The USB chip and driver set they use was bought in to save needing USB development expertise at a time when it was probably new to them.
I beleive its the same chipset and core driver code (both on the pc/mac end and on the DSP end) thats also used by novation and various others.
Also the USB 1.1 specification includes support for this type of device, whereas at the time, the USB 2 specification did not, so while USB2 is higher bandwidth, it was actually potentially less reliable than the slower USB 1.1 spec for streaming a small number of audio channels.
In reality, they should have gone for firewire on a purely technical level, however if you dont have in house firewire expertise, then that would have been far more risky as at the time I dont think there was any good off-the-shelf easy integration firewire chip+driver offerreing around. Im not even sure that there is even now.
Smaller companies are better off buying off the shelf support for things like this as developing reliable drivers at each end (pc/mac and the dsp) takes alot of time, even for a standard class device which this is at a basic level (if you hadnt noticed - Vista recognises the audio functionality of the Virus TI automatically, so no driver needed to use it as a standard sound card).
As for the latency, well TBH Ive never quite understood why it is so high, but then Ive not actualy written a USB audio device driver so have no idea what buffer sizes it is forced into using etc. For the latency to be that high then I'm guessing it must send/receive quite large audio data blocks over the wire along with alot of buffering in case your pc decide to ignore USB for a bit (as often happens with PCs and Macs).
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