[alsa-devel] proposal: snd_pcm_start_at()

[Pierre-Louis Bossart]

On 10/2/14, 9:34 AM, Tim Cussins wrote:
> Hi all,
>
> I'm Tim: I work at Linn Products Ltd - we make Network Music Players,
> amongst other things.
>
> As you might imagine, synchronised-start is important when multiple
> devices on the network are rendering the same audio. We'd be interested
> in contributing a small expansion of the alsa-lib API to support
> synchronised start.
>
> Assuming we can synchronise the audio clocks (I'm aware this is not
> trivial - It's not the topic of this post), we'd propose something like:
>
>      int snd_pcm_start_at(snd_pcm_t* pcm, snd_htimestamp_t* tstamp);
>
> and playback would begin as close to tstamp as possible. If tstamp is in
> the past, it would should return an error.
>
> Recent work by Takashi Iwai enables client code to set the clock type of
> timestamps using snd_pcm_sw_params_set_tstamp_type(). This context could
> quite naturally extend to tstamp argument of snd_pcm_start_at().
>
> Before I get stuck into working up the details under the hood, it'd be
> good to get some feedback/objections regarding this approach.

It's probably better idea to start PCM playback with a bunch of zeroes 
and then rely on existing timestamping to insert samples at the right 
location in the ring buffer - which you have to do anyway to compensate 
for drifts between your network clock and audio clock. This is a more 
predictable solution that abstracts away all the time needed to arm DMA, 
FIFOs, etc. The only hardware-dependent variable that would remain is 
the precision/granularity of the timestamping.
-Pierre