[Speex-dev] Implementation of a Speex based hardware VOCODER

Jim Crichton jim.crichton at comcast.net
Mon Oct 29 06:09:51 PDT 2007

It is not clear what you mean by "low bit rates".  If you are working over 
analog phone lines, then I would think that your speeds (including whatever 
overhead you add) should match available telephone modems in the V.32 or 
V.34 series, which would start at 4800.  Listen to Speex quality at 3.95 
kbps, and see if this is acceptable for you.  If it is, then Speex will 
offer great flexibility, while still reaching a bit rate below 4800.  Speex 
will also allow you to integrate the voice codec and telephone modem on a 
single processor.  Of course, you still need to find the telephone modem, 
and I am not aware of any free algorithms there.

If you want an even lower bit rate solution, then you can buy AMBE+ as an IC 
(search for AMBE-2000 or AMBE-2020).  I believe that these ICs are 
preprogrammed TI DSPs, and work in a range of 2000 to 4000 bps.  This is a 
closed solution, though, and more suited to a commercial product than to a 
research project.

I expect that converting Speex to an FPGA would be a tremendous amount of 
work, so a uController would be more practical.  There have been several 
messages lately about running Speex on the dsPIC processor.  If you want to 
run the encryption, voice codec, and telephone modem on a single IC, then 
you would need a more powerful processor.


Jim Crichton

----- Original Message ----- 
From: "Andre Melo Carvalhais Dutra" <amcd at ita.br>
To: <speex-dev at xiph.org>
Sent: Friday, October 26, 2007 9:53 AM
Subject: [Speex-dev] Implementation of a Speex based hardware VOCODER

> Hi everyone,
>     I´m a graduate student in a Brazilian Intitute of Technology, and I´m
> doing some academic research regarding secure voice transmission over 
> phone
> lines. One of our reserach goals is to implement a hardware vocoder, with 
> low
> bit rates, and a preferably free algorithm, to be used in this secure 
> voice
> system.
>     Actually, there is a functional system using a proprietary AMBE board
> (also a very expensive one) with a bit rate of 4800bps. When I found this
> project, I tought the vocoder hardware could be implemented based on the 
> Speex
> code, since it works with similar bitrates.
>     Is speex suitable for that? What are the main hindrances? I thought in
> using some high speed uController (code would be implemented in C and 
> ASM), or
> a low cost FPGA (in wich the vocoder would be implemented in some hardware
> description language, such as VHDL); what do you thing about this?
>     Thanks in advance. Wishing to hear from you soon,
>     André Melo Carvalhais Dutra.
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> Speex-dev mailing list
> Speex-dev at xiph.org
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