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Posted by John Navas on 01/21/08 15:10
On Sat, 19 Jan 2008 19:08:25 +0000 (UTC), Ilya Zakharevich
<nospam-abuse@ilyaz.org> wrote in <fmthr9$1k8i$1@agate.berkeley.edu>:
>[A complimentary Cc of this posting was sent to
>John Navas
><spamfilter1@navasgroup.com>], who wrote in article <cj44p3t4f03goqbh2taphug458m1ck0of4@4ax.com>:
>> The primary issue, as I've noted previously, is not speed, but that
>> Firewire is designed for continuous independent bus transfers, whereas
>> USB 2.0 is not, with all USB transfers controlled by the host by means
>> of polling. That can result in small USB transfer pauses when the host
>> gets busy. (Ever notice how a USB mouse pointer will sometimes move
>> erratically?)
>
>No. I do not see how the effect you describe can appear; device
>drivers should not be affected by the "system being busy"; an
>interrupt is an interrupt is an interrupt. I may be missing more
>technical details...
The USB bus isn't interrupt-driven -- it's polled (as I wrote), which is
why latency can be an issue. When the host is busy servicing device 1,
device 2 has to wait until the host is free (latency), which can be a
problem when streaming at high speed, as in the case of video. The part
you snipped on polling:
This is no problem with, say, a disk drive, or even a DVD burner
(given underrun protection), but when digital video is being streamed
there's often no good way to pause the stream, so when the host gets
busy, data can be lost. USB 3.0 is supposed to address this issue,
but how well it will work in practice is an open question.
--
Best regards,
John Navas
Panasonic DMC-FZ8 (and several others)
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