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.
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