Posted by John Navas on 01/21/08 15:06

On Sun, 20 Jan 2008 10:10:31 -0600, "Peter D" <please@.sk> wrote in
<13p6sjndnmlji49@corp.supernews.com>:

>USB 2.0 (aka USB Hi-Speed) is technically faster (480Mbps) than Firewire 400
>(400Mbps).

That's raw speed, which is relatively meaningless.

>In real life USB maximum transfers peak at about 2/3 of that speed. If you
>research actual test results, you'll often see speeds max out at 1/3 of the
>max of 480 Mbps.

That actually varies widely, a function of the USB transfer type and
signaling rate, in addition to source and target device characteristics.

>Becasue USB creates a network where every device "chats'
>with the central "host" (the computer in most cases) USB 2.0 requires more
>CPU prcesses than Firewire and the more peripherals that are connected and
>in use the slower the USB network.

The "host" is both the USB host controller and the host computer.
<http://www.usb.org/developers/usbfaq/>

USB's actual throughput is a function of many variables. Typically,
the most important ones are the target device's ability to source or
sink data, the bandwidth consumption of other devices on the bus, and
the efficiency of the host's USB software stack. In some cases, PCI
latencies and processor loading can also be critical.

When more devices are active on a given USB bus, total bus throughput
tends to go up. The problem is that latency for any given device tends
to go up as well.

>So avoid situations where you are
>transferring from a USB device to a USB device (USB scanner to USB external
>HD, USB camcorder to USB HD). Always transfer from a single USB device to a
>non-USB device if possible.

Most computers have at least two USB ports, and the most important thing
is to put slow devices on one port, and fast devices on another port.

Since USB doesn't support device to device transfers, transferring data
between two devices consumes double the amount of bus bandwidth in
addition to host overhead; i.e., device 1 to host, and host to device 2,
although the impact is greatly reduced if the two devices are on
different USB ports. This usually isn't an issue with digital video.

>And don't forget that if you have a USB keyboard
>and/or mouse connected you don't have a single device on the USB bus. Evbery
>time you use the mouse or keyboard, you slow the network.

More accurately, you increase latency for other devices on the same bus
by utilizing the bus.

>Firewire comes in two flavours. The original Firewire 400/IEEE1394(a) (100,
>200, or 400 Mbps) and new Firewire 800/IEEE1394b (800Mbps). There's also a
>'new' 3200 Mbps standard on the way. Actual speed is much closer to
>technical speed, and faster and more reliable than USB. Why? Because of the
>design. As well as significant design improvements that enhance and improve
>efficiency through hardware implementation and control, Firewire allows each
>device to control the network and each device can "speak" directly to
>another without the need for a central "host". This significantly reduces
>CPU load and increases transfer rates. Real life transfer rates on Firewire
>are typically 90% of the max technical speed. Even poorly
>configured/implemented Firewire can run at 80% of max speed.

The bigger issue for video transfer is reduced latency. There's more
than enough bandwidth with either USB or Firewire.

>Why Firewire is better than USB for Video:
>As well as the significant real life speed improvement of Firewire over USB,
>Firewire is also much better at sustained throughput, reducing (in fact in
>most cases eliminating) dropped frames commonly seen in USB transfers.

That's because of bus latency.

>A real test you can try:
>Without doing anything else on the computer, transfer 5 minutes of video
>using USB 2.0 and then Firewire and count the dropped frames. Now do it
>again while using the computer (surf the net, type a letter, typical use
>stuff). Now compare the droppped frames. I think you'll settle on Firewire.
>
>Some sources:
>http://en.wikipedia.org/wiki/USB
>http://en.wikipedia.org/wiki/FireWire

--
Best regards,
John Navas
Panasonic DMC-FZ8 (and several others)

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