On Sun, Oct 24, 2010 at 1:17 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > The lsusb output for your sound card shows that it doesn't distinguish > between 44.1 KHz and 48 KHz when listing its bandwidth requirements; it > asks for the same amount of bandwidth in either case. ÂIn particular, > it requires 192 bytes/frame for 16-bit transfers and 296 bytes/frame > for 24-bit transfers (a frame is 1 ms). ÂDon't ask me why the second > number is 8 bytes larger than it should be -- I don't know. > > The information in section 5.11.3 of the USB-2.0 specification shows > how to compute the amount of USB bus time based on the size of a > transfer. ÂFor 192 bytes isochronous OUT this comes to a little over > 153 us, and for 296 bytes iso OUT it comes to a little over 237 us. > IN transfers require about 1 us more. ÂYou can get reasonably close to > these numbers just by realizing that full-speed USB runs at 12 Mb/s or > 2/3 us per byte and that bit-stuffing requires all the times to be > increased by a factor of 7/6 -- and keep in mind that the protocol has > some overhead. > Hi Alan, There is something wrong with your calculations. Every ms FS sends about 12000 bits = 1500 bytes. 90% of this is 1350 bytes. I know this is approximate, but I have seen fs cameras using the maximum iso xfer size of 1023, so at least that is possible. To transfer 6 channels of 3 bytes per sample and 48Khz requires 6 * 3 * 48 bytes = 864 iso bytes per frame. My conclusion is this kind of transfer is legal per the USB spec. There may be iso bandwidth bugs in Linux, though - especially through HS hubs, I suggest a test directly connected to his UHCI or OHCI root hub. Regards, Steve -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
