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

Nicolai Heilemann nicolai_oliver.heilemann at student.fh-reutlingen.de
Thu Oct 28 05:51:36 UTC 2004


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
>
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