PORTS Forum

Ports and Interfaces => USB => Topic started by: sjalloq on April 14, 2011, 06:24:49 pm

Title: USB Audio Class spec clarification
Post by: sjalloq on April 14, 2011, 06:24:49 pm

Hi all,

first time post here.  I've been using USB Complete to get me up to speed with the basics and my first attempt at a USB device will be to implement an Audio Class device.

I'm currently stuck trying to understand a section of the Audio Device Class 1.0 spec.  I'm looking at Release 1.0, March 18, 1998, section 5.2.2.4.1.  The second to last paragraph on that page talks about a special case for the Set Feature Unit Control Request when the Channel Number is set to 0xFF.  It references a 'bmaProps() array' that is not mentioned anywhere in the document.  Neither is it mentioned in the Formats, Terminals or overall USB 2.0 specifications.  I'm assuming it is a typo but I haven't been able to decipher what it should be.

Anyone have any thoughts?

Thanks, Shareef.

Title: Re: USB Audio Class spec clarification
Post by: Jan Axelson on April 14, 2011, 10:00:48 pm

I think you're right - it's a typo and should be bmaControls.

Jan

Title: Re: USB Audio Class spec clarification
Post by: sjalloq on April 15, 2011, 05:49:36 am

Hi Jan,

thanks for that.  My first thought was that it must be bmaControls but I wasn't sure that made much sense in the context it was written.  Having re-read it a few times I think it's now clear.

My understanding is that the size of the parameter block must match the number of channels that have the relevant CS bit set in their bmaControl entry.  I originally assumed that every channel would have the same controls and hence your parameter block would always be num_chans in length.

Cheers.