[Asterisk-Dev] MELP and MELPe (1.2kbps) codecs for Asterisk

[Daniel Pocock]

>
> 2) Presently, even IAX2 with 20ms frames has about 12kbps of overhead 
> (IP,UDP,IAX2).  So for GSM you have about 25kbps.  Even using the 
> magical 0bps codec, you still end up using 12kbps on the wire, unless 
> you use larger frame sizes.  [IAX2 trunking, of course, helps a _lot_ 
> where trunking is viable].

The MELP frame size for 2.4kbps is 22.5ms, and for 1.2kbps it puts 
67.5ms into a frame.  Can IAX2 support these sizes?  I believe the 
overhead should be 3.55kbps with a 67.5ms frame size, total link 
capacity required = 4.75kbps (or 600 bytes per second).  Theoretically, 
that could work over a 9600 baud PPP connection in Africa, and still 
leave some spare bandwidth for instant messaging.

>
> 3) It's quality is probably not acceptable for commercial use;  It 
> certainly does not approach uLaw quality..  sounds more like speex at 
> 4kbps.
>

There are more audio samples here, at 1200 and 2400, various noisy 
environments (street, car, etc):

    http://www.signalogic.com/index.pl?page=codec_samples

The military use it.  If it's good enough to order 'through the bathroom 
window' precision bombings  on Osama bin Laden's hideouts, surely it's 
good enough for commercial use?

In practice, I think this codec would only be appropriate for very 
specific applications where bandwidth is extremely expensive or 
impractical (eg long range VHF radio, people who live out of reach of a 
DSL service, satellite links, etc).  It wouldn't be something that you 
would impose on a corporate user who has a 2mbps serial link at their 
city office.