Computer Audio Asylum

Music servers and other computer based digital audio technologies.

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Original Message

A little insight

Posted by Gordon Rankin on February 19, 2008 at 09:54:57:
Adal,

All devices on the USB link are sent packets. Each device specifies what is called MaxPacketSize for each endpoint.

In audio using ISO packeting the amount of data sent is specified by the SOF packet. You can specify this from I think 1ms to 32ms. But in audio we do as most do and set this to the maximum which is 1ms.

For 16 bits you need 4 bytes per sample (stereo) and 24 bits you need 6 bytes.

If we are doing 44.1/16 at 1ms increments we need 176.4 bytes per SOF packet. Since this cannot be done all OS's send 176 bytes 9 times and then 180 bytes ounce for a total of 1764 bytes over 10ms.

If we run 24/96 then we need 6*96= 576 bytes per 1ms SOF.

The PCM27xx series controllers have fixed buffer sizes of 2x48*4=384.

With the TAS1020/TUSB3200 you can specify the buffer size to what ever you want and control the DMA actually everything. So you can make the buffer allot larger and assure the lack of pops and clicks if there are errors in the bus.

The USB controllers only look at what their USB_ID is. They disregard all other communication seen on the link. With many computers and hubs the device will only see what it is suppose too.

Thanks
Gordon
J. Gordon Rankin