a wave- link : the tying of two channels of the 20/20 AD compressor to  
form from its average value. a shared gain curve.  
attenuation : a reduction in the amplitude of an audio signal. lock : two or more audio signals which have the same  
frequency and phase.  
balanced signals : two equal and opposite signals run  
through a twisted pair. The advantage of balanced signals is mS : millisecond: 1/1000 of a second.  
that common mode noise tends to be added equally to both  
portions of the pair; it can be removed by subtracting the  
negative from the positive signal. (Common mode noise  
rejection cannot be performed on unbalanced signals, which  
oversampling : a method of sampling an audio signal at  
many times the audio bandwidth (typically, 64-256 times) in  
order to reduce the complexity of the anti-aliasing filter. The  
oversampled data is then digitally filtered to reduce the  
consist of an audio signal and a ground return.)  
sample rate.  
bandwidth : the arithmetic difference between the upper and  
lower cut-off frequencies of an audio system.  
period : the interval of time required for a waveform to  
complete one cycle and begin to repeat itself. Period = 1/  
bit : binary digit: the smallest unit of digital information, frequency.  
equivalent to the result of a choice between two alternatives  
(yes or no, on or off , 0 or 1). A series of bits make up a binary  
number called a sample word.  
phase : the angular relationship between two signals, meas-  
ured in degrees, where 360° = the period of the waveform.  
When two identical signals are always of the same polarity,  
common mode noise :any signal which is common to both they are said to be in phase, or to have a phase of 0°. When  
the negative and positive polarity of a balanced signal. In a they are always of opposite polarity, they are said to be out of  
typical audio signal, common mode noise is introduced over phase, or to have a phase of 180°.  
the length of the cable by outside interference. A typical  
example is line hum.  
polarity : the positive or negative direction of an electrical  
force.  
CMRR : Common Mode Rejection Ratio: measures the ability  
quantization : the process of converting an infinitely-varying  
of a system to reject common mode noise. It is expressed as  
waveformintoafiniteseriesofdiscretelevels. Theinaccuracy  
the ratio of common mode noise entering the system to the  
introduced by this process is called quantization error.  
common mode noise leaving it.  
rms amplitude : root mean squared amplitude: a way of  
assigning a DC power equivalent to an AC waveform. For a  
dBfs : decibel full scale: this is the unit used to describe the  
amplitude of a digital audio signal where 0 equals digital full  
sine wave, this is 0.707 times the peak level.  
scale.  
sample rate : the periodic interval at which an analog audio  
signal is sampled, e.g. 44.1 or 48 kHz.  
digital headroom : does not exist, see dBfs.  
distortion : an unwanted change in a waveform as it passes  
signal-to-noise ratio : the ratio between the maximum peak  
through an electronic component or from one medium to  
signal and the wideband noise in the absence of signal. This  
another.  
number becomes more meaningful when distortion is added.  
dither : a low level signal which is added to digital audio to It is then expressed as S/(N+D).  
reduce quantization noise, in effect, by replacing it with noise  
which is perceived as less offensive.  
S/PDIF : Sony/Phillips Digital Interface Format.  
synchronize : to arrange events to occur simultaneously. In  
audio, the events are both the period and the phase of a  
signal.  
dynamic range : when used to describe digital-to-analog or  
analog-to-digital converters, dynamic range is the ratio of the  
full scale signal to the broadband noise (0-20 kHz) measured  
THD+N : the ratio of the rms amplitude of the distortion and  
with a -60dBfs signal. In other words, it is the S/(N+D)  
noise that is added to a signal to the rms amplitude of the  
measured with a -60 dBfs input signal and a 20 kHz low-pass  
signal itself. This can be expressed in percent or decibels. S/  
filter.  
(N+D) = 1/(THD+N).  
EBU : European Broadcast Union.  
time constant : the amount of time it takes for a signal to rise  
or fall 63% of its full scale. This only pertains to signals which  
emphasis : a form of equalization which increases high  
frequency signals in order to maximize the signal-to-noise  
rise and fall exponentially, such as the 20/20 AD meter time  
ratio. De-emphasis is the inverse of emphasis.  
constants or compression gain curves.  
equalization : the process of increasing or decreasing the  
amplitude of some frequency ranges of a signal with respect  
to others.  
word clock : a square wave used as a synchronization  
source. Its frequency equals the sample rate.  
26  
 
Definition of Terms  
AES : Audio Engineering Society.  
frequency : the number of vibrations per unit of time, ex-  
pressed in cycles per second (Hz).  
aliasing : a form of distortion which occurs when an A/D  
converter’s sample rate is less than twice the highest fre- harmonic distortion : the appearance of harmonics of the  
quency component of a signal. input signal at the output of a device.  
amplitude : the maximum departure of the value of a wave- link : the tying of two channels of the 20/20 AD compressor to  
form from its average value. a shared gain curve.  
attenuation : a reduction in the amplitude of an audio signal. lock : two or more audio signals which have the same  
frequency and phase.  
balanced signals : two equal and opposite signals run  
through a twisted pair. The advantage of balanced signals is mS : millisecond: 1/1000 of a second.  
that common mode noise tends to be added equally to both  
portions of the pair; it can be removed by subtracting the  
negative from the positive signal. (Common mode noise  
rejection cannot be performed on unbalanced signals, which  
oversampling : a method of sampling an audio signal at  
many times the audio bandwidth (typically, 64-256 times) in  
order to reduce the complexity of the anti-aliasing filter. The  
oversampled data is then digitally filtered to reduce the  
consist of an audio signal and a ground return.)  
sample rate.  
bandwidth : the arithmetic difference between the upper and  
lower cut-off frequencies of an audio system.  
period : the interval of time required for a waveform to  
complete one cycle and begin to repeat itself. Period = 1/  
bit : binary digit: the smallest unit of digital information, frequency.  
equivalent to the result of a choice between two alternatives  
(yes or no, on or off , 0 or 1). A series of bits make up a binary  
number called a sample word.  
phase : the angular relationship between two signals, meas-  
ured in degrees, where 360° = the period of the waveform.  
When two identical signals are always of the same polarity,  
common mode noise :any signal which is common to both they are said to be in phase, or to have a phase of 0°. When  
the negative and positive polarity of a balanced signal. In a they are always of opposite polarity, they are said to be out of  
typical audio signal, common mode noise is introduced over phase, or to have a phase of 180°.  
the length of the cable by outside interference. A typical  
example is line hum.  
polarity : the positive or negative direction of an electrical  
force.  
CMRR : Common Mode Rejection Ratio: measures the ability  
quantization : the process of converting an infinitely-varying  
of a system to reject common mode noise. It is expressed as  
waveformintoafiniteseriesofdiscretelevels. Theinaccuracy  
the ratio of common mode noise entering the system to the  
introduced by this process is called quantization error.  
common mode noise leaving it.  
rms amplitude : root mean squared amplitude: a way of  
assigning a DC power equivalent to an AC waveform. For a  
dBfs : decibel full scale: this is the unit used to describe the  
amplitude of a digital audio signal where 0 equals digital full  
sine wave, this is 0.707 times the peak level.  
scale.  
sample rate : the periodic interval at which an analog audio  
signal is sampled, e.g. 44.1 or 48 kHz.  
digital headroom : does not exist, see dBfs.  
distortion : an unwanted change in a waveform as it passes  
signal-to-noise ratio : the ratio between the maximum peak  
through an electronic component or from one medium to  
signal and the wideband noise in the absence of signal. This  
another.  
number becomes more meaningful when distortion is added.  
dither : a low level signal which is added to digital audio to It is then expressed as S/(N+D).  
reduce quantization noise, in effect, by replacing it with noise  
which is perceived as less offensive.  
S/PDIF : Sony/Phillips Digital Interface Format.  
synchronize : to arrange events to occur simultaneously. In  
audio, the events are both the period and the phase of a  
signal.  
dynamic range : when used to describe digital-to-analog or  
analog-to-digital converters, dynamic range is the ratio of the  
full scale signal to the broadband noise (0-20 kHz) measured  
THD+N : the ratio of the rms amplitude of the distortion and  
with a -60dBfs signal. In other words, it is the S/(N+D)  
noise that is added to a signal to the rms amplitude of the  
measured with a -60 dBfs input signal and a 20 kHz low-pass  
signal itself. This can be expressed in percent or decibels. S/  
filter.  
(N+D) = 1/(THD+N).  
EBU : European Broadcast Union.  
time constant : the amount of time it takes for a signal to rise  
or fall 63% of its full scale. This only pertains to signals which  
emphasis : a form of equalization which increases high  
frequency signals in order to maximize the signal-to-noise  
rise and fall exponentially, such as the 20/20 AD meter time  
ratio. De-emphasis is the inverse of emphasis.  
constants or compression gain curves.  
equalization : the process of increasing or decreasing the  
amplitude of some frequency ranges of a signal with respect  
to others.  
word clock : a square wave used as a synchronization  
source. Its frequency equals the sample rate.  
27  
 
3 Oak Park  
Bedford, MA 01730 USA  
Tel: 781-280-0300  
Fax: 781-280-0499  
 

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