[App_rpt] USB hubs & USB audio

[David Kramer]

I have successfully tested URIs with several high speed USB hubs.  But these
tests were performed without any other USB devices connected at the same
time so this may not be very usefull.

Skip points out the difference between full speed (12Mbps) and high speed
(480Mbs) devices.  This terminology is confusing.  Not all USB 2.0 compliant
hubs support high speed, so be sure the package says HIGH SPEED.

Very few hubs support multiple transaction translators, probably because the
chip sets are more expensive and almost nobody understands the need.
Theoretically, one TT is all that is needed provided you don't mix USB 1.x
and 2.0 devices on the same hub.  Keep in mind that USB supports several
data modes each with varying priorities and latency allocations.  Plus USB
physical layer is half-duplex so one-way communication never comes close to
480Mbps.  The TT tries to handle all the traffic so don't expect good
results if you plug a bunch of web cams or other bandwidth hungry devices
into your hub.

Hope this help.

David Kramer
DMK Engineering Inc.
310-544-1222


-----Original Message-----
From: app_rpt-bounces at lists.illiana.net
[mailto:app_rpt-bounces at lists.illiana.net] On Behalf Of Skip WB6YMH
Sent: Tuesday, July 22, 2008 1:22 PM
To: app_rpt at lists.illiana.net
Subject: [App_rpt] USB hubs & USB audio

> Jim Dixon and I have had disapointing results with USB hubs during the
> testing of USB fobs with them. We don't know why they don't work
> properly and we don't have the time to figure it out right now. There
> may be hubs which work just fine, but we just don't know as the root
> cause of the problem has not been uncovered.  If Steve Henke W9SH
> knows something I don't them maybe he should speak up on the list. 
> Steve  WA6ZFT 

The problem is probably complex.  USB 2.0 hubs come in two flavors single-
transaction translators (single-TT) and multi-transaction translators 
(multi-TT).  Most USB audio devices are "full speed" devices (12 
mbits/sec), not "high speed" devices (480 mbits/sec) so the USB hub must 
translate the data rate between the host and the device.  The single-TT 
devices only have a single translator which is shared between all ports.  
A multi-TT hub has a translator per port.  So in theory a milti-TT hub is 
what you should want for optimal bus usage.

An additional complication is that the Linux kernel must be multi-TT hub 
aware to properly schedule things.  I'm not current with Linux 
development, but I've read discussions in the past about improving the 
kernel's USB scheduler in this area.

Here are a couple of links to info which may be useful:

http://www.commsdesign.com/design_corner/showArticle.jhtml?articleID=1650536
2

http://www.linux-usb.org/usb2.html

73's Skip WB6YMH

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