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Tiny triangle waves?

Discussion in 'Recording' started by RandomGuest, Apr 19, 2002.

  1. RandomGuest

    RandomGuest Guest

    Feb 10, 2001
    Maybe Greg can help me here:

    I was asked by a colleague to help him xfer an older existing ProTools session into his Radar system, so he could mix it for a client (he doesn't own ProTools).

    The session was only 14 tracks (jazz), so we connected the AES/ebu outputs of two 888's to go digitally directly into the radar. Everything was clocked with my Lucid Genx6.

    To our consternation, we were getting loud digital pops, seemingly at random. Sounded a lot like a clocking problem, but further inspection showed they were occuring on the existing specific tracks at specific times. Nothing appeared to be out of the ordinary looking at the wave-forms in the edit window, but I increased the horizontal resolution click by click on the original tracks, until "magically" a perfect rectangular shape appeared. Two audible clicks would occur in this region: when the cursor first hit the front border of the "box", and then again as it passed by the back border. Increasing horizontal resolution to sample level showed that the box (which might be a few seconds in total duration) was actually a series of perfectly shaped triangle waves spaced extremely close together! The waves were inaudible, except at the onset and outset of the region. This seems to imply they are at a frequency beyond human hearing.

    Additional note: the triangle waves had either a completely positive or negative "polarity" (if that's the correct word?). That is, in any particular region, the waveforms extended from the center baseline only up or down, not both.

    My question is, what would produce such a phenomenon? I didn't think DC offset would look like that, but could it be something related? Each time the box-like shape of triangle waves occurs, it is apparently at random and unrelated to the musical activity of the track. It tended to occur mostly on the same three or four tracks (Left piano mic, rack tom mic, room mic). If it had been on just one particular track, I might guessed it might be a manifestation of a short circuit somewhere, but on multiple tracks that seems less likely.

    Fortunately, they are only two or three per song, often on non-critical tracks, so the music may be salvageable with some judicious editing. But I'm still wondering if anyone had ever seen anything like this before. I know I haven't.
  2. Sorry Littledog,

    I've never seen this before either. My first reaction would have been as yours, to look at the clocking. Beyond that you've got me stumped!

  3. RandomGuest

    RandomGuest Guest

    Feb 10, 2001
    hi Greg,

    Glad you're still here! Just a follow up: I did post the question on DUC and a couple of people had experienced the exact same phenomenon. The consensus seems to be that a hard drive problem cause this. Something interfering in the read/write process.
  4. sdevino

    sdevino Active Member

    Mar 31, 2002
    Its also possible this was the result of some type of oscilation on the interface. there are digital line drivers and receivers which are just specialized amplifiers in these circuits. Just the wrong combo of cable load, bad solder joint, too warm room temp etc could have setup an occaisional oscillation.

    Was the problem repeatable?
  5. RandomGuest

    RandomGuest Guest

    Feb 10, 2001
    The original tracks were out of state, and although I tried to get access to them, there was evidently some sort of conflict going on between the artist and the original studio, so i wasn't successful. So I don't know about repeatablility.

    All I know is, in all the years of doing Pro Tools here, I've never seen that in any of my tracks. Fortunately!

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