Re: CPU Overhead compared to USB

On Oct 14, 2005, at 12:07 PM, email@hidden wrote:

From: Thane Heninger <email@hidden>
Subject: CPU Overhead compared to USB

... We anticipate that we may be sending both isochronous and asynchronous data in both directions on the bus, streaming multiple, high frequency audio channels, while at the same time, running CPU intensive software on the host. Would we be better off with FireWire compared to USB? Would it make a big difference? ...

Thane Heninger
Harman Music Group

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The "peer to peer", double duplex topography, xx-synconous packet handling and the more efficient FireWire microprocessor @ 400 mbps, result in "overhead" performance almost equal to USB2 @ 480 mbps.

Consider: the FireWire 1394a (400 mbps) Bulk File Transfer Rates are actually greater than USB2.0 (480 mbps) even with the slightly "slower" microprocessor clock speeds.

In real world practical applications, direct connection (peripheral to host), performance and overhead is very comparable, USB2.0 being roughly equal to FireWire 1394a. Adding hubs and additional devices a USB2 group dramatically degrades performance compared to a FireWire group network.

Bandwidth aside, audio devices like headsets and speaker sets (2 to 4 channels) work fine on USB1 or USB2, primarily because these are usually directly connected to the host computer through a minimum of intermediate nodes (hubs). The connection of professional level, high resolution audio (6 or more channels, 24 bit/96k) requires many more node addresses than USB1 or USB2 can accommodate efficiently.

Although the address space is supposedly larger for USB1 & USB2 (127 nodes + host), I have yet to see a USB1 or USB2 chain of devices actually work properly beyond the maximum 54 nodes (as performed at a USB PlugFest) ... In fact the practical upper limit without a significant performance degradation of USB1 or USB2 is 15 address nodes. The practical upper limit without significant performance degradation of FireWire 1394a is 31 address nodes.

The popular audio device manufacturers, M-Audio, Roland Edirol, Tascam, etc., have leaned toward FireWire 1394a primarily because of the better bulk file transfer rates, peer to peer and double duplex topography ... and that practical address space requirement ... plus the cost of manufacture and initial costs of design and engineering. FireWire being effectively simpler and easier to work with while maintaining equal or superior performance ...

" ... We anticipate that we may be sending both isochronous and asynchronous data in both directions on the bus, streaming multiple, high frequency audio channels, while at the same time, running CPU intensive software on the host. ..."

Ed Karns

FireWireStuff.com