USB Adapt or be on asyncing ship or a dead DAC

[RayDiant]

I have done a bit of reading about USB audio and connecting to DACs. I understand there are 3 methods:

1. Synchronous - the master clock is the upstream device (usually a PC). Generally not the best option
2. Adaptive - the downstream device (usually the DAC) averages the speed of the receiving signal to better guess the clock speed. An improvement over Synchronous but still not ideal
3. Asynchronous - the DAC clock is the master, telling the PC when to send.

My question is why do we need all this clocking at all? Why doesn't a USB connection to my DAC work like a buffered data stream:

1. I send the header info (bit depth and sample rate are the most important) FLAC file to my DAC via USB
2. My DAC goes - "Ok I'm going to process a 24bit 96kHz FLAC.
   
3. The DAC tells the USB interface to start filling its buffer
4. As the buffer nears being full the DAC tells the USB to hold on a minute
5. The DAC, using its internal clock and processing, starts to unravel the FLAC file into audio
6. As the buffer starts to empty repeat steps 2 ot 5. The trick is to ensure that the buffer is kept full enough so the processing in step 5 can be continuous.

The only 'clock' constraint on the PC is that it can respond and deliver the file fast enough to keep the buffer full enough. In my mind this pushes all the clock timing issues into the DAC where you want it. This is also what has to happen if you have a network streamer connected via ethernet.

Mark