Removing "wow"

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steveB
Posts: 1
Joined: Tue Apr 13, 2010 12:24 am

Removing "wow"

Post by steveB »

Vinyl pressings that are slightly off-centre (too common) produce an audible "wow" (very low frequency, at the disc rotation speed), which increases in amplitude as the disc towards the centre (end of music).

It occurred to me that this could be removed by Fourier Transformation (into the frequency domain), eliminating the "wow" frequency in that domain, then re-transforming back to the time domain.

Does Goldwave have any tools to assist in this area?

Steve

cdeamaze
Posts: 87
Joined: Thu Oct 05, 2006 3:19 pm

Re: Removing "wow"

Post by cdeamaze »

This is exactly where GoldWave can help. If you know the precise frequency of the "wow", you can apply a notch filter to remove it. If you are not sure about the frequency contents of the "wow", you can study its contents by using Short Time Fourier Transform(STFT) where the STFT should be located at when "wow" occurs and last for the duration of "wow". Spectrum analysis, and spectrogram in particular, is the area you should look into.

DougDbug
Posts: 2067
Joined: Wed Feb 16, 2005 3:33 pm
Location: Silicon Valley

Re: Removing "wow"

Post by DougDbug »

(I don't understand what cdeamaze is suggesting...)

I don't know of any software to do this, but it might exist... I did find a WOW & Flutter VST plug-in. You might be able to use it to cancel-out you WOW. (See the GoldWave FAQ for help with VST plug-ins.)

I agree that, in theory, this shouldn't be too hard to fix. It does NOT require FFT. FFT is needed if you need to change speed/tempo or pitch independent of each other. This is not the case with WOW, where speed and pitch change together.

All you have to do is re-sample (move the samples around) at a variable rate. The tricky part would be to get the correct "depth" (which as you say, gets worse toward the center of the record) and to get (and keep) the correction time-aligned with the defect.

Hmmmm.... GoldWave's Expression Evaluator has a Frequency Modulation preset. I haven't studied expression, but I think it might work (Since it does not use FFT, the speed and frequency should modulate together.) Or, perhaps you can modify the expression if necessary. The modulation frequency is easy (33.33/60) but you'll have to experiment to find the correct amplitude of modulation for each track and to time-align the modulation.

I've read about people modifying their turntable or drilling a bigger hole in the center of the record. I think the turntable mod was done by removing the spindle, and maybe holding the record in place with some double-sticky tape, or by attaching some sort of guides that align the outside of the record. Re-drilling the record sounds very tricky... You'd have to make some sort of jig to drill a hole in the exact center of the record.

DewDude420
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Location: Washington DC Metro Area
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Re: Removing "wow"

Post by DewDude420 »

the amount of wow you get depends on your turntable. pivotal arms for example make wow noise....my tangent arm doesnt make as much. either way...these are usually subharmonic....so a highpass filter of 20hz should clean all this up....as well as remove low level runble/offset from warps and scratches...a simple solution.

dayepipes
Posts: 1
Joined: Thu Jan 06, 2011 12:18 am

Re: Removing "wow"

Post by dayepipes »

As a bagpiper I've been plagued by vinyl wow for years and years. Bagpipes with their constant pitch drone magnify wow worse than most other kinds of music. Coincidentally bagpipe records were made for many years on the cheap so that most that I have in my 50's through 80's collection have noticeable wow problems.

There are 2 kinds of wow: off-center wow, and oval wow. The off-center wow is fixable with Biblical technology. Oval wow will require software.

The best single solution I found for off-center wow since the 1950's was a strong light, a pencil and a rat-tail file. Put the record on the turntable, turn it on, engage the tone arm lift lever, and gently guide the tone arm over one of the outermost music breaks on the record. It may help if you can partially lower the arm to float a fraction of an inch over the record. Watch the grooves next to the tone arm and notice them moving slightly in and out as the record turns.

Now you might be lucky and find that the wobble is small enough, and the center hole enough larger than the spindle, that at the point in rotation where the grooves are the farthest out, you can touch the edge of the record with your thumbnail to bump it in, and that might be enough to center it. If so, you're done, remember to do this any time you record from the vinyl or play it for sensitive listeners.

If that's not enough you need to alter the spindle hole. Hold the pencil over the label (if you don't want to mark up your label, put some easy-peel masking tape over it all around) and touch the label every time the minimum or maximum diameter wobble of the break passes the arm.

You can then use the rat-tail file to enlarge the spindle hole on the side where the grooves were closest in to the spindle hole. Do it just a little at a time, and put the record back on pushing the area you filed up against the spindle.

With luck you'll stop just at the point where the grooves rotate at a perfectly constant distance from the spindle. You can make a small permanent mark on the label and remember to push that side up against the spindle. If you overshoot and enlarge the hole too much, every time you play the record you'll have to spend some time nudging it as it rotates till you get the grooves on center.

Welcome to my life with vinyl 1960-2011. :lol:

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