Steinberg's Midex 3 is a one-in, three-out USB MIDI interface employing their Linear Time Base technology, first used in the larger Midex 8. In conjunction with a suitable application, which currently means Cubase v5.0 r3 or above or Nuendo v1.52 or above, this is claimed to offer tighter MIDI timing by buffering the data within the interface and clocking it out very accurately. I tested the Midex 3 with Nuendo v1.52, running under Mac OS 9.04 on a 300MHz Apple Mac G3 with third-party USB card.
I experienced no problems installing the drivers. OMS recognised the Midex 3 straight away and happily shuttled MIDI data in and out, both in an LTB-supporting application (Nuendo) and an old, pre-LTB version of Cubase. Unlike some, I've never experienced serious MIDI timing problems with my old serial-port Altech Midiface, so I was keen to see what improvements LTB could offer. To test it, I ran the same sidestick part in turn through the Midiface and the Midex 3, then compared my K5000W's recorded audio output.
With no other MIDI parts running, there was a marginal improvement: notes triggered by the Midex 3 exhibited impressive timing variation of less than 1mS, while occasional Midiface-triggered notes were out by 3mS or so. On this evidence, I was expecting to notice a greater improvement when I loaded down Nuendo with additional audio and VST Instrument MIDI parts, the latter containing huge amounts of controller information. Surprisingly, however, the Midiface coped very well, and I had to burden Nuendo very heavily to make a noticeable difference: but in extremes, the Midex 3 did remain stable when the Midiface gave up.
In general, I suspect that LTB will be more important in large MIDI rigs than the small setups the Midex 3 is designed for. Whether or not your system suffers from timing instability, though, the Midex 3 is a very capable MIDI interface and offers good value for money in the UK.