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].
3) It's quality is probably not acceptable for commercial use; It
certainly does not approach uLaw quality.. sounds more like speex at 4kbps.
Posted: Tue Nov 23, 2004 11:01 am Post subject: [Asterisk-Dev] MELP and MELPe (1.2kbps) codecs for Asterisk
Quote:
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.
Quote:
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):
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.
Posted: Tue Nov 23, 2004 12:06 pm Post subject: [Asterisk-Dev] MELP and MELPe (1.2kbps) codecs for Asterisk
Daniel Pocock wrote:
Quote:
>
> 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.
IAX2 can support whatever frame size you want. There's a couple of issues:
1) In general, there's no way to configure this in *, and in practice,
there's a default frame size for each codec throughout * (30ms for iLBC,
20ms for everything else). This could be changed, though. For example,
I have a custom build of app_conference which works with 40ms speex
frames, for this reason.
2) With larger frame sizes, you obviously add latency to the conversation.
3) With larger frame sizes, you have worse effects from packet loss:
I.e. it's much worse to lose one 60ms frame, than to lose three
non-consecutive 20ms frames.
Also, one thing to consider other than those samples is how well this
codec performs in such lossy conditions.
as far as 67.5ms, that seems to be 540 samples, so you could probably
make things work with that, although since timestamps are in ms, you'd
have to round them off: i.e. 0, 68, 135, etc.
Posted: Tue Nov 23, 2004 6:46 pm Post subject: [Asterisk-Dev] MELP and MELPe (1.2kbps) codecs for Asterisk
On Tue, 2004-11-23 at 19:01 +0000, Daniel Pocock wrote:
[snip]
Quote:
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?
[snip]
I just tried yelling out of my bathroom window but watching CNN I have
not seen any place receive a warm fuzzy US bomb blanket. Guess they
still haven't found him ;)
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