Re: USB audio devices

[Patrick Shirkey <pshirkey@boosthardware.com>]

Karsten Wiese wrote:
> We can also vary the exact USB frame time.
> With UHCI 1.1 USB Hosts there is the SOF Register. 
> It is setable from 0 to 255, the default being 127.
> Using this SOF-Register, we can set the actual USB Frame Rate from 
> ((12000 - 127) / 12000)ms to ((12000 + 128) / 12000)ms. That is about -/+ 1%:
> More than enough to adjust the USB-Frame rate to let us get 44(44100) or 48
> (48000) Sample Frames everyy USB Frame.
> It really works here already with the us428: The trick is:
> We first make the USB-Frame longer until we capture 1 Sample Frame more  45 
> (for 44100). then the USB-Frame is shortened until we capture only 43 Sample 
> Frames for one USB Frame. and so on. 
> If we use a period size of multiples of 44 (for 44100) then :
> - Latency is almost at its best (only 1/44 less than optimal.
> - User Prog Scheduling Jitter is minimized.
> - Alsas keeps constant Period size.
> 

Hallaluhjah

Please please please make this standard for the usb audio driver.



-- 
Patrick Shirkey - Boost Hardware Ltd.
Http://www.boosthardware.com
Http://www.djcj.org/LAU/guide/ - The Linux Audio Users guide
Http://www.djcj.org/gigs/ - Gigs guide Korea
========================================


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