PyAv av.open() 指定使用的编解码器

Question

When PyAv is used to open the alsa audio device. How I can specify used codec and not the ffmpeg default one because that is wrong. By default it will use pcm_s16le and I need to use pcm_s32le . I can record from my device with following ffmpeg command:

ffmpeg -f alsa -acodec pcm_s32le -i dmic_sv alsaout.wav

but not with

ffmpeg -f alsa -i dmic_sv alsaout.wav

Which will give me following error:

[alsa @ 0x12061c0] cannot set sample format 0x10000 2 (Invalid argument)
dmic_sv: Input/output error

How to transfer the working command to PyAv av.open() function? There is stream_options but it doesn't seem to work. I tried

stream_options = [{'-acodec': 'pcm_s32le'}]
av.open('dmic_sv', format='alsa', mode='r', stream_options=stream_options)

And I get the same as above.

av.error.OSError: [Errno 5] Input/output error: 'dmic_sv'; last error log: [alsa] cannot set sample format 0x10000 2 (Invalid argument)

How to do this?

Answer 1

I'll answer my own question because I figured it out. I read ffmpeg source code and saw that when using alsa audio device and codec is not specified ffmpeg will default to use signed 16-bit pcm samples. Code here . By further exploring the source code the codec value comes from AVFormatContext::audio_codec_id struct field.

Now figuring out that PyAV using Cython to use FFmpeg and by reading PyAV source code of Container class I noticed it holds AVFormatContext in it's self.ptr variable. Then reading InputContainer source code and especially before calling avformat_open_input function to open the alsa device. Specifying the used audio codec is not supported by PyAV.

I forked the library and ended quickly hacking the solution for me. Now the question is would it be possible to add this feature to PyAV to force the codec used for the audio? In this case when the device is using pcm samples and relying ffmpeg to use choose the default one it will always use 16-bit samples and in my case I needed to use 32-bit samples.

Hopefully this helps someone and save them the trouble I went through:) I also posted this same answer for PyAV issue here .