Bmac-
condescension is always the best give away that a nerve has been struck.
Synchronous and Adaptive do not refer to the same thing. Are you confusing the terms Centrance currently sells an adaptive kit. When they started out they sold a synchronous kit. See the below definitions from the Well Tempered computer website.
what makes you think he would have any interest or desire to pay a competitor to license their technology to use in his own product, thereby undermining his own product? Does that idea make any sense to you at all?
It is used in those "world class" dacs because they have to purchase some interface because they can't write the codes required themselves. Since nobody is selling an OEM asynchronous kit that leaves a lot of business for Centrance. "The best thing is the thing you have until something better comes along."
From the Well Tempered computer
Isochronous transfer can be done with three possible types of synchronization in the USB audio device.
Synchronous
The clock is directly derived from the 1 kHz 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 48 kHz. This is a primary reason why many early USB audio devices only supports 48 kHz, they used this mode.
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 the bus still can affect 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 dependant 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
Take it easy.
Bianci
