Thursday, April 28, 2005

Investigating the Audio Advantage Micro

If you've installed Developer tools on your Mac you will find a useful little application called USB Prober.

If you have a AAM plugged in, and you run USB Prober, you find out what the Apple driver has to work with in configuring and sending it data.
Full Speed device @ 3 (0x19110000): ............................................. Composite device: "USB Audio"
Device Descriptor
Descriptor Version Number: 0x0110
Device Class: 0 (Composite)
Device Subclass: 0
Device Protocol: 0
Device MaxPacketSize: 8
Device VendorID/ProductID: 0x0D8C/0x0001 (C-MEDIA ELECTRONICS INC.)
Device Version Number: 0x0010
Number of Configurations: 1
Manufacturer String: 1 "C-Media INC."
Product String: 2 "USB Audio"
Serial Number String: 0 (none)

It's been a while since I did driver development, but let's break this down: it's a composite device which means it can be treated as two or more types of devices. For instance, it might be a outlet for audio data, and also be a volume control. The actual manufacturer of the USB interface chip was not Turtle Beach, but instead is somebody called C-Media Electronics, who've been issued a VendorID of 0x0D8C by the maintainer of a list of unique vendor IDs. and this is their first product using this ID.

Below this we have 2 interfaces. These are alternate pathways towards controlling the device. A driver will follow a very standardized series of calls over the USB bus to set up a communication pathway, these public interfaces lets the driver what format the device expects on the other side.

The first interface is given as Interface #0 - Audio/Audio Control
and a perusal indicates it is a standard USB protocol for controlling audio playback by giving a standard way to support muting, volume, etc. (I guess)

The second interface is given by Interface #1 - Audio/Audio Streaming
and basically says that this interface will accept a stream of PCM formatted audio data in an isochronous stream. The PCM is 2 channel stereo with a sampling rate of 48Khz. Very similar to what you get off of an audio CD except an audio CD is natively 44.1kHz. It's an isochronous stream which basically means the USB bus will try hard to allocate the stream a constant bandwidth, it's better for your mouse movement to be choppy than your audio playback.

But nothing about S/P-DIF. How does the PC driver send AC-3 to the receiver if it only supports PCM? Well there must be a way.