µ-law: pcm_mulaw files incorrectly shown as "s16", actually 8 bit per sample

[Marcus Müller]

Summary of the bug: ffprobe (6.1.2)/ffplay/ffmpeg incorrectly show

Stream #0:0: Audio: pcm_mulaw ([7][0][0][0] / 0x0007), 48000 Hz, 1 channels, s16, 384 kb/s

when inspecting a µ-law PCM file.

If you divide 384 kb/s / (48000 Hz), you get the actual (and correct for G.711 µ-law) 8 b per sample.

How to reproduce:

# get some example linear pcm
curl https://opus-codec.org/static/examples/samples/speech_orig.wav > orig.wav
ffmpeg -i orig.wav -c:a pcm_mulaw mulaw.wav
ffprobe -show_entries stream=sample_fmt -hide_banner mulaw.wav
# gives:
# Input #0, wav, from 'mulaw.wav':
#  Metadata:
#    encoder         : Lavf60.16.100
#  Duration: 00:00:10.80, bitrate: 384 kb/s
#  Stream #0:0: Audio: pcm_mulaw ([7][0][0][0] / 0x0007), 48000 Hz, 1 channels, s16, 384 kb/s
###### PROBLEM HERE: ######-----------------------------------------------------^^^
# [STREAM]
# sample_fmt=s16
# [/STREAM]

The sample_fmt should be s8, not s16.

[Balling]

Maybe

The μ-law and A-law algorithms encode 14-bit and 13-bit signed linear PCM samples (respectively) to logarithmic 8-bit samples. Thus, the G.711 encoder will create a 64 kbit/s bitstream for a signal sampled at 8 kHz

[Marcus Müller]

ah, I see, that kind of makes sense. Otherwise, that field is meaningless for anything actually compressed (and not just companded/quantized).

Hm, however, is it then correct to organize it as property of {{stream}}? It seems to be a property of the decoder.

[Marcus Müller]

(oh by the way, if you don't think this constitutes a defect, please close. I wasn't sure I should close it myself since your comment started with "Maybe")

[Balling]

Are you Marcus from Gnuradio project, lol

Anyway, I was wrong ​https://imgur.com/a/jo9NzKh mediainfo actually shows it as 8 bit in metadata.

s16 does not mean it is 16 bit. It just means it will be decoded as s16.

[James]

You can see that -show_entries stream=bits_per_sample reports 8, which is what's coded in the WAV header.

As Baling said, the sample format is what the decoder exports.