digital audio weiss engineering ltd. Florastrasse 42, 8610 Uster, Switzerland +41 1 940 20 06 +41 1 940 22 14 http://www.weiss.ch / http://www.weiss-highend.com GAMBIT ADC2 OPERATING MANUAL Software Version: 3.0 OPERATING INSTRUCTIONS FOR GAMBIT ADC2 24Bit / 192kHz A/D CONVERTER INTRODUCTION INTRODUCTION Congratulations on purchasing the Weiss Gambit Series ADC2 A/D Converter ! The ADC2 is a two channel A/D converter designed for most demanding audio applications. It uses state of the art A/D chips in Weiss Engineering's proven "correlation technique" configuration, which lowers converter imperfections and improves the SNR ratio. For best noise rejection, the analogue input signal is kept balanced from the input connectors up to the converter chips. A high quality microphone preamp is available as a standard feature. Additionally, there is a built-in digital twosegment compressor / limiter for optimal levelling, employing adaptive release timing, lookahead circuitry and sidechain-oversampling for mastering-quality dynamics processing. Large bar graphs, a well thought-out push button array and uncluttered rotary knobs optimize handling ergonomics. Synchronization · · · Internal sync selectable 44.1kHz, 48kHz, 88.2kHz, 96kHz, 176.4kHz, 192kHz. External sync from Wordsync Input (BNC) or from AES/EBU input. Wordsync output on a BNC connector. Metering · · · · · One input / output level bargraph per channel with short peak hold One gain reduction bargraph per channel. Two over LEDs Two numerical displays for peak hold and level settings. Reference for level setting can be altered Outputs Summary of ADC2 features: Inputs · · · Two balanced LINE and two MIC inputs on separate XLR connectors. MIC input with switchable phantom power. Input sensitivity is settable in each channel independently (or ganged) with two rotary encoders. Line Input sensitivity: -16dBu to +26 dBu. Mic Input sensitivity: -54dBu to +6dBu. Separate AES/EBU input on XLR for synchronization and/or audio input to the DSP, which allows for use of the ADC2 for redithering and limiting of digital signals. · · · AES/EBU on XLR in one or two wire format S/PDIF on RCA Firewire (to be implemented). DSP · POW-R dithering to 16 bits, settable to three different algorithms according to the POW-R standard. SNR enhancement. Level bar graph meter. Two segment compressor / peak limiter Double-precision processing 40bit floating-point · · · · · · · Daniel Weiss Engineering Ltd., Florastr. 42, CH-8610 Uster +41 1 940 20 06 +41 1 940 22 14 http://www.weiss. ch Page 2 of 2 weiss@weiss.ch OPERATING INSTRUCTIONS FOR GAMBIT ADC2 24Bit / 192kHz A/D CONVERTER OPERATION OPERATION ADC2 Block Diagram Fig. 1 ADC2 Block Diagram Schematic Input Control Panel This section describes how to use the input control panel (see Fig. 2). active, the compressor / limiter links1 the sidechains of the two channels. Typical applications for "ganged" mode are · · stereo (left/right) recordings with two microphones stereo insert limiting (during mixdown or mastering). Fig. 2 ADC2 Input Control Panel Ganged For stereo use, press the "ganged" key. This locks together channel 1 and channel 2 gain and threshold controls. Additionally, if "ganged" is active, If "ganged" is not active, channel 1 and channel 2 settings are independent and the compressor / limiter works unlinked. Thus the ADC2 can also be used in a studio recording / mixing environment as dual channel outboard compressor / limiter. A 3.3ms delay is used by the limiter for preview. 1 "sidechain linking " means that the same gain reduction is applied to both channels to avoid corrupting the stereo balance. Daniel Weiss Engineering Ltd., Florastr. 42, CH-8610 Uster +41 1 940 20 06 +41 1 940 22 14 http://www.weiss. ch Page 3 of 3 weiss@weiss.ch OPERATING INSTRUCTIONS FOR GAMBIT ADC2 24Bit / 192kHz A/D CONVERTER OPERATION Digital In Pressing the "digtial in" key activates the digital AES/EBU XLR input, the key lights red if active. As soon as a valid signal is present, the sampling rate is displayed with the sampling rate keys (see Fig. 6). If the digital input stage does not lock to any signal, none of the sampling rate keys are lit. If the "digital in" key is not lit, the analogue inputs are selected. To activate the microphone input for either channel, press the appropriate "mic" key (if "ganged" is active, pressing one "mic" key will select microphone inputs for both channels). Press "+48V" if phantom power is needed. It is usually advisable to use "low-cut" when recording with microphones. If "low-cut" is lit, low frequency content (traffic rumble, building tremors, air-condition hum and other low-frequency air movement like doors opening or closing near the microphone etc.) is filtered out. Peak Hold If "peak hold" is lit, the numeric display shows the peak hold value (in dBFS) which is selected by the gain key: "input gain" selects the input peak hold, "output gain" selects the output peak hold ("threshold" also selects output peak hold). See Fig. 1 for positioning of input and output peak hold in the processing chain. If input peak hold is selected, values equal to or above 0.0dBFS are indicated by ">=0". The output peak hold however can measure values above 0.0dBFS because of the internal floatingpoint data format used in the digital signal processor. This is very useful to set the output gain. Input Gain If "input gain" is lit, the rotary knobs control the analogue input sensitivity. This is adjusted in 1dB steps via a relais controlled attenuator. The selected value is shown in the numeric display. The level bargraph is switched between input and output in the same manner as the peak hold. Thus, if the "input gain" key is lit, the bargraph displays the input peak meter. If the "output gain" key is lit, the bargraph displays the ouput peak meter (settings can be different if "ganged" is not lit). Refer to Fig. 1 on p. 3 for positioning of input and output peak meter in the processing chain. Note: The reference for the input sensitivity display can be chosen by the user. For that purpose press and hold the "ganged" key and then switch the ADC2 on. After a few seconds release the ganged key. Wait for a few more seconds. Now you can select the reference with the righthand knob. This allows to set the reference to e.g. +4dBu. Factory setting is 0 dBu reference. When finished press the blinking "peak hold" key. The setting is retained in non-volatile memory. Output Gain If "output gain" is lit, the rotary knobs control the digital output gain. This gain can be used for optimal gain adjustment and to drive the peak limiter (see below). The gain ranges from - (off) to +18.0 dB, the resolution is optimized for fine control around 0.0 dB while still allowing complete fade-out. A special algorithm is used for click-free gain transissions (no "zipper" noise). To clear the peak hold value, double-click the "peak hold" key! Double-click resets both input and output peak hold. Daniel Weiss Engineering Ltd., Florastr. 42, CH-8610 Uster +41 1 940 20 06 +41 1 940 22 14 http://www.weiss. ch Page 4 of 4 weiss@weiss.ch OPERATING INSTRUCTIONS FOR GAMBIT ADC2 24Bit / 192kHz A/D CONVERTER OPERATION Threshold If "threshold" is lit, the rotary knobs control the threshold of the compressor section. The value is shown in the numeric display. If "peak hold" is active, the output peak value is displayed. The ADC2 has a two-segment dynamics compressor. The threshold can only be adjusted for the compressor section, which has a fixed ratio of 2:1. The limiter has a fixed ratio of :1 with a fixed threshold of 0.0 dBFS. Because the signal before the output gain never exceeds 0.0dBFS, the limiter is inactive if the output gain is £ 0.0dB. If the threshold is set to 0.0 and the output gain is below 0.0 the dynamics section is inactive. This setup is shown in the input vs. output graph in Fig. 3. -12.0 -6.0 0.0 +6.0 0.0 -6.0 -12.0 Fig. 4 Compressor with -3.0dB Threshold Fig. 5 Compressor with -6.0dB Threshold 0.0 -6.0 0.0 -12.0 -12.0 -6.0 -6.0 0.0 +6.0 The overall gain reduction applied by the compressor / limiter is shown in the yellow bargraph. Note that the compressor uses auto gain makeup: depending on the threshold, a gain is added after the compressor to ensure that 0.0 dBFS input results in 0.0 dBFS output (graphically speaking, auto gain make-up "fixes" the transfer curve at the (0.0, 0.0) point). -12.0 -12.0 -6.0 0.0 +6.0 Fig. 3 Compressor with 0.0dB Threshold To activate the limiter, set the output gain to a positive value. The boost will drive the signal into the limiter. The gain reduction applied by the limiter is visible on the yellow bargraphs. To soften the transmission from linear to limiter segment, the compressor can be activated by adjusting the threshold parameter. Fig. 4 shows a setting with a threshold of ­3 dBFS, Fig. 5 shows ­6 dBFS. The range of possible threshold values is listed on p. 13. Daniel Weiss Engineering Ltd., Florastr. 42, CH-8610 Uster +41 1 940 20 06 +41 1 940 22 14 http://www.weiss. ch Page 5 of 5 weiss@weiss.ch OPERATING INSTRUCTIONS FOR GAMBIT ADC2 24Bit / 192kHz A/D CONVERTER OPERATION Sync and Dither Control Panel If none of these two keys are lit, the ADC2 uses internal sync. POW-r Dither To requantize the input signal from 24bit to 16bit format, a special technique is used to avoid distortion, called dithering. Many dithering schemes are available today, both professional and homebrewed. The ADC2 uses the POW-r scheme, which is widely regarded as one of the best, if not the best2, and is used in a variety of digital equipment. The POW-r scheme offers three different presets which can be selected with the dither keys "pow-r 1", "pow-r 2" and "pow-r 3". The choice should be based on listening tests and preference of the recording engineer. Fig. 6 ADC2 Sync and Dither Control Panel Output Frequency If the analogue inputs are selected ("digital in" is not lit), the A/D sampling frequency can be set in the sync source block: Sampling quency Fre- 44.1 / 48 kHz key 44.1 kHz 48 kHz 44.1 kHz 48 kHz 44.1 kHz 48 kHz fs x 2 / fs x 4 key none none sf x 2 sf x 2 sf x 4 sf x 4 44.1 kHz 48.0 kHz 88.2 kHz 96.0 kHz 176.4 kHz 192.0 kHz Dithering is in "auto black"3 mode: if digital zero is detected at the ouput, dithering is switched off. This ensures that programme breaks are not filled with dither noise. "Auto black" mode is not available for analogue input signals! Table 1 Samplig Frequency Selection If "digital in" is lit, the sampling frequency keys display the sampling frequency of the AES/EBU input signal in the same fashion as is shown in Table 1. Sync Source As can be seen in Fig. 1, the ADC2 can be synchronized externally by other digital devices. It has two external sync inputs. External sync is only available for analogue inputs, ie "digital in" key is not lit. If "AES" is lit, the AES/EBU signal at the "DI 1 / SYNC" input is used for synchronization. If "BNC" is lit, the wordclock signal connected to "WORDSYNC in" is used for synchronization. Both sync inputs have a loop-through ouput to syncronize other devices. Also note the effect of the "DITHER MODE" dip switch setting in Fig. 1 ADC2 Block Diagram Schematic. If the dip switch is in the down position, requantisation to 16 bit and dithering is only applied to the S/PDIF RCA output, whereas the AES/EBU XLR output still carries all 24bits. This mode is useful for live recording to a 24bit device with simultaneous backup to a 16bit device (like DAT or CD-R). If the dip switch is in the up position, all digital outputs are dithered to 16bits. 2 For press reviews of POW-r visit www.weiss.ch/whatpresssays.html The term "black" comes from digital image formats, where black is represented by the value zero. 3 Daniel Weiss Engineering Ltd., Florastr. 42, CH-8610 Uster +41 1 940 20 06 +41 1 940 22 14 http://www.weiss. ch Page 6 of 6 weiss@weiss.ch OPERATING INSTRUCTIONS FOR GAMBIT ADC2 24Bit / 192kHz A/D CONVERTER OPERATION Why dither? If the 24bit digital signal is not dithered when recording to 16bit media (e.g. DAT, CD-R etc), it is truncated instead, which leads to quantization distortion (similar to clipping distortion). Fig. 7 shows a 24bit sine wave at ­85dBFS truncated to 16bit. To view the frequency characteristics of the three POW-r algorithms, refer to POW-R Noise Spectrum on p. 14. For more information on dithering in general, view the article "The Secrets of Dither" at www.digido.com . For more POW-r information, please visit www.weiss.ch/pow-r/pow-r.html . Fig. 7 1kHz Sine wave truncated to 16bit If the same sine wave is dithered instead of quantized, the quantization distortion disappears and is replaced by wide band shaped noise, which is more pleasing to the human ear (Fig. 8). Fig. 8 1kHz sine wave dithered to 16bit Daniel Weiss Engineering Ltd., Florastr. 42, CH-8610 Uster +41 1 940 20 06 +41 1 940 ...