[alsa-devel] What does snd_pcm_delay() actually return?

[Takashi Iwai]

At Mon, 9 Jun 2008 21:02:25 +0200,
Lennart Poettering wrote:
> 
> Takashi, Jaroslav,
> 
> could you please explain what exactly snd_pcm_delay() returns? 
> 
> Some applications (such as WINE) assume it is the time that would pass
> until we reach an underrun if we would stop writing any further data
> to the PCM device.
> 
> Other applications (such as most media players) use it for time
> synchronisation. i.e. assume that it is the time that passes until a
> sample I write to a PCM device now would take to be played.

As James already pointed, the correct answer is the latter.
In the driver implementation level, snd_pcm_delay() simply returns the
difference between appl_ptr and hw_ptr.  It means how many samples are
ahead on the buffer from the point currently being played.

However, if you stop feeding samples now, snd_pcm_delay() returns the
least time XRUN occurs.  So the first understanding isn't 100% wrong.


> Why does this make a difference? For all drivers/ioplug backends where
> audio is not directly written to the hardware buffer of the audio
> device the underrun might happen much earlier than the last sample
> written is heard. This seems to be the case of USB audio for
> example. If I fill up the hw buffer completely, I will be notified
> about a buffer underrun much earlier than the buffer size would
> suggest. 
> 
> Another example is the PulseAudio backend: for each client stream we
> maintain a private playback buffer. After that buffer there is the
> hardware buffer.  If the client buffer runs empty we'd like to signal
> an underrun. However, in that situation it still will take some time
> until the underrun is really hearable, because the audio first has to
> pass the second buffer in line, the hardware buffer.
> 
> Or in other words: in some backends (PCI/DMA) after a sample is read
> from the ALSA playback buffer and the read index is increased it is
> immediately heard. In others it will still take substantial time until
> it is heard. The current API doesn't expose any information about how
> long that additional time can be, and it is not clear if
> snd_pcm_delay() includes or does not include this extra time in its
> result.
> 
> The ALSA documentation claims that snd_pcm_delay() returns the
> "distance between current application frame position and sound frame
> position". That would point that WINE's interpretation is
> correct. (i.e. ignore the extra time)
> 
> However, the USB audio driver actually seems to return the total time
> delay as it is useful for synchronization, so follows what media
> players expect. (i.e. take the extra time into account)

The implementation of snd_pcm_delay() (at least in the driver level)
purely depends on the accuracy of PCM pointer callback of each
driver.  So, if the driver returns more accurate hw_ptr via pointer
callback, you'll get more accurate value of snd_pcm_delay().  In the
worst case, it may be bigger up to one period size than the real
delay.


Takashi


> I personally believe that the USB audio driver does it right because
> the most important use for snd_pcm_delay() is to achieve
> synchronization between audio and video. However, the other value is
> very important too. Not just for WINE, but also for my own work,
> PulseAudio.
> 
> Most of the time the small difference between those two values doesn't
> really matter, since the difference is very small and for classic
> PCI/DMI devices zero. However, with USB I am now encountering this
> problem, because I cannot reliably estimate from the data ALSA supplies me
> with when the next underrun would happen. Also, the WINE people
> are pretty vocal that I am not right with my interepretation of the
> sitaution.
> 
> Takashi, Jaroslav, could you please eleborate on this, and explain
> which interpretation is the correct one?
> 
> Also, regardless which one is the correct one, could we please add a
> second API function the allows clients to query the other value?
> 
> I have already raised this issue in differents words two times before
> [1]. I never got a comment from you on this. So, please please with
> cream on top, respond!
> 
> Thank you very much,
> 
> Lennart
> 
> [1] http://mailman.alsa-project.org/pipermail/alsa-devel/2008-April/007354.html
> 
> -- 
> Lennart Poettering                        Red Hat, Inc.
> lennart [at] poettering [dot] net         ICQ# 11060553
> http://0pointer.net/lennart/           GnuPG 0x1A015CC4
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