Synchronizing with the actual USB device audio clock
Synchronizing with the actual USB device audio clock
- Subject: Synchronizing with the actual USB device audio clock
- From: philippe wicker <email@hidden>
- Date: Tue, 20 Dec 2005 17:05:31 +0100
Hi all,
Could anyone explain me how the HAL is synchronizing the period to
which it calls the IOProc to the actual audio clock of the USB
device? To make my question a little bit clearer, let's assume a
theoretical device with only a mono ouput audio channel at a fixed
(non adaptative) and exact 44.1 KHz sample rate. This device would
need exactly 44100 samples each second to feed its DAC. How does the
HAL proceeds to provide no more no less than this (average) amount of
samples?
If this device had also the capacity to sample analog audio at the
same exact frequency, it should be requested by the USB host to send
an exact average 44100 samples each second. I've read here and there
that a USB "in/out" device was split into 2 devices by the HAL. What
about the average actual USB audio data flow rate in both directions?
On the very short term, I'm preparing the debug of the audio part of
the hardware. The first step is to validate the audio streaming
between the host and the USB interface. The basic idea is to
implement a hardware audio loop back, ie to copy the audio received
in the OUT ISO endpoint into the IN ISO endpoint. My hardware can do
that easily providing that the average actual USB audio data flow
rate in both direction is exactly the device sample rate. I think IN
and OUT data flow rate are both locked to the device audio clock but
wants to be sure...
Thanks
Philippe
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