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Thread: noise only on some notes (perhaps aliasing)

  1. #1

    Default noise only on some notes (perhaps aliasing)

    The problem:
    the audio of a DVD (a piano performance) that I produced is good on computer headphones and also on audio-monitors (through external audio-card), but when I play the DVD on a lcd-tv with its poor 2*10W-integrated-speakers (reading the dvd with a standalone dvd player) I hear an added noise. This noise arrives only when the piano plays some precise notes: it seems to me more or less F3 (349 Hz), G3 (392 Hz), F4 (698 Hz), G4 (784 Hz). The noise is not big, but when it arrives it sounds like something frying...
    Coming back to my computer the noise disappears on those notes, but I finally notice that they sound with an "elastic" intonation... a bit like tuning, a slow adjusting the pitch of those notes.

    The "story" of this audio-track: I recorded the audio in 44.1 kHz with NTSC timecode (29,97fps). I resampled the whole original media in 48 kHz and then I changed its speed to convert it into PAL timecode (25fps) with Apple Soundtrack Pro. I did all this to edit the audio in sync with separated PAL HD footage in FinalCutPro. I did the audio edit in FCP (to maintein the sync with the video clips edits). Then I exported the result as "separate channel" into Apple Soundtrack Pro to refine EQ and reverbs.

    A friend says that probably it is a problem of aliasing.
    Is it so? Or could be something different?
    What is your advice?
    Please help me soon.

  2. #2


    Caveat - I shall make a guess, but I may be entirely wrong!

    Are you able to upload three, ten second samples?
    1) one the original recording.
    2) one from the conversion to 48k.
    3) one from the final, rendered DVD.

    I wonder if the original recording exhibits these resonant frequencies? The f4g4 is probably only a harmonic of the f3g3 disturbance (i.e. one Octave up equals double the frequency).
    Does the 'tuning' effect go up? or down? Pianos are not easy to record; and I think with lots of compression they can start to sound out of tune. If the f3g3 range is, for whatever reason, over resonant; then the notes sound odd. And the 'frying' sound is occuring when the compression kicks in.

  3. #3


    Many Thanks!
    Please, download the zip from
    It contains 4 tests:
    1- the original wave 44.1k ntsc
    2- the resampled aiff 48k
    3- the retimed and edited step
    4- the PCM audio from the dvd
    I don't understand why through the computer the noise disappears, and through tv+standalone dvd player the noise appears...
    Perhaps I don't recognize an hidden problem when the audio is playing on the computer...

    On tv, the noise is manly at 5" and 8" of the sample I linked.
    Last edited by adrjork; 08-11-2010 at 12:33 AM.

  4. #4


    The 4 extracts I linked in the former reply, all contain the noise for my tv (so the original take in 44.1k too). I tried them mastered as audio-cd and dvd-video.
    So I decided to listen to a Deutsche Grammophon cd with the same piano piece I recorded (through the standalone dvd player and the same tv) and I must say that there's noise, in other notes, so in other frequencies, rarely, but there is!
    I notice that the noisy-notes on tv, in the computer (or on a good stereo system) are simply "harsh", they sound "harsh". That "harsh" produces noise on a poor stereo system (in my case the integrated speakers).
    My question is: why some frequencies are harsh when I record audio? Is for microphones I use, is for ambient reverb? Is for piano tuning? And what is the "name" of this problem? Is it aliasing? Does exist a filter to correct it?
    I'll try to do in this way: I'll take note at which pitch the noise appears on tv, and I'll try to confirm it using an EQ filter moving a frequency set with maximum dB and maximum Q to find the point of maximum resonance, then I'll cut that frequency bringing the EQ frequency at its minimum dB and mainteining same Q.

  5. #5


    You may find it useful to post your query (and link) to an alternative Internet Forum. Pianos are very difficult to record and you may need assistance beyond what you can get here.
    Having said that. Here are my comments/suggestions.

    1) I can hear the 'frying' sound on the original. What device and mic were used to record it?
    I suspect the recorder has an automatic volume controlling system. The frying sounds appear to occur just after loud passages. The auto system is probably pumping up the volume - which at that point should just be standard, quiet background noise.
    2) I looked as an audiogram using FFT. I recommend finding some similar software for your Mac. It helps analyse the frequency bands and confirm where the loud bit occur.
    3) The original recording does not sound clear in the bass. That may be due to the mic, or its position near the piano (or overuse of the sustain pedal, or the quality of the piano).
    4) The original recording appeared to me in mono. If so, then I also recommend finding a way to record in stereo; perhaps by using a separate recorder; such as a Zoom recorder.
    5) Cleaning up the original recording a little bit may be possible, but it won't be perfect.
    6) I would be careful when adding reverb. In this instance, it has probably magnified the imperfections.

  6. #6


    Thanks a lot for your answer.
    The piano was perfect (steinway D-274) perhaps the tuning was down a bit at that moment.
    The mics weren't professional and they was closed to the piano (near the soundboard and with semi-closed case) for reasons linked to the live performance.
    So in the recording the sound result "closed". Zoom H4 made the recording and hasn't auto-volume.
    You are right: the noise is also in the original take.
    Listening to the tv-speakers, the "frying" noise appears only on 3 frequencies: 350, 370, 395 Hz (F3, F#3, G3).
    Cutting them with EQ filter (-24dB and 72.0 Q for the three frequencies as well) resolves the problem, but I have a doubt...
    Listening to the same piano piece, same bars, but Deutsche Grammophon cd (!!!) on the same tv I can hear the noise ON THE SAME FREQUENCIES! Really less than in my case, but there is a little bit frying noise on the three frequencies 350, 370, 395 Hz.
    So... I understand that any case I HAVE TO correct my take, but could be possible that at 50% it's also a tv problem? I mean a problem of my tv-speakers on those freqs?
    What's your opinion?

  7. #7


    I agree. TV speakers are not usually of high quality and may over exaggerate some frequencies.
    I recommend re-examining where to place mics when recording a piano. I don't have a clue, but with accoustic guitars; putting a close mic facing the sound hole create creates too boomy a sound (perhaps due to 'standing waves').
    I briefly looked at the specs for the Zoom. It appears to have a switchable 'auto gain' system; but I appreciate you believe that was not turned on. I am surprised because I cannot otherwise explain where the frying/hissing sound comes from.

    As you can see, I cannot offer much help here.

  8. #8


    Believe me, you gave me a big help:
    "standing waves" is certainly the problem, due to the fact that mics were in the stomach of a semi-closed piano. And perhaps mics weren't good enough for a piano take. Zoom, instead, hadn't auto-gain activated.
    Anyway, before cutting definitively the 3 noisy freqs, I'll try to listen to some other tvs.
    Thanks for all, Tim.
    And thanks for this forum.

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