[App_rpt] USB hubs & USB audio

[David McGough]

So, I guess the next question is: Does anyone have a favorite multi-tt
hub? ....I have used this one in the past for other linux applications and
it seemed okay...I'll try one with the URI's....I guess the biggest issue 
here is that the hub you buy today won't necessarily be the one on the 
shelves tomorrow....A bit frustrating!

http://www.usbgear.com/USBG-217C.html

-David



On Tue, 22 Jul 2008, David Kramer wrote:

> 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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