[asterisk-users] Record in mp3

Tilghman Lesher tilghman at mail.jeffandtilghman.com
Fri Apr 24 13:21:50 CDT 2009


On Friday 24 April 2009 12:40:45 Danny Nicholas wrote:
> I’m sure someone will correct this if I’m wrong – Asterisk can’t make
> direct mp3 records because it’s not a supported codec.  Typically Asterisk
> records anything as a gsm, ulaw or alaw file, depending on the codec used
> to run the connection to the phone.

Well, even for the mp3 support that we do have, it's that we don't have
recording support for the format.  We do have read support, though, which
converts the format on the fly from the compressed mp3 format back into
uncompressed signed linear audio.  Given the patent protection on the MPEG
standard, distributing an MPEG-capable audio compression could raise
liability issues.

Secondarily, MPEG audio compression takes a lot of CPU.  Until the last few
years, desktop CPUs weren't even capable of doing realtime MPEG audio
compression, which is necessary if you're going to have the recording ready
by the time the audio input is terminated.  Above and beyond that, even modern
CPUs are limited in how many concurrent streams can be MPEG-compressed,
which may cause problems if you're encoding multiple channels to MP3 at the
same time.

Probably the best possible fix would be DSPs capable of audio MPEG-encoding.
Not only does this solve concurrency, but since DSPs are generally already
licensed for use with a particular codec, liability is addressed, as well.

-- 
Tilghman



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