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Cannot set channels number for the output of a USB audio 2.0 class compliant device
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Cannot set channels number for the output of a USB audio 2.0 class compliant device


  • Subject: Cannot set channels number for the output of a USB audio 2.0 class compliant device
  • From: Philippe Wicker <email@hidden>
  • Date: Wed, 30 Mar 2016 15:02:06 +0200

Hello all,

Don’t know if I should have posted this to the USB list but as it concerns the firmware of an audio 2.0 device I thought that there could be people here able to give me some hints about my problems.

I’m working on the firmware of a device which is a USB audio 2.0 class compliant device. This device supports sample rate from 44.1 to 192KHz (discrete values 44.1, 48, 88.2, 96, 176.4 and 192). And we want it to support for both Input and Output 8 channels (6 analog + 2 ADAT) at 192 and 176.4, 10 channels (6 analog + 4 ADAT) at 88.2 and 96, and 14 channels (6 analog + 8 ADAT) at 44.1 and 48. All the channels support a unique 24 bits resolution.

For now I’m trying to do that using alternate settings and I check it using the “Audio Devices” window of Audio MIDI Setup. Apart the AS Interface for the zero bandwidth I have 2 sets of 3 alternate interfaces (1 set for the input and the 2nd set for the output). The channels number is specified within the Class Specific AS Interface descriptors, and the resolution is specified within Type 1 Format descriptors.

Looking at the Audio Devices window, it works for the “Input” tab, but not for the “Output” tab.

The “Input" tab displays 1 popup where I can choose the sample rate, and a 2nd one where I can choose the number of channels.

On the other hand the “Output” tab correctly displays the sample rate popup, but gives me only the “14ch - 24bit integer” choice. If I set the “output” descriptors so that the alternate settings provides different resolutions (instead of channels numbers) then the right popup is correctly populated with the 3 resolutions.

So I cannot get a "8 in / 8 out” configuration which what I want to achieve. I have at best an "8 in / 14 out” config which is too much (not enough bandwidth at high sample rates).

Is this non-symetic behaviour the expected one? Any idea?

More generally, is there a better way to manage this kind of multw-configurations where the number of channels is correlated with the actual sample rate?

Thanks,
Philippe
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