USB interfaces; a general note

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    Ted Fletcher

    I have been using various USB interfaces since the days they were invented and recently I needed to research what’s ‘state of the art’ and what’s available to link monitors, microphones, record and playback. I was amazed at the variety of little boxes that are suddenly available, and then on closer inspection, I noticed how similar they all are and how the flowery descriptions don’t say very much except to claim some level of superiority.
    Clearly, all the ones I have seen are made in China and all seem to have identical functions with some slightly dodgy claims.
    I decided to try some critical tests on a few of them and the results are interesting.
    For the past couple of years I have been using an inexpensive ‘SubZero SZ-A12’ interface which seemed to work well and has excellent performance.
    It is capable of 24 bit 96KHz operation and it has excellent power supply noise suppression. The audio performance is good enough for most of my research work; response within 0.1dB across the selected range, harmonic distortion mostly down in the noise which is well down near the -100dB mark.
    Some of the new ones available are claiming 24 bit 192KHz so I bought a couple and hit them with the same tests……. Oh dear!
    Amplitude frequency response is generally OK, well it should be. The newer interface chips do have marginally improved noise levels, however the harmonic distortions are far from satisfactory, there is significant 3rd order distortion (around 0.1% at operating level) and it is much higher than the less obtrusive 2nd order distortion. But the worst performance is with crosstalk, L/R crostalk at 1KHz was very bad at about 40dB down. Checking frequencies up towards the upper limit of 20KHz the crosstalk gets to just a few dB! I can only assume that the layout engineers learn about digits but not about audio!
    I shall stick to my SubZero SZ-A12 unit; looking at some newer equipment proves the old saying ‘improvement means deterioration’!

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