[RE] Time sensitivities in ResE. Part 2: Latency
Once again, there are several reasons why latency matters in a
media-streaming network.
1) Control response ... in a normal CE system, the latency of most
importance is that between the time a consumer presses a button the remote
control and the time the appropriate response is perceived by that same
consumer. Response times of current consumer devices is on the order of 0.2
to 0.5 sec. Some devices take longer, but are disliked (controls on an
airplane entertainment system are typically pretty awful ... try them
sometime). Unfortunately, much of the response time is consumed by the CE
devices themselves, and any significant addition caused by the network would
be unacceptable ... so let's say that the network should hold control
latency to less than 100ms. Not too bad.
2) Perceptual feedback ... in system where the network is part of a musical
instrument or gaming feedback, the requirements are much more severe.
Average people start noticing problems at 8-10ms, and some professionals
much sooner. As a network latency, this is not much of a problem, but the
network isn't the only consumer of latency: in a typical system, the
keyboard or guitar uses some, the audio processor (mixer/effects) uses some,
and the amplifier uses some ... Alexei described this process in SG
presentations some time ago, and came up with the 2ms figure for maximum
network transit delay (did I get that right, Alexei?).
This last one is particularly significant, since this latency figure is
absolute, not statistical (any late data will cause glitches in the sound
since we can't do much buffering here).
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Michael D. Johas Teener ‹ mikejt@broadcom.com
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