Lexicon 2 BIT A-D CONVERTER - REV Скачать руководство пользователя страница 21

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Using the 20/20 AD

20/20 AD User Guide

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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
dither. Note harmonic distortion (spikes at multiples of 1kHz)
and elevated noise floor.

WIDE dither is added before truncation. Noise floor is ele-
vated further, but harmonic distortion is removed.

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HF dither is added before truncation. Noise floor in audio
range is lowered by 3dB — at the expense of increased noise
above 20kHz.

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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.

PONS dither. Noise floor is down almost to the untruncated
level in the critical band (2-4kHz), but significantly increased
at high frequencies.

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Содержание 2 BIT A-D CONVERTER - REV

Страница 1: ...20 20 AD 20 Bit A D Converter User Guide ...

Страница 2: ...asonable protection against such interference in a residential installation However there is no guarantee that interference will not occur in a particular installation If this equipment does cause interference to radio or television reception which can be determined by turning the equipment OFF and ON the user is encouraged to try to correct the interference by one or more of the following measure...

Страница 3: ...E 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 KEEP AWAY FROM LIVE CIRCUITS 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 conn...

Страница 4: ...rounding 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 D...

Страница 5: ... 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 input...

Страница 6: ...d 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 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 p...

Страница 7: ...meter LEDs to hold at the highest level received Press again to deactivate ATTEN Press to attenuate balanced input signals by 6dB Press again to turn attenuation off Do not use this control on unbalanced signals The 20 20 AD front panel controls are divided into three functional groups for conversion compression and synchronization output formatting THRESH When the compressor is on indicates that ...

Страница 8: ...compres sor THRESHOLD Sets the level at which com pression starts none 1 dB SCALE DISPLAY Normal 1 dB Scale Gain Reduction Normal 1 dB Scale Gain Reduction none 1 dB SCALE DISPLAY Normal Normal Normal Blank 1 dB Scale Gain Reduction Switches ON Meters 1 2 Meters 3 4 4 Channel Mode 2 Channel Mode 1dB SCALE GAIN REDUCTION NORMAL OVL 0 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 11 OVL 0 3 6 12 18 24 30 3...

Страница 9: ...S WC SAMPLE RATE EXT SYNC WIDE HF 48 AES WC PRES 20 BIT PL DN 44 1 SPDIF AES LOCK Alternate Modes ALT ATTEN Mutes the outputs Note that the level meters still work To unmute hold ALT and press ATTEN again ALT 4 CHAN Activates output of a digital sinewave at the AES ports ALT DISPLAY Activates remote mode front panel lockout When acti vated the 1dB SCALE and DISPLAY LEDs flash and front panel contr...

Страница 10: ...h be tween the input signal and the setting of this switch will be indicated on the front panel by the BAL LED Analog Input 3 pin XLR connectors In 2 channel mode only inputs 1 and 2 are active AC Power Standard 3 pin IEC power connector Sync Input 3 pinXLRconnector professionalformatAES EBUbalanced digital input 75Ω unbalanced BNC connectors for word clock in out The input and output connectors c...

Страница 11: ...Digital Input AES EBU XLR A3F Balanced RS 422 Digital Output S PDIF RCA Unbalanced 75Ω Consumer Digital Output S PDIF Consumer Digital Consumer Digital Audio Optical Output Word Clock BNC Unbalanced 75Ω Analog Input XLR pin 2 high Connectors 2 high 3 low 1 ground 1 ground 3 low 2 high Female Male XLR Pinout pin 2 high by convention ...

Страница 12: ...able 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 124Ω 12 2 pF ft S PDIF Consumer Digital Audio Belden 9259 RG 59 U 22 AWG conductor 242 O D 75Ω 17 3 pF ft Belden 8218 27 AWG conductor 150 O D 75Ω 20 5 pF ft Maximum recommended length 32 ft 10M S PDIF Consumer Digital Audi...

Страница 13: ...utton 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 positive return shield polarity unbalanced screen and positive open or return polarity return In a typical studio environment each piece of equipment is connected to a star ground through the safety ground on the AC ...

Страница 14: ... PAIR 3 2 1 3 2 1 XLR CONNECTOR XLR CONNECTOR 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 grou...

Страница 15: ...e only two pins to open simply remove the shunting block U30 U31 W18 W17 U42 U48 U49 J13 W27 W26 W16 Analog Jumpers Digital Jumpers W2 W9 W10 W11 W12 W13 W14 W1 Jumper locations shown with factory settings Note that the 20 20 AD s 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 li...

Страница 16: ...tive 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 powe...

Страница 17: ... 1dB scale dependent on the setting of the1 dB SCALE switch ATTEN 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 ...

Страница 18: ...ork reliably Refer to Page 7 for recommended cables 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 75Ω resistor through the leftmost jumper in J13 If the clock source is not capable of driving high through 75Ω most TTL and CMOS drivers are not it may be necessary to open this jumper In this case it is important t...

Страница 19: ...F 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 Internal and External modes External Mode AES WC EXT SYNC WC AES...

Страница 20: ...ise 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 ...

Страница 21: ...k 8 00k 10 0k 12 0k 14 0k 16 0k 18 0k 20 0k HF dither is added before truncation Noise floor in audio range is lowered by 3dB at the expense of increased noise above 20kHz 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 22 0k 24 0k 20BIT dither is added before truncation This is similar to HF dither but at a lower le...

Страница 22: ...rt 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...

Страница 23: ...s the gain curve is not linear but changes smoothly This reduces high frequency artifacts which can result from aggressive compression 0 Level U n i t y G a i n Output Threshold 24dB Input 0 Level 110dB 104dB The settings of the GAIN and THRESHOLD knobs determine the compression ratio according the formula In our example the compression ratio is 24 24 6 1 33 1 This is not a very intense compressio...

Страница 24: ...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 GAIN THRESHOLD 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 T...

Страница 25: ...or 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 is automatically reduced just enough to assure that the compressor output never exceeds 0dB After the overload has passed the compressor gradually brings the gain back up to 4dB ...

Страница 26: ...etting your margin greater than a few dB unless of course there is a specific audio effect you are trying to achieve 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...

Страница 27: ...ineachset 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 whi...

Страница 28: ... 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 Realigning Two DAT Recordings Digital Sinewave Generator 3 To turn off the sinewave generator press and hold down 1...

Страница 29: ...to noise ratio De emphasis is the inverse of emphasis equalization the process of increasing or decreasing the amplitude of some frequency ranges of a signal with respect to others frequency the number of vibrations per unit of time ex pressed in cycles per second Hz harmonic distortion the appearance of harmonics of the input signal at the output of a device link the tying of two channels of the ...

Страница 30: ...to noise ratio De emphasis is the inverse of emphasis equalization the process of increasing or decreasing the amplitude of some frequency ranges of a signal with respect to others frequency the number of vibrations per unit of time ex pressed in cycles per second Hz harmonic distortion the appearance of harmonics of the input signal at the output of a device link the tying of two channels of the ...

Страница 31: ...3 Oak Park Bedford MA 01730 USA Tel 781 280 0300 Fax 781 280 0499 ...

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