I've been doing some online research on digital audio.
It all started when I purchased the device pictured below, a Turtle Beach Audio Advantage Micro, at a CompUSA store for $30.
What interests me about this product is I have a set of Logitech Z-5500 digital, 5.1, speakers, which have an unused optical input. The Audio Advantage has an optical output. So I can plug the AAM into my Powerbook 12" and get beautifully clean sound avoiding the auditory ugliness of a ⅛" mini-jack. And it works! iTunes sounds clearer than ever.
But there is something else I would like.
As of Mac OS X 10.3, the DVD Player application has an interesting option. If you have a S/P DIF output (which for all intents and purposes, the TOS-Link on the AAM is), and the driver for that output supports what is called bypass mode, the DVD Player can be set to not decode the AC-3 soundtrack on your DVD, it can be lazy and not convert the beautiful 5.1 soundtrack into old school stereo. Instead, the raw AC-3 file will get spit out the S/P DIF and into your stereo receiver, and if your receiver can handle it (which the Z-5500 most certainly can), you will get surround sound of the highest quality instead of lame stereo with a Pro-Logic II effect to make it sound sort of like surround sound.
So, the DVD Player supports it, the AAM supports it, my speaker system supports it. What doesn't support it? The Apple provided driver does not know about the AAM, and therefore cannot tell the DVD Player to send AC-3 streams through. The AAM supports this bypass mode on the PC, my desktop Dell sounds great, because Turtle Beach has written a driver for Windows XP. They rely on Apple's generic audio driver for the basic stereo support we Mac users get. If someone in the know were to write a Mac driver for the AAM, you could get surround sound out of a Powerbook, Mac Mini, eMac, iMac or iBook for $30+. PowerMac G5's already have an optical output.
As it happens, I have written a handful of USB drivers in my professional life, but never for OS X, and never involving audio. So I am investigating how I would go about writing a driver, and I will test out using the blog form to publish my finds here, if I abandon the effort someone might find my entries useful.
Of course, I'll get my copy of Mac OS 10.4 (Tiger) in a few days, and it's USB audio driver might support the AAM, and I'll say forget it.