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RE: address@hidden

From: grumpy steve
Subject: RE: address@hidden
Date: Mon, 13 Sep 2004 23:22:15 +0100

Neil Bryan B. Cazar wrote:

>>3.) it then passes through a notch filter which filters out the
>>noise. the noise is the hissing sound you hear when you play cassette tapes
>>or old records. the output is then converted back to a wav file.
A lot of the noise from old gramophone records is so-called “surface noise”, cased by small scratches and detritus jammed into the groove.  If you were to look at the waveform, you would see the smooth music signal interrupted by numerous spikes.  Because these surface noise spikes are short, the noise is very broadband, and no amount of linear filtering can separate out the signal from the noise.  But non-linear filtering can, in certain cases, do quite a bit better.  I heard a demonstration several years ago of a system that tackled surface noise by running a linear prediction filter on the signal.  The linear predictor was set to guess what the value of the next sample would be, and would normally do a pretty good job, leading to a prediction that was close to the actual sample.  The one thing the linear predictor could not predict was spikes, so when one of those came along, the prediction would differ a great deal from the true sample.  During such periods, the audio output was taken from the output of the linear prediction filter rather than from the original programme material.  This had the effect of shaving off the spikes, leaving a smooth waveform behind.  I have no doubt that the example used in the demonstration was carefully chosen to show the technique to its best advantage, but even allowing for this, the results were remarkable.


I have often thought of trying this sort of thing out for myself, but have never had the time.  Octave would certainly make an excellent environment for such an experiment.




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