Call recording format

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Asterisk Users 6 Comments

What format are the actual calls in? Are they in G.711u/a format or
are they in something else (perhaps gsm?) format? I’m asking to find
out if Asterisk would need to transcode them.

On Mon, Nov 22, 2010 at 6:47 AM, Vilius Adamkavicius
> Hi All,
> We have a requirement to record over 60 simultaneous calls. Our recording
> facilities are implemented using Monitor() over AMI. The thing we have
> noticed that making 60 simultaneous call recordings using wav CPU load is
> significantly higher (around 2 times more) than using gsm. Even writing call
> recordings to /dev/null makes a big difference in CPU load.
> What could be the reason for this? Is Asterisk updating wav headers every
> time it writes?
> What would be recommended hardware setup for over 60 simultaneous call
> records?
> Regards,
> Vilius.
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6 thoughts on - Call recording format

  • Hi Joel,

    We have a meetme on which we are landing two G.711 alaw calls, one coming
    from TDM another from SIP. Once we those parties are in the conference we
    are adding one more leg using Local channel and starting to record it.

    Surely it would be logical if it would be less overhead recording alaw wav
    since we are using alaw on both parties, but its not.


  • On Mon, Nov 22, 2010 at 8:47 AM, Vilius Adamkavicius

    Ignoring your real questions, and asking an alternate question:

    Why not just record in gsm?

    If your answer is that you have to play these back on Windows, you can
    build an on-the-fly gsm-to-wav converter using sox.

    My understanding is that recording in wav doesn’t exactly make you
    have higher audio quality in your recordings, although the experts at
    codecs could better answer that.

  • Hi David,

    Looking at MOS G.711alaw wav most definitely has the higher score than gsm.
    Moreover recording in gsm is more CPU intense than wav. Therefore your
    suggestion to do more CPU intense recording and afterwards use system
    resources to convert it back to wav is not a solution. Also some of our
    customers require call recordings to be done in wav.


  • wav with signed linear payload?

    I wonder what would happen if you record it as “.sl” (raw signed linear)
    and convert it to wav at the end of the call (while mixing).

  • WAV or wav? One of these has GSM-encoding inside a WAV formatted
    envelope. That said, I wouldn’t expect that to have any noticeable
    CPU utilization above that of GSM. If you are using the non-GSM
    version of WAV, then I am as baffled as you – hopefully someone who
    knows more about this can help.

    On Mon, Nov 22, 2010 at 7:58 AM, Vilius Adamkavicius

  • We are using wav, not WAV. I believe WAV is the one with GSM. Its a very
    good idea to compare WAV against wav, will run some tests and come back with
    outcome, will try Tzafrir’s suggestion as well.

    Thanks guys