Alsaloop: sync mode PLAYSHIFT + Loopback on playback side

[Jaroslav Kysela]

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

-- 
Jaroslav Kysela <perex at perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.