Adaptive and asynchronous USB

Vincent Kars

WBF Technical Expert: Computer Audio
Jul 1, 2010
USB audio is send over the USB tot the DAC
As USB runs at a fixed rate e,g, Full Speed runs at 12 MHz, the amount of data is controlled to generate a quasi real time stream.

There a two methods to synchronise the DAC and the PC.

In adaptive mode the sample rate is guessed from the rate the data is pouring in.
The DAC has to adapt its speed to the sender to avoid buffer over/under run.

In asynchronous mode the DAC tells the PC how many data to send.
This allows for using a fixed clock at the DAC.
By design asynchronous USB allows for a implementation with a very low intrinsic jitter.

Jim Lesurf measured the DacMagic feeding it over the USB using its own adaptive interface and using the Halide Bridge, a asynchronous interface.

More can be found here

Bit more detail on USB audio
Jul 18, 2011
One more information I could share is, the implementation of feedback end point of asynchronous USB interface is also very important too. Nice end point implementation will command the computer to output in higher or lower speed which is adequate for the current core DAC consumes. If we add a good buffer mechanism between the isochronous down stream of USB samples and core digital-analog-convertion module, the result jitter will be very very minimum.

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