Re: [codec] #16: Multicast?

Brian Rosen wrote:
> Engineers who believe delay is a "less is better" quantity need to be
> educated that it is not.  It is a threshold.

Considering that the network delay is not a constant, you no longer have an 
absolute cliff. So reducing the delay means you can increase the distance 
without falling off the cliff.

	Jean-Marc

> On 5/11/10 12:30 PM, "Ben Schwartz" <bmschwar@fas.harvard.edu> wrote:
> 
>> On Tue, 2010-05-11 at 16:38 +0200, Christian Hoene wrote:
>>>  Until 400ms quality is still acceptable (about toll quality). The
>>> ITU-T G.107 quality model reflects this opinion.
>>>
>>> However, in the recent years, new results have shown that the impact
>>> of delay on conversation quality is NOT as strong as assumed.
>> I have been unable to locate the paper that is the source of these
>> claims.  However, I will note that
>>
>> (1) The results conflict with common sense.  A round-trip delay of 800
>> ms makes normal conversation extremely irritating in practice.  I'm not
>> surprised these results don't show up in laboratory tests, because fast
>> conversations with interjections and rapid responses typically require a
>> social context not available in a lab test.
>>
>> It's possible that the ITU regards "extremely irritating" as
>> "acceptable", since effective conversation is still possible.  In that
>> case, I would say that the working group intends to enable applications
>> with much better than "acceptable" quality.
>>
>> (2) Tests may have been done in G.711 narrowband, which introduces its
>> own intelligibility problems and reduces quality expectation.  Higher
>> fidelity makes latency more apparent.  Similarly, the equipment used may
>> have introduced quality impairments that make the delay merely one
>> problem among many.
>>
>> (3) I presume the tests were done with careful equipment setup to avoid
>> echo.  The perceived quality impact of echo at 200 ms one-way delay is
>> enormous, as shown in
>>
>> http://downloads.hindawi.com/journals/asp/2008/185248.pdf
>>
>> Using an echo-canceller impairs quality significantly.  Imperfect echo
>> cancellation leaves some residual artifact, which is also irritating at
>> long delays.
>>
>> The tests (even in the paper above) were performed using a telephone
>> handset and earpiece.  High-quality telephony with a freestanding
>> speaker instead of an earpiece demands especially low delay due to the
>> difficulties with echo cancellation.
>>
>> --Ben
>>
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> 
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