Hello,
Has an SDP profile been established for Codec2 yet? Since I am
trying to build it (experimentally) into my open source firmware
for SIP phones ( http://devel.0cpm.org/firmerware/ ) I would like
to know how to describe the codec in negotiations.
If nothing exists yet, would the following be suitable?
m=audio 1234 RTP/AVP 80 81
a=rtpmap:80 PCMA/8000
a=rtpmap:81 x-codec2/8000
a=fmtp:81 bps=2550,2400,2000
The PCMA is just here to show how alternatives would look
around Codec2, if any are available.
The "/8000" refers to the sample rate, not the bitrate.
(Even if G.722 was mistakingly set to 8000 in the past.)
Only a wideband version of Codec2 would change that value.
The payload-format-specific parameter "bps=" contains a
comma-separated list of bitrates that can be synthesised into
speech.
The actual format used can be derived from the packet size,
even under audio/red that is still the case. There may be
problems due to ptime= however; if 2 packets of bitrate N
would have the same size of 1 packet of bitrate M it may
not be possible to distinguish what was sent.
2550 = 17 * 5^2 * 3^1 * 2^1
2400 = 5^2 * 3^1 * 2^5
2000 = 5^3 * 2^4
1200 = 5^2 * 2^4
The most likely problem would be that 2 packets at 1200
have the same size as one packet at 2400. This could be
solved in SDP in two ways. First, by setting a maximum
to ptime to avoid that, along the lines of:
m=audio 1234 RTP/AVP 80 81
a=rtpmap:80 PCMA/8000
a=rtpmap:81 x-codec2/8000
a=maxptime:20
a=fmtp:81 bps=2550,2400,2000
This would be subject to RFC 3267 support and with ptime
scaled in milliseconds it may not completely solve the
matter. The second solution would be to split the
description into individual offers:
m=audio 1234 RTP/AVP 80 81 82 83
a=rtpmap:80 PCMA/8000
a=rtpmap:81 x-codec2/8000
a=fmtp:81 bps=2000
a=rtpmap:82 x-codec2/8000
a=fmtp:82 bps=2400
a=rtpmap:83 x-codec2/8000
a=fmtp:83 bps=2550
I feel the latter to be the best option, as it cannot
cause problems when five packets at one rate could
never match seventeen of another, etc. An SDP profile
could formalise on this approach by requiring a single
entry only in the bps= section.
Even if this is a bit bulky SDP, it evades a need to
include tags and/or lenghts in all packets in the
RTP stream.
I used MIME subtype "x-codec2" because there is no
standardised name yet. Is Codec2 a working name, or
is it the final name for the codec? To me, David
appears to be fighting a giant, so I could imagine
Goliath as a codec name... which would be ironic,
given the packet sizes ;-) Anyhow, that's not for
me to decide, merely to ask about.
I hope this is helpful.
Cheers,
Rick van Rein
OpenFortres / 0cpm
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