Asynchronous vs Synchronous

What's the diff between an asynchronous and synchronous DACS? Appears that asynchronous ones are "better". Can someone explain the difference in really basic terms? Talk down to me, please! I have a Bryston BPA-1/BDA-1 combo. I'm guessing that my Bryston is not asynchronous.
This describes it well.
Outstanding! Thanks.
I own the BDA-1. It’s USB input is not asynchronous and included for convenience. However it remains a very good DAC and has plenty of SPDIF inputs. I like to use an outboard USB converter for computer audio.
I salut to Mapman too for this link.
Thanks pal.
Mesch - does the Bryston BDA-1 support DSD (whatever that is)?
In synchronous USB D/A converter clock comes from the
computer. Usually D/A converter runs from internal clock that is synchronized with incoming USB clock by PLL (Phase Lock Loop) but it is less than perfect. This scheme is used in pretty much every CDP, except here incoming USB clock can be very jittery.

Asynchronous USB uses fixed internal DAC's clock for the D/A converter. This clock is independent (asynchronous) to USB data rate. For that reason data has to be buffered. DAC has to request different number of samples each frame to keep data buffer between underflow and overflow.

Because in asynchronous USB DAC D/A converter is fed from the data stored in a buffer it operates from stable internal clock reducing jitter.
Rockyboy, The BDA-1 does not support Direct Stream Digital.
Bryston makes an external usb to spdif converter the BUC-1 that can be used with the BDA-1. I own the bdp-1/bda-1 combo too. I haver used the M2tech Hiface in the past with good results.
From all the responses, I'm concluding that I don't need to worry about whether my DAC is asynchronous or synchronous, since a PC is not involved in my system. FWIW, I'm linking my BDP & BDA with an AES/EBU cable.