Subject: Re: MCI to offer video services over the Internet

From: [email-masked] (Geoff Huston)
Date: Wed, 20 Mar 1996 19:47:51 +1000

For those folk such as myself who only got off satellite links about 3 years ago for
international telephony, 0.5 of a second (0.65 to be precise) is a huge problem -
not only do you have a very significant echo cancellation problem, which current
equipment still has not completely solved, the delay of 0.5 second makes
any useful interactive conversation close to impossible for us carbon-based
lifeforms in the antipodes. I recall a study which placed a threshold of some
0.15 - 0.2 second as the point at which people had to adjust speech to accomodate
for the delay, but I've completely forgotten the reference.

Thanks,

   geoff

At 4:29 AM 20/3/96, [email-masked] wrote:
>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...
>-- 
>				Valdis Kletnieks
>				Computer Systems Engineer
>				Virginia Tech

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From: Richard Chycoski <[email-masked]>
Date: Wed, 20 Mar 96 08:09:33 -0800

> From: [email-masked] (Geoff Huston)
> ...
> I recall a study which placed a threshold of some 0.15 - 0.2 second
> as the point at which people had to adjust speech to accomodate

The threshold of detection is under 100 milliseconds. This has dictated some  
of the parameters for ISDN and ATM. The 500 millisecond delay limit is more  
the threshold of "tolerance", since more than 500 milliseconds of echo of  
one's voice tends to confuse the speaker.

Neither of these is necessarily important in the delivery of "phone service"  
across the Internet. Intelligent muting or the use of push-to-talk can reduce  
or eliminate this problem, especially when the communications is  
computer-to-computer (rather than when interconnecting into the public  
telephone network).  How many people find the delay of "vat" to be a real  
problem when talking across the MBone?

Also, the availability of cheap long distance can significantly increase the  
"tolerance" of most ears... (:-)

- Richard Chycoski
  Senior Systems Consultant
  Academic Computing Services
  Simon Fraser University
  [email-masked]

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From: Ian Duncan <[email-masked]>
Date: Wed, 20 Mar 1996 17:13:27 -0500 (EST)

> one's voice tends to confuse the speaker.

As I remember it the base perception threshold is 50ms, the "echo
confusion" tolerance limit is 200ms and the threshold of perception of
conversational delay even given adequate echo cancelling is 500ms. 

> across the Internet. 

All three are interesting and important constraints on how one builds
voice over IP protocols applications. 

> How many people find the delay of "vat" to be a real  

I have, under certain conditions. Try doing a simple three (or more) way 
meeting over 150-250ms delay connections, particularly when one or more 
of the participants isn't habituated to the technological limitations.

/id