Re: Several beginner q's about Sox encoding (mp3/aac)?

Erich Eckner <[email protected]> Tue, 21 Feb 2017 06:52:49 +0100
Newsgroups	gmane.comp.audio.sox
Message-ID	<[email protected]>
Hi,

> I have a Raspberry Pi running Debian Jessie, as well as a MEMS I2S mic and
> a Pi Camera. My mic records 32bit raw PCM audio. I am trying to stream
> video and audio to Youtube Live. I am using Sox for audio recording and
> encoding.

I doubt, your microphone really records 32 bit (even more expensive
equipment usually records 24 bit "only"). The data stream is 32 bit (as
is sox' internal stream), but the actual data is only less - maybe 24 or
16 bit.

> YouTube Live only accepts mp3 or aac encoded audio.
> 
> I have installed sudo apt-get install libsox-fmt-mp3, so my Sox is able to
> encode mp3.
> 
> I am using this command to record and encode audio:
> 
> rec --buffer 2000000 -c 1 -r 44100 -b 32 test.mp3
> 
> /usr/bin/rec WARN formats: mp3 can't encode to 32-bit
> 
> Input File     : 'mic_mono' (alsa)
> 
> Channels       : 1
> 
> Sample Rate    : 44100
> 
> Precision      : 32-bit
> 
> Sample Encoding: 32-bit Signed Integer PCM
> -------
> 1. I get the warning: " mp3 can't encode to 32-bit" - what should I do
> about this?

Some settings can't be encoded into mp3. In the past I've hit the limit
for sample rate (48k) - it seems, the bit depth is also limited.
Try reducing it to a sane value (see above).

> 2. My resultant MP3 file has small clicks and popping noises throughout. I
> know this is not a problem with my mic, as I've produced AAC encoded audio
> with FFMPEG that was perfectly clean. Any idea why this is happening?

This is probably due to a slow cpu / computing intensive encoding. I had
same issue with an old laptop and some other recording tool, when
recording to mp3, but not to wav.
I'm not sure how to circumvent this. Probably a Raspi is too slow at all?

> 3. Is it possible to compile SOX with AAC encoding support? I want to try
> AAC encoding to see if that eliminates the clicks and pops.

I have no idea about this, but maybe it's possible.

Regards,
Erich

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