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[digi-voice] Digital voice over high-speed packet

Ken Linder (kc7rad) kc7rad at radstream.com
Thu Oct 28 20:01:40 UTC 2004

Thanks all for the info...

I have been experimenting with a codex called speex (www.speex.org) and can 
get decent results down to about 5kbps, just after the first night.  I am 
sure I can get it down less than that.

There is a LOT of header overhead with AX.25 and that is something that will 
have to be death with for my little project.  I was hoping to overcome this 
problem by going to very high speeds.  I just purchased a Kantronics TNC, 
supposedly capable of up to 32kbps and even 56kbps with a little circuit 

I have been looking around for some alternate protocol that gives a larger 
data segment.  Any suggestions???


----- Original Message ----- 
From: "Nicolai Heilemann" 
<nicolai_oliver.heilemann at student.fh-reutlingen.de>
To: "TAPR Digital Voice Mailing List" <digi-voice at lists.tapr.org>
Sent: Wednesday, October 27, 2004 11:51 PM
Subject: Re: [digi-voice] Digital voice over high-speed packet

> I have done some LPC tests on a 9k6 channel using RAT (Robust Audio Tool 
> by the University College of London) with focus on speech quality 
> assessment. The result where not too uplifting. (Or lets face it: I proved 
> that it works in principle, but don't expect more ;) ).
> In general a 9k6 channel is able to transport speech if you compress it 
> down to 2kBit or so, (not more as with AX.25 you have an awfull lot of 
> packet header overhead), but as you can expect with such a high 
> compression the speech is extremely vulnerable to packet losses (which 
> will occur quite often as you cannot use balanced mode but have to send 
> everything as UI frames as you would end up with no real time any more 
> otherwise), so it simply sounds quite poor.
> Anyway it would be interesting to use more sophisticated codecs (CELP or 
> MELP-based or so) which deliver a higher speech quality by themselves than 
> LPC and see whether this would lead to satisfying results.
> If you are interested in some detailed results, I am happy to share, and I 
> also would be awfully interested in the outcome of your experiments of 
> course.
> Vy 73 es 55 de
> Nicolai, DL9TS
>> At 01:09 AM 28/10/2004, you wrote:
>>> Hi all,
>>> Is there anyone on the list that knows whether anyone is doing anything 
>>> with digital audio over packet?  Any info would be appreciated.
>> I have heard of the occasional station using Speak Freely running LPC-10 
>> over 9600 bps packet with reasonable results (on a clear channel).
>> 73 de VK3JED
>> http://vkradio.com
>>digi-voice mailing list
>>digi-voice at lists.tapr.org
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