User manual LEXICON 20 BIT A-D CONVERTER REV 0

[. . . ] User Guide 20/20 AD 20 Bit A/D Converter Lexicon Precautions The 20/20 AD is a rugged device with extensive electronic protection. However, you should observe the same reasonable precautions that apply to any piece of audio equipment: · · · · Always use the correct line voltage. Do not install the 20/20 AD in a closed, unventilated rack, or directly above heat-producing equipment such as power amplifiers. Never attach audio power amplifier outputs (speaker outputs) directly to any of the 20/20 AD's connectors. [. . . ] As with the Word Clock input, use only 75 RG-59/U or similar cable. 14 20/20 AD User Guide Using the 20/20 AD 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 PRES lights are on, but the LOCK light is off, the selected sync source is present, but is not within the 41-51kHz lock range. External Mode WC AES / WC: Internal and External modes PRES AES LOCK EXT SYNC 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 WC PRES States AES LOCK EXT SYNC AES / WC: WC AES / WC: AES LOCK EXT SYNC 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 WC AES / WC: AES LOCK EXT SYNC PRES 15 Using the 20/20 AD Lexicon 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 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 "replacement" 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. Psychoacoustically Optimized Noise Shaping (PONS) 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 to higher frequencies. This is called psychoacoustically optimized noise shaping (PONS) because it takes advantage of the variations in the ear's sensitivity to noise within the 20 Hz - 20 kHz range. 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 -60. 00 -70. 00 -70. 00 -80. 00 -80. 00 -90. 00 -90. 00 -100. 0 -100. 0 -110. 0 -110. 0 -120. 0 -120. 0 -130. 0 -130. 0 -140. 0 -140. 0 -150. 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 -160. 0 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 2channel 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. -60. 00 -60. 00 -70. 00 -70. 00 -80. 00 -80. 00 -90. 00 -90. 00 -100. 0 -100. 0 -110. 0 -110. 0 -120. 0 -120. 0 -130. 0 -130. 0 -140. 0 -140. 0 -150. 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 -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 WIDE dither is added before truncation. [. . . ] lock : two or more audio signals which have the same frequency and phase. 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 sample rate. period : the interval of time required for a waveform to complete one cycle and begin to repeat itself. [. . . ]