[Speex-dev] Speex over 56.6K modem

Tom Grandgent tgrand at canvaslink.com
Wed May 4 05:44:55 PDT 2005

I use Speex with dialup modem users.  Even if the dialup modem 
is "56K", you should assume that upstream bandwidth available is 
only 20-30kbps at the most.  I use 16kHz wideband mode and VBR 
quality 2.  Also, I send 80ms (4 frames) per packet, because 
there is an overhead of approximately 33 bytes per packet 
due to UDP (8 bytes), IP (20 bytes), and PPP (~5 bytes) headers.  
If you sent 20ms (1 frame) per packet, this overhead would be 
13.2kbps, which is totally unacceptable for a dialup modem.  
80ms packets result in a more reasonable 3.3kbps overhead.

Also, you should have a decent jitter buffer for received 
packets so that if packet timing is not entirely smooth, 
enough audio is buffered to avoid dropouts.

In my program using Speex, I've been able to have a dialup 
modem user send audio and video to two people at the same time 
and not have any breakups in the audio.  (It was *very* low 
bitrate video.) :)

Hope this helps,


Tay YueWeng <yueweng at yahoo.com> wrote:
> Dear all,
> Have anybody uses speex codec over 56.6K bit/s line
> that hear clearly in both direction before? I perform
> a testing with one side using 56.6K dialup modem and
> the other side using broadband. The user that uses
> 56.6K/s modem is able to hear the broadband user
> voice.  But the broadband uses hardly understand
> 56.6K/s modem user's voice. I am using 16KHz encoding.
> I am not sure if the the modem is half or full duplex.
> Of course, there is nothing wrong Speex but I am just
> wondering what is the mimimal bandwidth requirement
> for speex under 8KHz and 16KHz? 
> So far, I have enabled VBR and DTX. What more should I
> do to optimize the bandwidth utilization? 
> Is Speex codec design with 56.6Kb/s bandwidth
> consideration?
> It seems to me that the amount of bandwidth
> utilization for 8KHz and 16KHz is almost the same
> after compression, is this normal?
> Thanks,
> YueWeng

More information about the Speex-dev mailing list