Computer Audio Asylum

Music servers and other computer based digital audio technologies.

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Original Message

RE: USB audio standards

Posted by Charles Hansen on May 9, 2011 at 08:18:19:
It's not just USB. Everything has to work together.

USB 1.1 and Class 1 Audio are "kind of" the same thing because they are used hand-in-hand almost all the time. A packet is sent every millisecond. The maximum packet size is 1024 bytes.

To keep the math easy, we can divide the sample rate by 1000 to get the sample rate per millisecond. So 96/24 requires (96 bits per millisecond) x (24 bits per channel) x (2 channels) / (8 bits per byte) = 576 bytes per packet.

192/24 would require exactly twice the data rate or 1152 bytes, which exceeds the 1024 byte limit. Even 176.4/24 would require 1058.4 bytes per packet, again exceeding the 1024 byte limit.

In theory one could have reduced resolution formats such as 176.4/23 or 192/21, but there is no software or music player that will handle that resolution.

Once you go to Class 2 Audio under USB 2.0, the data rate is increased by a factor of 40x. This allows just about anything under the sun with regards to audio. In theory one could have a pro setup with 16 channels of A/D at 192/24 and 16 channels of D/A at 192/24 all on one USB line. I wouldn't want to have to do the programming for that, but there are guys that do!