On Thu, May 14, 2009 at 1:58 PM, Laurent Pinchart<laurent.pinchart@xxxxxxxxx> wrote:> Hi Hal,>> first of all, thanks for your answer.>> On Thursday 14 May 2009 20:18:07 Hal Murray wrote:>> > I need an API to transfer audio data from userspace to kernelspace. I>> > initially thought about ALSA, but it turns out some assumptions made>> > by ALSA are not fulfilled by my system. One of the most serious>> > problems is that the UAC gadget driver doesn't have any audio clock.>> > The only hardware clock available is the USB device controller>> > interrupts generated at the USB transfer rate, and those are much>> > faster than the audio sample rate. This will cause buffer underruns>> > that I need to handle.>>>> You don't need a clock. The data will come to you at the right rate. All>> you need to do is pass it on when you have enough to fill up a buffer. The>> buffer size is fixed. It's part of the spec for the device you are>> emulating.>> I'm not emulating any device. The buffer size is up to me, and I actually have> a fixed number of small buffers, but that shouldn't make a difference.>Hi Laurent, The problems might be clearer to you if you do the output side first.You have to define one or more alt/interfaces with isoc OUT sizes tomatch the sample rate. Some rates are straight forward and don't caremuch about the buffer/packet sizes like 45,000 HZ. However the 44100hz rate (and 22050 and 11025) made popular by CDs is interestingbecause it doesn't divide by 1000, the FS frame rate. So a driver mustoutput 9 packets of 44 samples (times the number of bytes in samplewidth times stereo/mono) and 1 of 45 samples. Typically USB chipsdetermine the sample rate by the interface and the number of bytescoming per frame. This is a challenging project, compounded by the audio usb class spec.In my opinion audio is the worst, hardest to understand spec. Secondis the HID-PID spec. Regards, Steve_______________________________________________Alsa-devel mailing listAlsa-devel@xxxxxxxxxxxxxxxxxxxx://mailman.alsa-project.org/mailman/listinfo/alsa-devel
