[Speex-dev] rtp payload lenth

Pawel Cyrta apolinaire at poczta.fm
Fri Sep 14 08:33:49 PDT 2007

Hello to all speex developers,

I have question regarding payload length of narrowband speex in RTP.
I were watching tcpdump of the xlite softphone and have found that
it uses weird payload length namely 75 Bytes
I went through various source and without success.

To be clear:
For 8000Hz sample in 20 ms that is 160 samples per frame.
This makes 50 frames per sec.

modes     bit-rate 
    8 kbit/s    (narrowband mode 3) = 20 bytes per frame
  11 kbit/s (narrowband mode 4)  = 28 bytes per frame
  15 kbit/s (narrowband mode 5)  = 38 bytes per frame
  18,2 kbit/s (narrowband mode 6)  = 46 bytes per frame
  24,6 kbit/s (narrowband mode 7)  = 62 bytes per frame

And for VBR   frame size is from a set of {6,11,16,21,29,39}

So, X-lite uses 75 Bytes and this does not look as multiplication of 2 
Additionally, timestamp between consecutive rtp frames is exactly 20ms, so
this implies that this 75 Bytes is only one frame plus maybe bits of 

Can one of you tell me what kind of mode this is?

Thank you in advance.


ps. IMHO  some kind of table that I've shown should be included in RFC
so in Table 1 in draft-ietf-avt-rtp-speex-03 there should be additional 
Namely frame size.

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