[Openal] AL_*_OFFSET resolution

[Chris Robinson]

On Friday 07 December 2007 09:15:50 am Ben Swanson wrote:
> I am attempting to sync an animation to a sound file using the
> AL_SEC_OFFSET property.  The problem I am having is that the resolution of
> the command seems to be 4096 samples, which leads to a choppy animation. 
> Is there a way  to change this (the sound file is in ogg vorbis format)?
>
> Ben

Audio isn't usually a good thing to exclusively time animation by, really. As 
you've noted, there's an approximately 4K frame update size (* 2 channels * 2 
bytes per sample = 16K bytes, a bit high but still somewhat reasonable for 
display sync).

What implementation are you using? If you're using the Sample Implemention 
(Linux, FreeBSD, Solaris, etc), then part of the problem is the mixer 
inefficiency which pretty much requires a large(r) buffer or else it'll miss 
sound card updates. In this case, you can try OpenAL Soft ( 
http://kcat.strangesoft.net/openal.html ) which has a few advantages: first 
being that it has a much cleaner mixer, so smaller update sizes should be 
possible.. and also (at least when using the ALSA backend) it uses ALSA in 
mmap mode which lets the mixer write however many samples it needs directly 
to the audio buffer when it needs. The update size basically becomes dynamic 
which would hopefully be good enough for sync'ing to animation and the like. 
The OSS backend doesn't currently do anything like that yet, but I may try 
something in the future.

Also be careful with buffer queues, since the offset is from the start of the 
queued buffers. So when you unqueue a buffer, the offset decreases according 
to the size of the removed buffer.