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Posted by John Navas on 01/19/08 15:17
On Sat, 19 Jan 2008 07:41:03 +0000 (UTC), Ilya Zakharevich
<nospam-abuse@ilyaz.org> wrote in <fms9if$18im$1@agate.berkeley.edu>:
>[A complimentary Cc of this posting was sent to
>Dave Martindale
><davem@cs.ubc.ca>], who wrote in article <fms15f$20c$1@swain.cs.ubc.ca>:
>> >[AFAIK, maximal practically measured transfer rate of USB2 is
>> > 33MB/sec. This (was claimed to) coincides with the maximal
>> > theoretical transfer rate with some particlar small size of
>> > buffer/window.]
>>
>> However, DV (and HDV) have a mean data rate of 25 Mbits/sec, or about 3
>> MBytes/sec. That's less than 10% of the measured 33 MB/sec, so
>> bandwidth should not be any more of a problem with USB2 than it is with
>> Firewire, at least on a USB2 controller that is not currently being used
>> by anything other than the video camera.
>
>Sure. You could notice that this was already mentioned in the message
>I replied to.
>
>[I still hope I can spot a person who KNOWS the reasons for abysmal
>performance of USB - comparing to Firewire...]
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?) 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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