Dual-mono / multi-mono dilemma

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Good post @ada. To pick up one point you mentioned: I believe the 40-bit format Devialet uses is floating-point, and it is probably used to avoid overflow or underflow in DSP operations (in other words, it is not meant to provide 40-bit audio resolution throughout the audio path). In any case the 40-bit floating-point format would "only" have 31 or 32 bits of precision.

Also, the ADH stage is based on a PCM1792 DAC which has 24-bit resolution, so whatever goes into the Devialet will presumably be at 24-bit resolution when it hits the ADH in both master and companion. And as you said, that should be enough resolution for all practical purposes when human hearing is involved.

In addition, the 32-bit -> 24-bit (in master) conversion, followed by the 24-bit -> 40-bit internal native format (in companion) conversion, can't be a good thing.

I don't see why it should be a problem. If the ADH stage operates at 24-bit resolution, you have to have the truncation from 32 to 24 bits at some point. Extending the data from 24 to 40 bits won't lose any further precision, and it will presumably be followed by truncation back to 24 bits for the ADH stage in the companion(s). The point is, I think, that the Devialet is inherently limited to 24-bit resolution by the ADH DAC so the fact that it will notionally accept and process 32-bit input doesn't mean that it is "giving you" 32-bit resolution at the analog output whether you're talking about a single stereo unit, a master or a companion.