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USB audio standards

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Posted on May 9, 2011 at 04:30:14
Roseval
Audiophile

Posts: 1802
Joined: March 31, 2008
I always thought that USB audio class 1 is limited to 24/96 and USB audio class 2 to 24/192.
After trying to decipher
http://www.usb.org/developers/devclass_docs/audio10.pdf
http://www.usb.org/developers/devclass_docs/Audio2.0_final.zip
it looks like this is not the case.
There is no specific limit in class 1 or 2 as far as sample rate is concerned.

Class 1 is tied to Full Speed = 12 MHz = 12 Mbit/sec raw
Given an empirical value of 40% net throughput = 4.8
24 bit
96000 sample rate
2 channel
4608000 bit/s
4.4 Mbit/s

Indeed 24 / 96 is what just fits in the Full Speed bus

Class 2 is tied to High Speed = 480 Mbit/sec raw
Given an empirical value of 40% net throughput = 192 Mbit/s

A 2 channel 24/384 (17.5 Mbit/sec) fits in without a problem.
A 24 / 96000 60 channel recording (132) likewise.

Another limitation might be the OS.
According to Antelope Audio both OSX and Linux allows for 384kHz (Antelope Zodiac Gold) using native mode driver. Don't know id this is the upper limit.
Win7 is limited to 96

It looks to me that there is no limit in resolution in the USB audio standard but in practice the bus (full speed) and the availability of drivers put constrains on the resolution.


The Well Tempered Computer

 

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RE: USB audio standards, posted on May 9, 2011 at 08:18:19
Charles Hansen
Manufacturer

Posts: 6984
Joined: August 1, 2001
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!

 

RE: USB audio standards, posted on May 9, 2011 at 13:04:52
Roseval
Audiophile

Posts: 1802
Joined: March 31, 2008
Thanks

Vincent
The Well Tempered Computer

 

RE: USB audio standards, posted on May 9, 2011 at 20:52:22
Charles Hansen
Manufacturer

Posts: 6984
Joined: August 1, 2001
And I forgot to say that what I described is the "isochronous" mode. This mode is designed for audio use. There is a minimum fixed bandwidth reserved for signal transmission, so that you don't get problems with dropouts because the bus was too busy. There is also a disadvantage in that there is no mechanism to check that the packet arrived without corruption and therefore also no mechanism to re-send bad packets. But these are not problems in the real world.

Asynchronous and adaptive modes are both sub-categories under the isochronous mode. So is synchronous, but that is generally only used for A/D converters when the data is being sent to the computer.

 

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