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Sound not centred?

 
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treefrog



Joined: 22 Nov 2006
Posts: 28
Location: Austria

PostPosted: Sun Apr 08, 2012 9:18 am    Post subject: Sound not centred? Reply with quote

Hi, when recording via the phono input of my ADC > Weiss INT 203 firewire > MacBook Pro (Amadeus 2.0.7) I got this recording of the noise (no sound input) only (vertical scale maximised).

Can anybody explain the strange pattern of the left channel lying besides the centre line? Also the right channel is not completely centred.

https://secure.flickr.com/photos/72351879@N07/6910079738/in/photostream
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Sonic Purity



Joined: 10 Nov 2007
Posts: 82
Location: Pasadena, California, U.S.A.

PostPosted: Sun Apr 08, 2012 10:27 pm    Post subject: Reply with quote

(Raises hand) I can explain!
One phrase: D.C. Offset. An imperfection in your A to D converter such that the zero point is shifted (offset) by a fixed amount (hence equivalent to a fixed direct current signal). In some cases, it might drift with temperature over time.

If it is small enough, it may not matter. If it is larger, it may induce a click or pop at the start and end of sound files when played. If youíve ever run someone elseís Internet video and heard a loud pop at the beginning and/or end, that video had a whole lot of D.C. offset in its soundtrack.

Amadeus has a handy function for removing D.C. offset. Iím still using Amadeus Pro 1, and in that version, it is located at:

Effects > Denoising > Suppress DC Offset

I donít know whether the function has moved in AP 2 or not. I recall Amadeus II having a similar function and probably in the same place.

Again, yours may not be severe enough to matter. For my purposes, iím OK with 1/2% D.C. offset or less, as shown in Waveform Statistics when the whole waveform is selected. (I just looked at this in AP 1 and it is unclear to me whether Waveform Statistics of that version is showing a percentage or a raw number.)

If it is found to matter, select the whole sound and apply that function, and life should be good. There probably isnít anything to be done about it within the A to D converter, though if the maker of yours has any information about ways to adjust/null out D.C. offset on its product, you could pursue that and potentially make the issue vanish.
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treefrog



Joined: 22 Nov 2006
Posts: 28
Location: Austria

PostPosted: Tue Apr 10, 2012 6:36 am    Post subject: Reply with quote

Thanks, SonicPurity. I also blamed DC.offset, but Amadeus' Waveform Statistics says DC-offset = 0.0000

Well, we are dealing with a low level noise here (-48 dB), so it may not matter.
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