Hi folks, I'm wondering whether anyone really knows what is going on when you connect the iphone to an "iphone combatible" stereo receiver or DAC via a USB cable? Does the iphone send the original bitstream of whatever music is being played? Does it decompress and send a PCM signal? Does it transcode to lossless AAC and send a new bitstream? Does it do something really crazy and apply its own DAC to generate a crappy analog signal, then reencode it digitally?
I ask because when I recently bought equipment for an aftermarket car system, I did a lot of research looking for the perfect headunit that, among other things, has an iphone-compatible USB input and a good quality DAC. Then, after spending ~3000 on the whole kit and caboodle with install, it occurred to me that the best DAC in the world might not make a lick of difference because Apple might bork the signal coming out of the iphone.
I understand that Airplay works by transcoding a signal to 16/44.1 lossless AAC, which won't do anything to improve sound quality if the music was compressed lossy to begin with, but generally doesn't hurt sound quality (unless you're playing hi-res music, in which case the extra bit depth will be truncated and extra samples will be lost). Does the iphone's USB out work the same way? My real concern is that Apple did something nonsensical like applying the iphone's own DAC to all music leaving the iphone by wire, and re-ADCing that analog signal for the USB-out. Please someone tell me that this isn't so, and that my new car stereo will be playing the original bitstream or some equivalent transcoding of it, unborked.