Extensible wave format support

[Zdeněk Softić]

I'm trying to identify a WAVE PCM audio file with multiple channels (3 and more) and it seems it's not identified correctly.
Mutagen gives me invalid WAVE encoding (it returns -2 instead of 1 which is PCM).

I was using sample files downloaded here https://www.mmsp.ece.mcgill.ca/Documents/AudioFormats/WAVE/Samples.html.

import mutagen

files = [
    "M1F1-int16-AFsp.wav",
    "addf8-Alaw-GW.wav",
    "8_Channel_ID.wav",
    "M1F1-float64WE-AFsp.wav",
]
​
for fn in files:
    with open(fn, "rb") as f:
        info = mutagen.File(f).info
        print(f'{fn.rpartition("/")[-1]:25} encoding {info.audio_format:2}, channels {info.channels}, bits_per_sample {info.bits_per_sample:2}, sample_rate {info.sample_rate:5}')

M1F1-int16-AFsp.wav       encoding  1, channels 2, bits_per_sample 12, sample_rate  8000
addf8-Alaw-GW.wav         encoding  6, channels 1, bits_per_sample  8, sample_rate  8000
8_Channel_ID.wav          encoding -2, channels 8, bits_per_sample 24, sample_rate 48000
M1F1-float64WE-AFsp.wav   encoding -2, channels 2, bits_per_sample 64, sample_rate  8000

I have looked into mutagen code and it indeed does not seem to support extensible wave format.

I am not fluent in audio formats, so thanks a lot for any insight :)

[Philipp Wolfer]

The value is kind of correct, just that I think mutagen should read the values unsigned, and currently reads it signed. For WAVE_FORMAT_EXTENSIBLE this format info is supposed to be FFFE (65534), which if you read this as a signed value is -2.

See https://learn.microsoft.com/en-us/windows/win32/api/mmreg/ns-mmreg-waveformatextensible and https://github.com/tpn/winsdk-10/blob/master/Include/10.0.16299.0/shared/mmreg.h

[Philipp Wolfer]

#596 fixes the issue. With the patch these files properly return info.audio_format == 65534