[PATCH] alsaloop: reduce cumulative error caused by non-atomic samples calculation

[Jaroslav Kysela]

Dne 09. 03. 20 v 21:29 Ruslan Bilovol napsal(a):
> When doing loopback between two audio card with
> same sampling frequency, I noticed slow increase
> of pitch_diff.
> 
> When I changed order of get_queued_playback_samples()
> vs get_queued_capture_samples(), I noticed same drift
> of pitch_diff but if was decreasing this time.
> 
> This seems to be caused by non-atomic consecutive
> snd_pcm_delay() invocation for playback then for
> capture. snd_pcm_delay() measures delay between
> read/write call and actual ADC/DAC operation.
> 
> So while we get this value for playback path in
> get_queued_playback_samples(), next call to
> get_queued_capture_samples() will happen a little
> bit later so snd_pcm_delay() may return incorrect
> value.
> 
> Be interleaving get_queued_{playback,capture}_samples()
> order, we divide this small error between playback
> and capture paths. I do not see any issues anymore
> with one-way drift of pitch_diff.
> 
> Signed-off-by: Ruslan Bilovol <ruslan.bilovol at gmail.com>

Nice catch and implementation. Applied.

				Thank you,
					Jaroslav

> ---
>   alsaloop/pcmjob.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/alsaloop/pcmjob.c b/alsaloop/pcmjob.c
> index b252486..1b7925a 100644
> --- a/alsaloop/pcmjob.c
> +++ b/alsaloop/pcmjob.c
> @@ -1951,8 +1951,16 @@ int pcmjob_pollfds_handle(struct loopback *loop, struct pollfd *fds)
>   	}
>   	if (loop->sync != SYNC_TYPE_NONE) {
>   		snd_pcm_sframes_t pqueued, cqueued;
> -		pqueued = get_queued_playback_samples(loop);
> -		cqueued = get_queued_capture_samples(loop);
> +
> +		/* Reduce cumulative error by interleaving playback vs capture reading order */
> +		if (loop->total_queued_count & 1) {
> +			pqueued = get_queued_playback_samples(loop);
> +			cqueued = get_queued_capture_samples(loop);
> +		} else {
> +			cqueued = get_queued_capture_samples(loop);
> +			pqueued = get_queued_playback_samples(loop);
> +		}
> +
>   		if (verbose > 4)
>   			snd_output_printf(loop->output, "%s: queued %li/%li samples\n", loop->id, pqueued, cqueued);
>   		if (pqueued > 0)
> 


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