Re: Playing 5.1 sound on a stereo device with OpenAL Soft

On 05/11/10 17:40, Chris Robinson wrote:
> On Friday, November 05, 2010 1:59:43 am Martin Lambers wrote:
>> You're right, it is indeed an ffmpeg problem. Some audio streams are
>> apparently *not* decoded to signed 16 bit sample formats. The ffplay
>> player contains code to convert sample formats in this case, but uses an
>> internal ffmpeg API for this purpose that is not part of the installed
>> header files. So, no luck for alffmpeg and my project...
> 
> Seems ffmpeg was changed (again). When I originally wrote the code, 
> avcodec_decode_audio2 guaranteed writing samples as signed 16-bit PCM, hence 
> why the function takes int16_t* for output samples. But now it seems the 
> AVCodecContext contains a sample_fmt field. And indeed, the WMA codec outputs 
> 32-bit float samples. This is easy enough to support with AL_EXT_FLOAT32, 
> though 32-bit int and double sample formats would need conversion (double 
> could work with AL_EXT_DOUBLE, but there's no multi-channel support for it).

Thanks for these hints. I now take the samples from ffmpeg in the format
it provides and let OpenAL do the conversion if supported. My
application is a 3D video player <http://bino.nongnu.org/>, and I think
this approach should work fine with most real world video files
(multichannel sound streams with double format samples are probably only
used for very special purposes).

> This is one of the biggest problems I've found with using ffmpeg. It regularly 
> breaks backwards compatibility, so you need to pick a specific version and 
> stick with it. GStreamer is nicer for streaming playback (especially with an 
> OpenAL sink <https://bugzilla.gnome.org/show_bug.cgi?id=615615> to integrate 
> it with the rest of the OpenAL app), although it's not so nice for general 
> decode-to-buffer functionality.

Yes, GStreamer seems a little bit too high-level for me - it seems
difficult to control the whole thing, including timing. OTOH, ffmpeg
seems a little too low-level for me...

Martin
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