Re: Linux USB Audio question -- asynchronous isochronous mode

Borislav Trifonov wrote:
> The USB Audio standard uses isochronous transfer in one of three modes:
> synchronous, adaptive, or asynchronous.  I'm developing hardware that
> will use the asynchronous mode to communicate with the PC, so that it is
> the one doing flow control.
>
> My question stems from something someone told me about Windows, that it
> actually will resample the data if asynchronous mode is used, which is
> bad and not in the spirit of the standard specification.  So I want to
> confirm that Linux will _not_ do that when asynchronous USB Audio is
> used, and the data will be bit-perfect -- the PCM data the player
> application sends to the system is exactly what comes out the USB port.

snd-usb-audio never resamples.

The driver reports the actual sample rate (as reported in the async
feedback packets, and measured relative to the USB clock) in
/proc/asound/card*/stream*, but this is just for debugging purposes.

The sample rate that is queried/set by user space is always assumed
to be measured by the device's own clock.


HTH
Clemens


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