[speex-dev] frame size

Joost Witteveen joost at iliana.nl
Tue Dec 9 09:33:32 PST 2003

> Framesize always refers to the decoded data frame size in samples.
> Framesize is dependent on the encoding mode
> Narrowband (8kHz): framesize = 160 samples = 320 bytes of PCM 

> The size of the encoded data depends on the quality setting, so if you
> know for instance that you are using quality 3 on narrowband, that is
> 119 bits of encoded data per frame which is rounded to 15 bytes per
> frame (without counting the ogg headers).

and craming 160 samples into 15 bytes is amazing indeed.

But, it looks like it's not so very usefull:

Each package looks like (tcpdump -x -vvv output, with SpeexNarrow-8k{sw})

18:09:36.966518 kato.10018 > foko.5000:  [udp sum ok] udp 32 (DF) [tos 0x10]  (ttl 59, id 0, len 60)
            4510 003c 0000 4000 3b11 2196 3e83 8ce6
            3ed8 13ca 2722 1388 0028 4893 8062 b576
            0000 7800 6cee c076 19c0 227f a415 aebc
            d7e5 eab3 e9cf 8ba9 fbb7 c039

So, each UDP package with 20 bytes speex-data, we send:

20 bytes speex
12 bytes ogg headers (and others?)
28 bytes UDP/IP headers (2 IP numerbers, 2 portnumbers, checksum, etc, etc)

and, if it goes over the phone, each package has a few ppp headers.

<p>Am I overlooking something, or does this fixed frame size mean that
the "8kbps" in reality means at least 24kbps?

Or is there something ohphone/gnomemeeting/etc can/should do to
put more than one fram inside each UDP package?

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