Subject: Loss repair for Audio

From: [email-masked]
Date: Thu, 21 Mar 96 14:02:38 +0000

Dear Henning,

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>I think it's important to consider the advantages of receiver-only loss 
repair:
>- - independence of the audio tool used at the sender
>- - different styles/qualities of loss repair are applicable
>- - no overhead data, no additional processing at the sender
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Receiver only repair is suitable for low loss and small packet sizes. It 
cannot cope with high levels of loss. There are many instances in the 
literature that describe mechanisms which work for small packets 16-32ms, and 
for relatively low levels of loss around about 20% if you are prepared to 
expend a lot of processing power. 

The point is that in order to repair speech at the receiver, you are assuming 
that the speech characteristics have not altered. As the packet size 
increases, so does the probability that the speech characteristics have 
changed, from voiced to unvoiced etc. This means that a receiver only repair 
mechanism will not work for large packet sizes or high loss no matter what you 
do.

Redundancy is a mechanism aimed at the higher loss levels, when a low 
bandwidth and communications quality version of the speech is transmitted 
piggy-backed onto the original. The extra bandwidth is minimal 13 bytes per 
20ms worth of speech, and there are some follow-on ideas to reduce the 
bandwidth further. Bandwidth consumed is not such a worry for the Mbone 
compared to per packet processing - ref Dave Clarks rfc.

Our recent human perception studies have show that a single copy of redundancy 
shows a significant improvement for packet sizes of 40ms duration and for loss 
rates of upto 50%. The improvement can be easily increased if multiple copies 
of the redundancy are sent.

We also have detailed results from both intelligibility and speech quality 
experiements that show only a minimal decrease in speech quality at 80ms if a 
copy of redundancy can be received - the probability of which increases if 
multiple copies are sent.

We use a simple receiver-only based mechanism to repair speech for low loss 
levels. It is expected that a range of solutions are suitable for packet loss 
repair, within which receiver-only mechanisms are suitable for low loss rates 
and small packet sizes, and redundancy-based mechanisms are suitable for large 
packet sizes and high loss rates. (We also assume that loss repair is for 
transient loss recovery, longer-term loss should be dealt with by reducing the 
load on the network).

I will get someone to translate your paper for me. It sounds good.  

If you would like a copy of an earlier paper of ours, then see

http://www-mice.cs.ucl.ac.uk/mice/rat

We have 2 new papers in the pipe-line, which I will send you details of 
soonish.

Regards

Vicky

P.S. You don't happen to work with Henning Schulzrinne do you?

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>We have developed a receiver-only loss concealment technique which is based on
>time-scale modification of correctly received audio.  Time-scale modification
>means in this case "stretching" the signal (_without affecting the pitch 
>period_)
>to fill the gap caused by lost packets.

>As no new information is introduced into the signal, the clicking, tinny and
>reverberating sounds of "classical" waveform substitution algorithms can be
>minimized.  The main limitation is of course the length of the lost speech
>segments, which can be concealed (up to 50ms). 

>Henning

>P.S. As we haven't finished our paper yet, I can only give a reference in 
>German:
>     http://www-nt.e-technik.uni-erlangen.de/~sanneck/da.ps.gz
___________________________________________________________________
           Henning Sanneck

           Research Institute for Open Communication Systems
	   GMD FOKUS, Hardenbergplatz 2, D-10623 Berlin, Germany
Phone    : ++49 / (0)30 / 254 99 - 175
Fax      : ++49 / (0)30 / 254 99 - 202
e-mail   : [email-masked]
WWW      : http://www.fokus.gmd.de/usr/san
___________________________________________________________________

> 
> RAT (Robust-Audio Tool) is a new audio tool which uses redundancy to repair
> loss. It is available by ftp. It includes packet loss protection.
> 
> See
> http://www-mice.cs.ucl.ac.uk/mice/rat/index.html
> 
> for details of where you can get hold of RAT binaries.
> 
> Regards
> 
> Vicky
>

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