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20/20 AD 20 Bit A/D Converter User Guide[...]

Lexicon Copyright © 1994 Lexicon Inc. 100 Beaver Street Waltham MA 02154-8441 USA 617-736-0300 Fax: 617-891-0340 All Rights Reserved This publication is protected by copyright and all rights are reserved. No part of it may be reproduced or transmitted by any means or in any form, without express prior consent in writing from Lexicon. Precautions T[...]

20/20 AD User Guide 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 in these instructions violates safety standards of design manufacture and intended use of the instrument. Le[...]

20/20 AD User Guide Contents Introduction Controls and Connectors ...................................................................... 1 Unpacking • Power • Mounting .......................................................... 1 Front Panel Controls .......................................................................... 2 The Converter • T[...]

20/20 AD User Guide Introduction The 20/20 AD is an analog-to-digital converter which can be configured for two 20-bit channels, or four 18-bit channels. Twin on-board signal processors 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 [...]

1 Controls and Connectors 20/20 AD User Guide After unpacking the 20/20 AD, save all packing materials in case you ever need to ship the unit. Thoroughly inspect the 20/20 AD and packing materials for signs of damage. Report any shipment damage to the carrier at once. The following accessories are included with the 20/20 AD: 1. Power Cable 2. User [...]

Lexicon 2 Controls and Connectors 1dB SCALE ALT PK HOLD ATTEN 1 THRESH UNBAL THRESH UNBAL 2 3 THRESH UNBAL THRESH UNBAL 4 4 CHAN ~ EMPH OVL 0 3 6 12 18 24 30 36 42 60 OVL 0 3 6 12 18 24 30 36 42 60 MUTE + • – M C P 20/20 AD Front Panel Controls The Converter 4 CHAN Push on-push off selection of 2-channel or 4-channel mode; lit when in 4-channel[...]

3 Controls and Connectors 20/20 AD User Guide The Level Meters When the compressor is off , the behavior of the level meters is determined by the settings 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[...]

Lexicon 4 Controls and Connectors Synchronization and Output Formatting Alternate Modes SAMPLE RATE 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 Clock is selected as an exter- nal sync source, the AES Channel Status Sample Rate bit will be set to the sample rate valu[...]

5 Controls and Connectors 20/20 AD User Guide PUSH DO WORD CLOCK IN DO 3 + 4 1 + 2 OUT AES SYNC IN V ~ PUSH 4I N 3 UNBAL BAL PUSH PUSH 2I N 1 UNBAL BAL PUSH Digital Output Two 3-pin XLR connectors, professional format AES bal- anced digital outputs. Two consumer format optical (fiber-optic) outputs. Two consumer format unbalanced coaxial RCA output[...]

Lexicon 6 Controls and Connectors Audio Connections XLR Pinout Signal Mating Connector Description AES/EBU XLR A3M Balanced RS-422 Digital Input AES/EBU XLR A3F Balanced RS-422 Digital Output S/PDIF RCA Unbalanced 75 Ω Consumer Digital Output S/PDIF Consumer Digital Consumer Digital Audio Optical Output Word Clock BNC Unbalanced 75 Ω Analog Inp[...]

7 Controls and Connectors 20/20 AD User Guide AES/EBU Digital Output This interface requires balanced connections using high-quality, low-capaci- tance, controlled-impedance, data communication, twisted-shielded pair cable. Microphone cable may introduce a significant amount of jitter into the signal, causing distortion. Use commercially-available,[...]

9 Using the 20/20 AD 20/20 AD User Guide Analog Inputs 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, an[...]

Lexicon 10 Using the 20/20 AD Jumper settings (as set at the factory) W18 CHASSIS GROUND ANALOG IN AES SYNC IN WORD CLOCK IN AES OUT S/PDIF OUT XLR CONNECTOR SHIELDED TWISTED PAIR BNC CONNECTOR 75 OHM COAXIAL 600 OHM W1 W9 W11 W13 W2 W10 W12 W14 W17 110 OHM W16 J13 75 OHM SYSTEM GROUND RCA CONNECTOR W26 W27 CHASSIS CONNECTED TO EARTH GROUND THROUGH[...]

11 Using the 20/20 AD 20/20 AD User Guide The following figure indicates the locations and factory settings of these jumpers 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 (or upper) two pins. (Note that the jumpers in J13 have onl[...]

Lexicon 12 Using the 20/20 AD 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. Note that when an UNBAL light goes on, there is a good chance[...]

13 Using the 20/20 AD 20/20 AD User Guide 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 in the DIGITAL COMP/LIMITER section is set to OFF. In normal operation, each level meter will indicate the level at the output of the converter in dB re[...]

Lexicon 14 Using the 20/20 AD 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). Before switching between Internal and[...]

15 Using the 20/20 AD 20/20 AD User Guide 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 sour[...]

Lexicon 16 Using the 20/20 AD About Dither Dither is a low-level pseudo-random signal which is added to digital audio to reduce quantization noise, in effect, by replacing it with a “ nicer sounding ” noise. 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 t[...]

17 Using the 20/20 AD 20/20 AD User Guide -160.0 -150.0 -140.0 -130.0 -120.0 -110.0 -100.0 -90.00 -80.00 -70.00 -60.00 0.0 2.00k 4.00k 6.00k 8.00k 10.0k 12.0k 14.0k 16.0k 18.0k 20.0k Plot of noise + distortion vs. frequency for the 20/20 AD in 2- channel mode with a -60dBfs sinewave at 1kHz. 20/20 AD in 2-channel mode, truncated to 16 bits without [...]

Lexicon 18 Using the 20/20 AD Automatic Offset Removal 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[...]

19 Using the 20/20 AD 20/20 AD User Guide ___| threshold |___ | threshold| – gain ratio = 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 medi[...]

Lexicon 20 Using the 20/20 AD 0dB -3dB -6dB -8dB -10dB -12dB -14dB -16dB -18dB -20dB -22dB -24dB -26dB -30dB -36dB 1 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 2 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 34 4.00:1 3.00:1 2.00:1 1.66:1 1.50[...]

21 Using the 20/20 AD 20/20 AD User Guide When RELEASE is set to 0, the compressor acts like an amplifier with a non- linear transfer function. For example, setting GAIN to 6dB and THRESHOLD to -14dB produces a close approximation of a generic tape saturation curve. This mode is also useful for overload protection of some types of percussive instru[...]

Lexicon 22 Using the 20/20 AD 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 b[...]

23 Using the 20/20 AD 20/20 AD User Guide In 2-channel mode, when the LINK switch is off, each of the two channels is 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 “ stereo link ” in analog compressors. When you are recording a stereo signal [...]

Lexicon 24 Using the 20/20 AD 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[...]

25 Using the 20/20 AD 20/20 AD User Guide Operating the 20/20 AD in the Digital Mode The 20/20 AD's Digital Input mode can be used to make use of the digital compressor, or to noise shape the dither spectra before recording 20-bit source material to a 16-bit recording device*. In Digital Input Mode, format in = format out. No format conversion[...]

27 20/20 AD User Guide Digital Audio Inputs and Outputs AES, S/PDIF: XLR, 3-wire balanced RCA, 2-wire unbalanced coaxial line Optical, optical fiber Word Clock: BNC, 75 Ω , unbalanced Front Panel Controls Converter Gain (4) 1dB SCALE/ALT PK HOLD ATTEN/MUTE 4 CHAN/~ EMPH DISPLAY/RMT LINK/ tone burst ON/VID Compressor Gain Compressor Threshold Comp[...]

Definition of Terms 29 20/20 AD User Guide Definition of Terms AES : Audio Engineering Society. aliasing : a form of distortion which occurs when an A/D converter’s sample rate is less than twice the highest fre- quency component of a signal. amplitude : the maximum departure of the value of a wave- form from its average value. attenuation : a re[...]