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A D 2 4 0 2 - 9 6 - O P E R A T I N G M A N U A L

Appendix

3

Word Length Reduction

The AD2402-96 is equipped with two state of the art world length reduction systems: The
Benchmark NN™ (Near Nyquist) system, and the Benchmark NS™ (Noise Shaped) system.
Unlike most competitive systems, the Benchmark NS™ system is based upon the most
current psycho-acoustic models. Furthermore, both Benchmark systems are unique in that
they were optimized while factoring in the noise contribution of the recording environment.

The Benchmark NN™ and NS™ word length reduction systems are the result of a
cooperative effort between Benchmark Media Systems, Inc., and the Audio Lab at the
University of Waterloo in Ontario Canada. We would especially like to thank Stanley P.
Lipshitz Ph.D., John Vanderkooy Ph.D., and Robert A. Wannamaker Ph.D. for their
pioneering research and for their significant contributions to the early stages of this project.
Special thanks are also in order to Robert Wannamaker for creating and modifying the
mathematical algorithms and filter coefficients which are at the core of the Benchmark
NN™ and NS™ systems.

We have taken a new approach by optimizing word length reduction for use in three
different levels of ambient noise. The ambient noise in a live recording situation is very
different from ambient noise level in a studio, and neither can be considered insignificant
when reducing 24-bit recording to 16-bits. All prior word-length-reduction systems ignored
the effects of this ambient noise and were optimized for noise-free input signals. The
Benchmark NN™ and NS™ systems were mathematically optimized while calculating the
effects of ambient noise. Three levels of input noise were used and three different curves
were produced. NN3™ and NS3™ represent optimal solutions where system noise is
limited only by the 24-bit A/D conversion process. NN2™ and NS2™ represent optimal
solutions where the ambient noise is 6 dB higher than the converter noise floor. NN1™
and NS1™ represent optimal solutions where the ambient noise is 12 dB higher than the
converter noise floor. When properly used, this optimization can improve the dynamic
range of a finished 16-bit recording by several decibels.

What is the Appropriate Setting to Use?

In general, NN3™ or NS3™ should be used for extremely low-noise studio recording
environments, NN2™ or NS2™ should be used for live recording, while NN1™ and

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Содержание AD2402-96

Страница 1: ...BENCHMARK MEDIA SYSTEMS INC AD2402 96 AD2K TWO CHANNEL 96 kHz ANALOG TO DIGITAL CONVERTER 11 09 04 Revision Absolute Input Polarity Correction pg 17 OperatingManual Revision 1 ...

Страница 2: ...4 0 2 9 6 A N A L O G T O D I G I T A L C O N V E R T E R Operating Manual BENCHMARK MEDIA SYSTEMS INC 5925 Court Street Road Syracuse NY 13206 1707 Phone 315 437 6300 Fax 315 437 8119 www benchmarkmedia com ...

Страница 3: ...onnection 9 What is FSD 24 Front Panel 10 How is FSD Different from 0 dBFS 24 Power Switch 10 How is FSD Different from Over 24 On LED 10 The Fallacy of Over Indicators 25 Low Battery LED 10 Do Over Indicators Have a Place 25 Input Level Switch 11 Design Philosophy 27 Lock LED 12 Specifications 28 Word Length Mode Switch 13 Sample Rate Switch 13 Jitter and it s Effects 31 Meter Scale Switch 15 Con...

Страница 4: ...Why NN3 for Cascaded 16 bit 37 Dither 37 Will Dither Damage My Speakers 40 How is the Performance Measured 40 Avoid Truncation without Dither 40 Digital Word Length Increases 41 Additional Information 41 Word Length Reduction Curves 43 ...

Страница 5: ...word lengths can be set at 24 20 or 16 bits The 20 and 16 bit word lengths are re dithered from 24 bits using either the Benchmark NN or NS word length reduction systems The NN and NS systems are optimized for the selected sample rate and for the number of bits being removed NN3 and NS3 can achieve near 20 bit performance on a 16 bit recorder The NN1 NN2 NS1 NS2 settings are optimized for noisier ...

Страница 6: ...D2402 96 digital input Then the user should select the D to D function and monitor each word length reduction setting to determine which best complements the recording Digital Outputs Four digital output jacks provide a unique set of features Two outputs are 75 Ohm unbalanced and two outputs are 110 Ohm balanced All outputs can operate in either professional or consumer status formats All outputs ...

Страница 7: ...ls Two digital cables are required for two channels Using the Dual Cable mode a standard DAT machine can record one channel at 96 kHz and an ADAT DA88 or other digital multi track devices can record up to 4 channels at 96 kHz Caution Tapes produced in Dual Cable mode will appear to be compatible with standard playback and editing equipment but actually are not compatible Do not use this mode unles...

Страница 8: ...ignal interconnection to a microphone preamplifier whether the preamplifier has balanced or unbalanced outputs For more information on proper cable wiring to receive a signal from an unbalanced source known as forward referencing see page 15 of the Benchmark Media Systems Inc application note A Clean Audio Installation Guide The Clean Audio Installation Guide PDF file is available for download fro...

Страница 9: ...rsion for absolute accuracy Power Supply Connection The AD2402 96 is equipped with an industry standard 4 pin XLR power connector and may be powered from a 12 V battery or from an external DC power supply The input voltage should be between 11 and 18 VDC Pin 1 Ground Pin 4 11 to 18 VDC at 0 8 A Pins 2 and 3 are not connected internally 9 ...

Страница 10: ... The green On LED will light whenever the power switch is on and the input voltage is high enough for normal operation The On LED will not light if the input voltage is less than 10 5 V An illuminated On LED is your assurance that the input voltage is high enough for normal operation Low Battery LED The AD2402 96 is equipped with a red Low Batt LED The low battery light is calibrated for use with ...

Страница 11: ... gain control on your microphone preamp This switch is used specifically to calibrate the AD2402 96 to a line level source mixing console or microphone pre amp It is NOT intended as a gain control for adjustment during a recording session Set the switch in accordance with your pre amp or mixer before beginning the recording session 24 dBu is a very hot level and is often only available at the outp...

Страница 12: ...402 96 is operating at 96 kHz the converter will lock to either a 48 kHz or 96 kHz digital audio signal If the lock light is off the AD2402 96 is operating as clock master using the internal crystal reference Important Note An external reference is normally not required for 2 channel recording The lock LED will not be on when the AD2402 96 is operating as a stand alone clock master device This is ...

Страница 13: ...t 20 bits NS1 and NS2 provide 21 bit and 22 bit performance respectively For the best performance use the maximum word length that is compatible with your digital audio equipment Maintain 24 or 20 bit word lengths as long as possible If WLR to 16 bits is required do so at the latest possible point and time Avoid processing 16 bit signals In general NS2 will produce the quietest 20 bit signal and N...

Страница 14: ...Channel 1 will be directed to the Main outputs Channel 2 will be routed to the Aux outputs Two tracks will be required for each channel Caution In Dual Cable mode each digital output is dedicated to a single audio channel instead of a stereo pair Dual Cable recordings require 88 2 and 96 kHz playback and editing equipment Furthermore Dual Cable recordings must always be dubbed and transferred from...

Страница 15: ... 16NS1 16NN3 16NN2 16NN1 or 16TPDF 18 18 24 20 16 16NS3 16NS2 16NS1 16NN3 16NN2 16NN1 or 16TPDF 20 20 24 24 16 16NS3 16NS2 16NS1 16NN3 16NN2 16NN1 or 16TPDF 24 20 20NS2 20NS1 20NN1 20TPDF 24 24 24 Meter Scale Switch The Meter Scale switch selects one of three digital meter functions The down position sets the meter scale to 6 dB steps The center position sets the meter scale to 1 dB steps The up p...

Страница 16: ...al sample will light the FSD LED Short single sample digital clipping events are often audible and all FSD events should be avoided The AD2402 96 has a very large dynamic range especially when operating at 24 bit output word lengths It is wise to use some of this dynamic range to provide more headroom as insurance against clipping Leave some extra headroom between your highest anticipated peak and...

Страница 17: ...recorders can accept either professional or consumer formats However many consumer recorders cannot accept professional formats In most cases the S PDIF setting will allow simultaneous output to both professional and consumer devices on both XLR and BNC or RCA coax cables Consumer status bit formats do not currently have the capability of identifying 88 2 or 96 kHz sample rates Consequently it is ...

Страница 18: ...0 dBFS Minimum sensitivity is 24 dBu at full scale 0 dBFS PLEASE NOTE An internal absolute polarity inversion error was discovered some time after the creation of the AD2402 96 AD2K The following table is revised from the original and should be used instead of the original table Balanced Inputs Pin 1 Shield Pin 2 Signal and Pin 3 Signal Unbalanced Inputs Pin 1 Shield Pin 2 Signal Ground Pin 3 Sign...

Страница 19: ...nector This BNC connector provides a digital audio input to the AD2402 96 The digital audio input has two functions It can provide a phase and frequency reference to the AD2402 96 sampling circuitry or it can serve as an input for Digital to Digital processing functions All digital inputs and outputs are transformer coupled The transformers isolate the AD2402 96 from RF interference provide protec...

Страница 20: ...erence is present but lock has not been achieved the LOCK LED will flash slowly Note Using an external reference will not degrade or alter the low jitter performance of the AD2402 96 converter at any of the fixed sample rates The jitter performance of the fixed sample rate modes is maintained even when the reference has relatively high levels of jitter However the VAR variable speed mode has a wid...

Страница 21: ...ording capability Or one digital audio source can be looped through one or more AD2402 96 converters to provide a large digital to digital fan out for dubbing If the loop feature is not being used a 75 Ohm termination should be connected to the Loop connector a 75 Ohm termination is supplied with the AD2402 96 Digital Outputs Four digital output jacks provide a unique set of features at 1 Vpp and ...

Страница 22: ...eak level will be controlled by the fast 7 8 msec decay time constant Selecting Meter Range The AD2402 96 has two meter ranges one with 6 dB steps and one with 1 dB steps The 6 dB scale allows monitoring for signal presence as well as coarse adjustment of levels The 1 dB scale allows highly accurate adjustment of digital levels In addition the bottom two steps of both scales are expanded In either...

Страница 23: ...the 7 8 msec delay the first time constant releases its control of the meter segments If a higher peak should occur during the 7 8 msec interval this new peak will be displayed and the 7 8 msec timer will restart Thus no peaks are ever missed and all are visible Slow Decay Time Constant The slow time constant is active whenever the peak hold function is off After the first time constant releases c...

Страница 24: ...ed to describe only the minimum and maximum digital code An FSD meter must have knowledge of the digital word length in order to work properly A 16 bit FSD code will not register FSD when feeding a FSD meter that is expecting a 20 bit word length The reason for this is that the 16 bit word will have 4 trailing zeros appended to it to make it a 20 bits word and the 20 bit FSD meter will interpret t...

Страница 25: ...odes until it is severely clipped Given a complex musical signal the reliability of an over meter can best be described as hit or miss If you have an over meter try this Feed a 10 kHz tone into any A to D converter at a level equivalent to 3 dBFS As shown in the graph below peak voltages will reach almost 1 5 times clip level This clipping will cause severe harmonics at 20 kHz and distortion will ...

Страница 26: ...ases this clipping is audible But the CD will sound louder and sometimes that is all that seems to matter A suggestion Keep the original recording clean by using a FSD meter Create a final 20 or 24 bit mix entirely with FSD meters Then and only then transfer to 16 bits using an appropriate dither process and adjust the levels using an Over meter This way if the clipping proves objectionable you ca...

Страница 27: ... the tone having the highest amplitude Again bench tests can ignore critical information Because bench tests have often failed to detect audio defects some have discounted their value and have relied solely on listening tests We believe listening tests are a valuable tool for evaluating a finished product and for confirming the validity of a careful and thorough bench testing development cycle The...

Страница 28: ...96 kHz single or dual cable output Analog inputs balanced XLR Input level for 0dBFS 14 to 24 dBu switch selectable Metering 9 segment fully digital with peak hold Digital Input 75 ohm loop through high Z D to D word length reduction format conversion D to D Reference AES EBU or S PDIF format Power 12 to 18 VDC 4 pin XLR connector Current drain 800 mA max Fuse 5x20 mm 2 A Low Battery indicators 11 ...

Страница 29: ...A D 2 4 0 2 9 6 O P E R A T I N G M A N U A L 16 BIT NOISE BUILDUP VS NN AND NS VS NUMBER OF PASSES 20 BIT NOISE BUILDUP VS NN AND NS CURVES VS NUMBER OF PASSES 29 ...

Страница 30: ...A D 2 4 0 2 9 6 O P E R A T I N G M A N U A L 30 ...

Страница 31: ...agnetic interference As a result the AES interface will add code dependent jitter which may be 100 to 1000 times greater than the jitter at the converter s sample and hold This is potentially inconsequential This interface jitter accumulates after the audio is in the digital domain and therefore does not degrade the audio signal provided it is removed prior to D to A conversion But unlike interfac...

Страница 32: ...highly dependent upon very precise digital filters These digital filters provide a brick wall that stops unwanted high frequency signals In the case of 44 1 kHz conversion this brick wall filter is constructed such that it stops all frequencies above 22 05 kHz one half of the sample rate Frequencies above 22 05 kHz cannot be represented in a 44 1 kHz digital system If these frequencies are not rem...

Страница 33: ...ter the response of the filter For example if converter clock in the AD2402 96 were derived directly from the AES EBU receiver as it is in many converter products the stopband attenuation at 100 kHz would be degraded from 117 dB to only 28 dB The 100 kHz signal that should have been removed is now splattered all over the audio band see graph FIGURE 5 100 KHZ STOP BAND ATTENUATION VS JITTER AT THE ...

Страница 34: ...o jitter92 pdf Jitter Specification and Assessment in Digital Audio Equipment By Julian Dunn Presented at the 93rd AES Convention October 1992 http www audioprecision com publications audiotst jan96 jan961 shtml Digital Audio Transmission Why Jitter is Important An Audio Precision Application Note By Dr Richard Cabot http www digido com jitteressay html Everything You Always Wanted To Know About J...

Страница 35: ...and NS systems We have taken a new approach by optimizing word length reduction for use in three different levels of ambient noise The ambient noise in a live recording situation is very different from ambient noise level in a studio and neither can be considered insignificant when reducing 24 bit recording to 16 bits All prior word length reduction systems ignored the effects of this ambient nois...

Страница 36: ... and NS processes have been optimized for certain levels of ambient noise contribution it is important to point out that the processes do not rely on this noise for dithering TPDF dither is always applied to the 24 bit signal prior to word length reduction The NN and NS processes are always fully dithered to insure full randomization of the quantization noise and to insure that the quantization no...

Страница 37: ...stem will remain inaudible during quiet portions of a recording even when the playback system is adjusted to achieve high peak sound pressure levels Dither that is audible will tend to mask musical details This masking effect of dither increases as the audibility of the dither increases While it is desirable to keep the dither inaudible it is also necessary to apply enough dither to fully randomiz...

Страница 38: ...R A T I N G M A N U A L 16 BIT NS WORD LENGTH REDUCTION CURVES FOR 44 1 KHZ The 16 bit 44 1 kHz NS3 noise shaping shown above provides a 14 dB improvement over 16 bit TPDF in terms of noise audibility See table on page 28 38 ...

Страница 39: ...N U A L 16 BIT NS WORD LENGTH REDUCTION CURVES FOR 48 KHZ The 16 bit 48 kHz NS3 has a 17 dB advantage over 16 bit TPDF 16 BIT NS WORD LENGTH REDUCTION CURVES FOR 96 KHZ The 16 bit 96 kHz NS3 provides a 28 dB improvement over 16 bit TPDF 39 ...

Страница 40: ...to all frequencies within the 0 to 22 05 kHz bandwidth of a 44 1 kHz digital system The audibility of the added noise is greatly reduced when it is concentrated at frequencies where our ears are least sensitive Near Nyquist systems reduce noise audibility by concentrating most of the noise energy between 18 kHz and the Nyquist frequency 1 2 of the sample rate while maintaining a relatively flat an...

Страница 41: ...is Processed Every time a digital signal is processed its word length increases Processing includes even simple operations such as level changes and the mixing of two signals Word lengths expand dramatically when more complex operations such as equalization sample rate conversion and effects processing are applied Long word lengths created by digital signal processing must be shortened before they...

Страница 42: ...A D 2 4 0 2 9 6 O P E R A T I N G M A N U A L Another dither article entitled The Secrets of Dither authored by Bob Katz can be found at http www digido com ditheressay html 42 ...

Страница 43: ...A D 2 4 0 2 9 6 O P E R A T I N G M A N U A L Additional Word Length Reduction Curves 16 BIT NS WORD LENGTH REDUCTION CURVES FOR 88 2 KHZ 16 BIT NN WORD LENGTH REDUCTION CURVES FOR 44 1 KHZ 43 ...

Страница 44: ...A D 2 4 0 2 9 6 O P E R A T I N G M A N U A L 16 BIT NN WORD LENGTH REDUCTION CURVES FOR 48 KHZ 16 BIT NN WORD LENGTH REDUCTION CURVES FOR 88 2 KHZ 44 ...

Страница 45: ...A D 2 4 0 2 9 6 O P E R A T I N G M A N U A L 16 BIT NN WORD LENGTH REDUCTION CURVES FOR 96 KHZ 20 BIT WORD LENGTH REDUCTION CURVES FOR 44 1 KHZ 45 ...

Страница 46: ...A D 2 4 0 2 9 6 O P E R A T I N G M A N U A L 20 BIT WORD LENGTH REDUCTION CURVES FOR 48 KHZ 20 BIT WORD LENGTH REDUCTION CURVES FOR 88 2 KHZ 46 ...

Страница 47: ...A D 2 4 0 2 9 6 O P E R A T I N G M A N U A L 20 BIT WORD LENGTH REDUCTION CURVES FOR 96 KHZ 47 ...

Страница 48: ...cause undesired operation SafetyInformation Do NOT service or repair this product unless properly qualified Only a qualified technician or authorized Benchmark Media Systems Inc distributor should perform servicing For continued fire hazard protection fuses should be replaced ONLY with the exact value and type as indicated on the rear panel and on this page below Do NOT substitute parts or make an...

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