Re: Alsaloop: sync mode PLAYSHIFT + Loopback on playback side

On 01. 10. 21 10:35, Pavel Hofman wrote:

Hi,

I added support for "Playback Pitch 1000000" ctl elem to UAC2 gadget (not submitted to USB yet) and now I am working on alsaloop support for this ctl elem. The changes are simple (tested to work perfectly, patch to follow), but during the work I hit the following issue with playback Loopback "PCM Rate Shift 100000".

If the snd-aloop device is on playback side (i.e. capture from soundcard -> Loopback), the required sync mode is PLAYSHIFT. That means Loopback ctl elem "PCM Rate Shift 100000" should be controlled (by a reciprocal). That is simple by a patch like this:

diff --git a/alsaloop/pcmjob.c b/alsaloop/pcmjob.c index 845ab82..619bf35 100644 --- a/alsaloop/pcmjob.c +++ b/alsaloop/pcmjob.c @@ -1061,7 +1061,13 @@ static int set_rate_shift(struct loopback_handle *lhandle, double pitch) int err;

      if (lhandle->ctl_rate_shift) {

•           snd_ctl_elem_value_set_integer(lhandle->ctl_rate_shift,
  

0, pitch * 100000);

•           long value;
  

•           if (lhandle->loopback->play == lhandle)
  

•                   // playback => reciprocal
  

•                   value = 1/(pitch) * 100000;
  

•           else
  

•                   value = pitch * 100000;
  

•           snd_ctl_elem_value_set_integer(lhandle->ctl_rate_shift,
  

0, value); err = snd_ctl_elem_write(lhandle->ctl, lhandle->ctl_rate_shift); } else if (lhandle->capt_pitch) { snd_ctl_elem_value_set_integer(lhandle->capt_pitch, 0, (1 / pitch) * 1000000); @@ -1205,15 +1211,18 @@ static int openctl(struct loopback_handle *lhandle, int device, int subdevice) int err;

      lhandle->ctl_rate_shift = NULL;

•   // both play and capture
  

•   openctl_elem(lhandle, device, subdevice, "PCM Notify",
  

•                   &lhandle->ctl_notify);
  

•   openctl_elem(lhandle, device, subdevice, "PCM Rate Shift 100000",
  

•                   &lhandle->ctl_rate_shift);
      if (lhandle->loopback->play == lhandle) {
  

•           // play only
              if (lhandle->loopback->controls)
                      goto __events;
              return 0;
      }
  

•   openctl_elem(lhandle, device, subdevice, "PCM Notify",
  

•                   &lhandle->ctl_notify);
  

•   openctl_elem(lhandle, device, subdevice, "PCM Rate Shift 100000",
  

•                   &lhandle->ctl_rate_shift);
  

•   // capture only
      openctl_elem(lhandle, device, subdevice, "Capture Pitch 1000000",
                      &lhandle->capt_pitch);
      set_rate_shift(lhandle, 1);
  
  
  

However, IIUC how the Loopback device works, the "PCM Rate Shift 100000" ctl elem applicable to device=0 on playback side is that of the capture side, i.e. for device=1. The patch above would pick the playback-side device=0 ctl elem in pcmjob.c:openctl_elem. Hard-coding the device=0 -> device=1 is possible, but Loopback supports more devices.

Please what solution for picking the correct "PCM Rate Shift 100000" ctl elem for the PLAYSHIFT sync mode would you recommend?

Hi,

I would not touch the controls associated to the capture PCM by default. It would be possible to add another alsaloop option and code to configure the rate shift control identifier separately for this use case. The user should avoid the double pitch control (playback + capture) for the loopback devices.

Jaroslav