USB
Synchronization modes in Isochronous transfers:
In all cases the data from the bus goes into a buffer and gets clocked out by a clock, how that clock is generated and how it interacts with the bus is the differences between the modes.
Synchronous: in this mode the readout clock is directly derrived from the 1KHz frame rate. There is a PLL that takes in the start of frame signal and generates a clock. Using this scheme its rather difficult to generate 44.1, but very easy to generate 48KHz. This is a primary reason why many early USB audio devices only supports 48KHz, they used this mode. As you can guess this mode is very susceptible to jitter on the bus, pretty much anything that causes the output from the host to be jittered (PS noise, vibrations, interference etc) AND things that can cause jitter on the interconnect (interference, reflections, ground noise etc) will wind up with jitter on the readout clock. This is a VERY poor mode to use for decent quality audio.
Adaptive: in this mode the clock comes from a separate clock generator (usually implemented as a PLL referenced by a crystal oscillator) that can have its frequency adjusted in small increments over a wide range. A control circuit (either hardware or firmware running on an embedded processor) measures the average rate of the DATA coming over the bus and adjusts the clock to match that. Since the clock is not directly derived from a bus signal it is far less sensitive to bus jitter than synchronous mode, but what is going on on the bus still can effect it. Its still generated by a PLL that takes its control from the circuits that see the jitter on the bus. Its a lot better than synchronous mode, but still not perfect by a long shot. This is the mode that MOST USB audio devices use today.
Asynchronous: in this mode an external clock is used to clock the data out of the buffer and a feedback stream is setup to tell the host how fast to send the data. A control circuit monitors the status of the buffer and tells the host to speed up if the buffer is getting too empty or slow down if its getting too full. Note this is still isochronous, the host is continuously sending samples, there is no "per packet handshake" going on. Since the readout clock is not dependent on anything going on with the bus, it can be fed directly from a low jitter oscillator, no PLL need apply. This mode can be made to be VERY insensitive to bus jitter.
Courtesy: http://www.audioasylum.com/forums/pcaudio/messages/7719.html
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