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Re: address@hidden

From: Quentin Spencer
Subject: Re: address@hidden
Date: Mon, 13 Sep 2004 08:16:12 -0500
User-agent: Mozilla Thunderbird 0.7.2 (Windows/20040707)

Neil Bryan B. Cazar wrote:

hello guys! i'm a newbie when it comes to octave. i would like to ask
for some help if some of you can give me a source code in octave that
does the following tasks.
1.) first the program would load a wav file.

I think there may be functions for this kind of a thing in the octave-forge package (, but I don't know what they are, so perhaps someone else on the list can respond.

2.) if i'm not mistaken, the wav file is in time-domain format. it
needs FFT (fast fourier transform) for it to be in the frequency domain.

In theory, yes, an FFT converts from time to frequency domain, but it depends on what you want to do in the frequency domain. Filtering is usually done in the time domain, but if you want to look at the spectral content of the signal, what you really want is a function such as "pwelch" in octave-forge that estimates power spectral density.

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.

The problem here is that the noise on most old recordings is what is referred to as "white noise", meaning that it contains equal energy at all frequencies (the same way white light contains energy at all frequencies). As a result, you can't really just filter it out. Recordings made on very old equipment had limited ability to reproduce high frequencies, so sometimes old recordings are "cleaned" by filtering out the very high frequencies, but otherwise there's not much you can do (and some audiophiles would prefer to leave the noise there so you don't mess with the music). White noise exists in all recording media, really. It's just that modern equipment has been able to push the level of the noise down to a level that it's barely audible, if at all. Anyway, if you really want to try filtering recordings, I would suggest a low-pass filter, rather than a notch filter.

I hope this helps.

Quentin Spencer

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