[Speex-dev] SPEEX tech specs

Jean-Marc Valin jean-marc.valin at usherbrooke.ca
Thu May 3 04:30:01 PDT 2007

B. Mitchell Loebel a écrit :
> Thank you. You're right ... my error ... I meant to say 12 bytes
> (including the 2 bytes for VAD). And it is 10ms/frame. No matter ...
> thank you for the SPEEX specs. In terms of quality, what SPEEX bit rate
> compares with G.729 at 8kbps data rate please? 

Haven't done formal testing and it depends on whether it's G.729 or
G.729A. I'd say probably 8 kbps or 11 kbps.

> Is there some reason why
> you chose the 20ms frame rate? 

It's a good value I think. Speex is designed for VoIP, so 10 ms is
pretty much useless because you'd be sending about 32 kbps worth of
overhead (each packet has 40 bytes for IP+UDP+RTP headers).

> Do you keep that same frame rate for the
> different bit rates? 


> The faster frame rate would seem to better adapt to
> multiple speakers cutting in and out ... yes/no?

I don't think we should really worry about speakers interrupting each
other at 10 ms interval. If that happens, you've got much more important
problems than the codec :-)

> Btw, 6 bytes/frame at 50 frames/second seems to be 2.4kbps so I'm
> guessing that the lower rate you mentioned is because of some overhead
> for VAD in your 6 bytes.

Well, technically 2.15 kbps would mean 5.375 bytes/frame, but then you
need to round up to the next whole byte -- unless you put multiple
frames in a packet, in which case it matters.


> At 07:11 PM 5/3/2007 +1000, Jean-Marc Valin wrote:
>> B. Mitchell Loebel a écrit :
>> > How many bits do you have in a frame please and what is your frame
>> rate?
>> > For example, G.729 (ACELP) has 12 bits/frame including VAD and the
>> frame
>> > rate is 100/second ... I'm looking for the comparable figures for
>> I'm sorry but if G.729 used 12 bits/frame at 100 frames/second, it would
>> have a bit-rate of 1.2 kbps. G.729 actually uses 10 *bytes* (i.e. 80
>> bits) per frame. Speex has 20 ms frames (50 frames per second) and
>> supports bit-rates that range from 2.15 kbps (6 bytes per frame) to 44
>> kbps (110 bytes per frame).
>>         Jean-Marc
> ---
> B. Mitchell Loebel, CEO, VP Engineering                              408
> 425-9920
> InstaFlash International Corporation
> (formerly Minute-Tape International Corporation)

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