Re: ISDN & sound question

Iain Campbell (iain@dcs.gla.ac.uk)
Sat, 29 Oct 1994 14:52:55 -0400


>I finally got my 64K ISDN working, but so far my incoming audio is still very
>broken up with CUSM. I thought with audio using only 32K, it would be "loud
>and clear".

Your audio needs a *constant* 32Kbs bandwidth to be available for clean
transfer. That means it has to fight for *half* of your 64K line against
all the video streams.

If you look at the Cornell reflector you'll see that everyone is pumping a
good 20Kbs average, each. That is the case even at a busy times when the
averages are forced down by the CU-SeeMe protocol.

It is easy to see that you only have a chance of clean audio when you've
got a *single* video stream going at the same time as your audio.

>Also, I notice in MacPPP I can only choose 57,600 (not 64K). Am I losing
>something?

I'm guessing here, but it looks like your ISDN interface-box-thingy is
connected to your serial port...yes? If so, then 57,600 is a very common
maximum rate for serial interfaces. That's be the fault of both your Mac
and your ISDN inter-boxy-whatsit.

However, if you have an ISDN *card* in your machine, you should be able to
do the "full boohna", ie 64K.

>Can I improve it?

The favourite, and easiest, method is to pause your video whilst sending
audio - that reduces the number of video-packets you're sending and that
are competing with the audio packets.

I would also recommend closing as many video windows as possible.
Remember, they all use bandwidth on your precious 64K link. Closing a
window instructs the reflector to stop sending you that stream.

My general experience is that audio with video is a no-go on anything less
than 100Kbs (ie about 'AppleTalk' speed).

I believe that some of the newer ISDN cards allow use of both of the 64K
channels (that make up the "Basic Rate Interface"), simultaneously.
However, that requires software at your end and the other end of the ISDN
link that will support two IP routes to your machine. You'll pay twice as
much for your connection as well!

Perhaps someone on the list could confirm this?

Also, I was under the impression that the ISDN channel size in North
America was 54K and not the 64K as it is elsewhere. Is this the case only
in some areas? ..or is it simply misinformation?

Cheers,

Iain.

____________________________________________________________________________
Iain Campbell email: iain@dcs.gla.ac.uk
Computing Science talk: iain@mull.dcs.gla.ac.uk
University of Glasgow tel: +44 141 339 8855 ext.6292
Glasgow G12 8QQ fax: +44 141 330 4913
SCOTLAND web: http://www.dcs.gla.ac.uk/~iain
____________________________________________________________________________