I'm not a fan of USB devices for MIDI -- and especially not when combined with audio. Since you have a G4, why not use a PCI based interface instead? With the right card you'll have much, much lower audio latency (USB
and Firewire have serious latency issues) -- and a PCI based MIDI device will not suffer from the jitter involved in USB transport.
Here's an explanation from Mix Magazine (Jan, 2001):
So what’s the problem with MIDI? According to Jim Wright at IBM Research, a longtime member of the MMA Technical Standards Board and chairman of the organization’s working group concerned with new transports, USB has timing problems that make it problematic for MIDI. He has conducted tests comparing “classic” (i.e., serial, parallel, PCI or PCMCIA) interfaces against USB interfaces, looking at their round-trip latency (the amount of time it takes for a MIDI event to get in and out of the interface) and their jitter (the variation in the latency). He found the latency in the USB interfaces to be between seven and eight milliseconds, about three times that of the classic interfaces. This is not in itself an insurmountable problem, because musicians adjust to small latencies in sound sources quite well—a bass player and a lead guitarist standing seven feet away from each other usually have no trouble staying together.
But the jitter in USB interfaces was also much higher than the older interfaces—about twice as high, meaning (to continue our analogy) that the two players could at any given moment be five feet away from each other, and the next moment be 10 feet away—and constantly moving. In another analogy, which Wright likes to use, imagine playing a slightly arpeggiated guitar chord: The jitter could make it sound as if one of your fingers jerked slightly while you were playing the chord. And for tight grooves and thick MIDI data streams with lots of aftertouch or controllers, this level of jitter is really unacceptable. Wright also found that when you add audio to the USB stream, the jitter goes up another 50%—so it’s three times what MIDI musicians have had to deal with in the past.
You can read the whole thing
here
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And with regard to Firewire:
Firewire is great in those circumstances where a PCI card isn't possible -- and it's got plenty of throughput so you can run multitrack material that wouldn't be possible with, say, a USB (1.1) interface -- but because the standard Firewire buffer size is large and takes a while to fill up, a dedicated PCI card interface will deliver much lower roundtrip latency times.
Hotshot chip designer Joe Bryan of Universal Audio recently wrote, “Most ADCs and DACs have only about 27 to 34 samples of delay in their internal digital (downsampling) filters, plus a few samples in the hardware interfaces. That means the total A/D/A delay is around 64 samples, or 1.33 ms at 48 kHz...”
However, FireWire “transfers data every 125 microseconds, but the hardware actually has significantly larger buffers, and the ‘standard’ AM824 protocol [used in mLAN and other implementations] is moving towards using several hundred samples of buffering to overcome multidevice bridging and merging issues. These delays are unacceptable in professional audio applications, so there is still a need for a low-cost, low-latency, multichannel, digital audio transport.” Bryan says he's working to form an industry group to create a better transport standard.
