[alsa-devel] [Alsa-user] Ticks when playing to USB DAC at high sample rates

Daniel Mack zonque at gmail.com
Wed Nov 7 10:01:14 CET 2012

On 07.11.2012 00:54, Jeffrey Barish wrote:
> I guess that snd_usb_pcm_prepare is called by the core usb system to prepare 
> the snd driver.  Anyway, snd_usb_pcm_prepare calls configure_endpoint with a 
> substream.  Then configure_endpoint calls snd_usb_endpoint_set_params with the 
> data endpoint that corresponds to the substream.  snd_usb_endpoint_set_params 
> calls data_ep_set_params with the data ep and data_ep_set_params sets nurbs to 
> 2 for the data endpoint.
> snd_usb_endpoint_set_params has code to call sync_ep_set_params when the 
> endpoint is type sync.  sync_ep_set_params sets nurbs to 4 (why?).  However, 
> snd_usb_endpoint_set_params never invokes the call to sync_ep_set_params 
> because ep->type is always data.  Back up: snd_usb_endpoint_set_params is 
> called from configure_endpoint.  It is called from an if block contingent on 
> the substream's having a sync endpoint.  It does.  However, the call to 
> snd_usb_endpoint_set_params passes subs->data_endpoint as the first argument, 
> so snd_usb_endpoint_set_params simply repeats the operations it performed as a 
> result of the previous invocation.  I presume that the first argument to the 
> second call to snd_usb_endpoint_set_params should be subs->sync_endpoint.

Eh. You clearly spotted a bug here. The rally interesting thing is that
that it didn't bite us earlier, as it's in since 3.6.

>  I 
> made that change.  Now freqm does change.  However, I am getting tons of 
> ticks.  Now it sounds as if the payload size is always wrong.
> Oh, 44.1 kHz works.  48 kHz works.  88.2 and 96 don't work.

That's a different issue. Would you like to prepare a patch for the
issue above please?


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