Asynchronous ALSA - callback and latency questions

Asynchronous ALSA - callback and latency questions

[Bram Bos]

Hello all,

I’m using ALSA for building a performance musical instrument, so I’m after
 low-latency audio output.
I have some questions (since I'm fairly new to ALSA and to Linux they may  be a
big n00by, so please bear with me)…

1. I’m using asynchronous audio output (i.e. using a callback). Sometimes the
system simply stops calling the callback function without giving an error. In
case of an xrun I would expect there to be an underrun message (which I
sometimes also get) so I can handle it properly. So I guess something else is
causing the issue? Any pointers?

2. What does “File descriptor is in a bad state” mean, when it is returned by
snd_pcm_writei? I notice that I can often avoid getting it by increasing the
size of my buffer but the message doesn't make sense to me.

3. Occasionally my application grinds to a halt with a “I/O is possible” message
– presumably this is triggered by the ALSA callback mechanism?

4. My last question is a bit more conceptual: what determines the playback
latency of my audio stream? The period size? The total ring buffer size? I
assume it should be the difference between the hardware playback pointer and the
software write pointer, but I can’t figure out how they are determined.

Thanks for any help.



System info: Raspberry Pi B+ (non-overclocked), Raspbian Wheezy, USB PnP CM108
audio device
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Re: Asynchronous ALSA - callback and latency questions

[Clemens Ladisch]

Bram Bos wrote:
> I’m using ALSA for building a performance musical instrument, so I’m after
>  low-latency audio output.
>
> 1. I’m using asynchronous audio output (i.e. using a callback).

Don't.  Many devices do not support this, and asynchronous callbacks run
in a signal handler, where you must use only signal-safe functions.

> 2. What does “File descriptor is in a bad state” mean, when it is returned by
> snd_pcm_writei?

That the device is neither running, nor in an underrun state.

> 4. what determines the playback latency of my audio stream?

The buffer size.  When you fill the buffer completely, the last sample
must wait for all previous samples to be played.


Regards,
Clemens
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Re: Asynchronous ALSA - callback and latency questions

[Bram Bos]

> > I’m using ALSA for building a performance musical instrument, so I’m after
> > low-latency audio output.
> >
> > 1. I’m using asynchronous audio output (i.e. using a callback).
>
> Don't.

That's an interesting insight. I reckoned using a callback was the most elegant
solution since that's what I'm used to for DirectSound, iOS, OSX, WMME, ASIO
etc.

So what would be the most bulletproof way of dealing with streaming audio
instead? Can I do polling in a non-blocking state? I think blocking-state audio
is out of the question since I do need to handle MIDI I/O and UI events at the
same time.

Thanks
Bram
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Re: Asynchronous ALSA - callback and latency questions

[Takashi Iwai]

At Tue, 9 Sep 2014 10:44:01 +0200 (CEST),
Bram Bos wrote:
> 
> 
> > > I’m using ALSA for building a performance musical instrument, so I’m after
> > > low-latency audio output.
> > >
> > > 1. I’m using asynchronous audio output (i.e. using a callback).
> >
> > Don't.
> 
> That's an interesting insight. I reckoned using a callback was the most elegant
> solution since that's what I'm used to for DirectSound, iOS, OSX, WMME, ASIO
> etc.
 
If you want to go in that direction, just use JACK.  All you needed
are found there.


Takashi
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Re: Asynchronous ALSA - callback and latency questions

[Bram Bos]

> > > > I’m using ALSA for building a performance musical instrument, so I’m
> > > > after
> > > > low-latency audio output.
> > > >
> > > > 1. I’m using asynchronous audio output (i.e. using a callback).
> > >
> > > Don't.
> >
> > That's an interesting insight. I reckoned using a callback was the most
> > elegant
> > solution since that's what I'm used to for DirectSound, iOS, OSX, WMME, ASIO
> > etc.
>
> If you want to go in that direction, just use JACK.

And the next best alternative would be blocked-mode/polling through ALSA, then?

I'd prefer to have as few software interface layer dependencies as comfortably
possible, so if I can get away with using only ALSA without requiring JACK that
would be a plus for me.

Thanks
Bram
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Re: Asynchronous ALSA - callback and latency questions

[Clemens Ladisch]

Bram Bos wrote:
>>>>> I’m using ALSA for building a performance musical instrument, so I’m
>>>>> after
>>>>> low-latency audio output.
>>>>>
>>>>> 1. I’m using asynchronous audio output (i.e. using a callback).
>>>>
>>>> Don't.
>>>
>>> That's an interesting insight. I reckoned using a callback was the most elegant
>>> solution since that's what I'm used to for DirectSound, iOS, OSX, WMME, ASIO etc.
>>
>> If you want to go in that direction, just use JACK.
>
> And the next best alternative would be blocked-mode/polling through ALSA, then?

Yes.  (That's what Jack uses; with full duplex, you already need to handle
multiple devices.)


Regards,
Clemens
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