Recording conferences and MP3


I was interested in other peoples experiences: We run a conference bridge (meetme) and want to record calls. we will need to have those files in MP3 format eventually.
Out of the box and Debian/Asterisk box doen not support MP3 encoding. right now we encode to wav files, transfer these to our windows servers and convert them to MP3 there. but this seems cumbersome.

I would like your opinions on:
[ul][li]what is the best format to record to in Asterisk (WAV, SLN, GSM, etc). I’m interested in quality of the recording and very much in the overal impact to the system performance.[/li]
[li]working with MP3 on asterisk machines. what is the best way to convert files to MP3, which libraries to use for that (examples?) and what is the impact on system performance when converting to MP3?[/li][/ul]


MP3 is over-engineering for telephone quality audio. Make sure you use a low bit rate, as you will otherwise just be wasting disk space.

The best recording format is the format that goes over the wire to the phone. In particular, do not use GSM unless that is the over the wire format or you want it as the final format.

Thank you for your response!
In this case I am just focusing on recording for playback on a PC (logging the conference). I will not be playing back over the phoneline.
What do you see as a reasonable alternative to MP3? we chose that because it is infact a lot more disc-space efficient than using the cold WAV files produced by Asterisk…

GSM. Also you may find that Media Player supports some of the more exotic telephone codecs.

The quality issue was that the source material was too low quality to justify standard rate MP3, so, if you use MP3, you should use the lowest rate that the coder allows, or the next lowest. Obviously check for subjective quality, as well.

GSM is more compact than low rate MP3, but is not suitable for music.


the GSM format does not natively play on Window Media Player… And I do not want to force my clients to install codecs.

I encode to 8kHz Mono MP3 with a bit rate of 8kbps (one hour of recording is about 3.5MB). I would say that is reasonable… playback quality if very clear.

The other thing I have been looking at has been the Asterisk SLN format, but that seems almost identical to WAV with the downside that Asterisk is about the only thing out there that seems to read it :wink:

So for now: record to WAV, copy to windows server, encode to MP3 there.

I would be curios to hear about experiences converting to MP3 on Asterisk (which libraries and performance impact)…

Thanks again!


We batch process with sox to convert to mp3, But this is only really possible as recording are only done on certain machines also they are quieter as sox does up the load a bit :wink:


If 8kbps MP3 is acceptable, it is more compact than standard rate GSM - it’s just that a lot of people would have used MP3 at its standard rate.

I think you will find that, like SLIN, Windows will take GSM with a WAV wrapper. The important thing to remember is that WAV is not a particular codec, but a file structure for carrying endoded audio and associated meta data. I think Asterisk WAV is just such a wrapper on SLIN. SLIN is the bare audio data,with no supporting meta data. Windows only supports MP3 directly, rather than in a WAV wrapper, because its popularity means that people expect that.

Hey david,

Thank you again, people will tend to think that wav = wav = wav… I am aware that wav comes in many flavours.

What exactly to you mean by a ‘wav wrapper’? and how would I wrap GSM in a that?

Windows Sound Recorder can save as GSM with a WAV wrapper, so GSM is certainly supported by Windows. What I am not sure about is the availability of command line tools to do this on WIndows. I’m not sure if Audacity has a command line mode.

By a WAV wrapper, I mean that there is WAV type metadata and a .WAV extension. Even the simple WAV formats are WAV wrappers around signed linear or unsigned linear.

MP3 is special in that many Windows tools will accept MP3 files without a WAV wrapper, although Sound Recorder can create MP3 WAV files.

Windows can also support G.723.1 WAV files, at 5.333kbps.