Subject: Delay and Loss on the Mbone

From: [email-masked]
Date: Wed, 20 Mar 96 18:24:47 +0000

Dear Pete,

THE reference for the perception of delay is:

Brady P.T. 'Effects of Transmission Delay on Conversational Behaviour on 
Echo-Free Telephone Circuits' Bell System Technical Journal, pp 115-134, 
January 1971

The perception depends upon whether echoes are present (such as might be 
produced by 2/4 wire hybrid tansformers, today commonly found at the junction 
between the exchange and the local subscriber) or not. So without echoes, 
400ms round-trip, with echoes considerably less.

People using Mbone audio don't suffer from echoes in the usual telephone sense 
of the word, and so tolerable delay can be much larger. 

The big problem with Mbone audio though is not delay or delay variance, but 
LOSS. Loss can render speech unintelligible for even low loss rates with the 
packet sizes used over the Mbone.

Here at UCL we have been trying to come up with methods of repairing the loss. 
We interleave a second low bandwidth version of the coded speech, so that loss 
can be repaired something reasonable. (This performs far better than any of 
the things you can do at the receiver, such as using noise, or the previous 
packet, or sample interpolation etc. etc.)

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

------- Forwarded Message

Date: Wed, 20 Mar 1996 10:25:26 +0200
From: Petri Helenius <[email-masked]>
To: [email-masked]
Cc: [email-masked], [email-masked], [email-masked], [email-masked], 
    [email-masked], [email-masked]
Subject: Re: MCI to offer video services over the Internet
ce.cc.vt.edu>

Valdis Kletnieks writes:
 > On Tue, 19 Mar 1996 13:43:33 PST, [email-masked] said:
 > > Hmm. Doing telephony over satellite - seems like one or two such
 > > hops and you're dead, given that the latency will creep over 1/2 second.
 > 
 > Hmm.. I must have blinked here and missed a sarcasm alert.  Wasn't the 
original
 > reason for geosyncronous satellites to provide long distance telephone 
service?
 > 
 > Remember.. 0.5 seconds latency may be an eternity for silicon-based life, 
but
 > for us carbon-based critters, it's still a blink of an eye...

And for many of the 'multimedia'-applications jitter (delay variance)
and bandwidth are actually more important than the actual latency. Usually
low jitter comes with either intelligent queueing or high bandwidth. I would
also count low loss as one of the factors since I've yet to see data-
interleaving audio application ...

Pete

------- End of Forwarded Message

---

From: Henning Sanneck <[email-masked]>
Date: Wed, 20 Mar 1996 22:41:46 +0100

Hi,
> packet, or sample interpolation etc. etc.)

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

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
___________________________________________________________________