Strange noise sources in recordings

Discussion in 'Music' started by Leo Kerr, Feb 22, 2006.

  1. Leo Kerr

    Leo Kerr Screenwriter

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    Yesterday, I was playing with the FLAC encoder/decoder to see how it worked, if it was really bit-for-bit lossless, et cetera.

    Anyway, I was looking at the resulting files in Adobe Audition / Cool Edit, and noticed a couple of things noise signals that got recorded.

    Now, I've long been familiar withe familiar evil 15,750Hz tone from NTSC clocks. Another recording I was looking at had a similar looking, 15,625Hz tone that lines up nicely with the PAL line clock.

    The sound-track disc for The Dream Is Alive however, had a very powerful tone right at 18,550Hz.

    What the was that?

    Three things came to mind:

    1. noisy frequency limiter to keep >20kHz signals from the D->A converter,
    2. a noisy synthesizer's internal clock, bleeding out, or
    3. wasn't CBS (I believe) talking about a copy protection scheme in the mid 1980s where they would either embed a tone or notch-filter the signal somewhere in the 18kHz range?

    The problem with any of these was that it ocassionally came and went, but the way it behaved, it seemed unlikely it was tied to a particular track.

    Curious, I checked a variety of other recordings. The cleanest ones were ones with the AAD SPARS codes - 1960's era recordings.

    And someone who ought to be really embarrassed would be Sony Classical / Lucasfilm. Episode III: Revenge of the Sith sound-track CD has both the NTSC clock, the PAL clock, and something at 13,310Hz.

    So. Any ideas where these other signals might have come from? I've not come up with any nice multiples or reasonable guesses as to where the 13.3kHz or the 18.55kHz tones would be coming from.

    Leo Kerr
     
  2. NickSo

    NickSo Producer

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    I don't know what the source of the noise is, but I think I often hear that tone in a few recordings.

    Sometimes its not prevalent all the way through the song, rather only when there are vocals, that might be a clue.
     
  3. Leo Kerr

    Leo Kerr Screenwriter

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    Most of these signals are pretty low down in the noise - rarely exceeding -88 - 90dBFS - the one exception was the 18550Hz tone which was rising up to, if I remember, about -75dBFS.

    And of the tracks I checked, none of them had vocals. It's mostly kind of just bizarre; not that these are affecting my enjoyment of the recordings. It's mostly just a, "hm. Where did that come from?" kind of thing.

    Leo
     
  4. ChristopherDAC

    ChristopherDAC Producer

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    That is a poser. 13.3 kHz seems to me just a fraction too low to be the line-clock of a monitor running 540p24 [1125/2*24 would be 13.5 kHz], and 18.5 is a fraction too low to be the line-clock of one running a 787.5-line raster [1.5×NTSC, used in some of the ATV proposals] at 24 fps, or those would be my choice of culprits. 720(750)p25 would be 18 750, still a smidgen too high — I have heard of this being used in off-line editing.
     
  5. Leo Kerr

    Leo Kerr Screenwriter

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    Note that The Dream Is Alive was recorded in 1985, mastered and released to CD along with The Blue Planet in 1990. I really expected to see an NTSC clock on that one, but as I recall (my notes are elsewhere as I type this) it didn't.

    Now, while they're not terribly audible (if at all - the Sith signals were a little above the noise floor,) the fact that they're there ought to be an embarrasment to someone at Lucasfilm (except Ben Burtt is now beyond embarrasment,) or Sony (although after the Sony/BMG Rootkit fiasco, they seem to be beyond embarrasment, too.) In the modern area of super high fidelity recording, they ought not have artificial noise bleeding in above the noise floor! :p)

    Leo
     

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