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6. Distribution & DSP issues

6.3 Constant gain A/D converters

In digital audio systems with ‘constant gain’ to keep the digital levels constant if analogue gain is changed (see chapter 7.3), the A/D converter device has DSP built-in to adjust the digital gain before the audio signal is sent to the network. In such a system, analogue gain is dedicated to the function of driving the A/D converter at the most efficient level (with the lowest quantization noise floor), leaving digital gains in the DSP processes to manage mixing input levels fully independently from the analogue gain. When constant gain is activated, a certain headroom is reserved to allow for digital make-up of the analogue gain. Figure 604 presents constant gain level diagrams with a 6 dB safety headroom and +/- 12 dB make-up range, supporting +24 dB digital gain in the DSP receivers (eg. mixers). To provide the three extra bits required for the worst case situation make-up, the network must support at least 27 bits to allow the use of constant gain without a decrease in resolution. A 24-bit digital audio protocol could be used, but then only a worst case resolution of 21 bits can be supported.

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