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Old 2005-07-23, 09:45 PM
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guygee guygee is offline
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Join Date: Feb 2005
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Re: An urgent message about digital noise reduction

Chazuke - I really do not know of any decent free tools out there, but you might try the "Goldwave" wave editor. It is shareware, but you can downloaded a limited demo (3000 commands total, 150 commands/session). The new version looks like it has various filtering tools:

Remember that broadband noise is much harder to remove (without damaging the music) than other types of noise. For example, "digipops" are usually of short duration and can often be penciled out without leaving an artifact. Same for a single scratch in an otherwise pristine vinyl rip. Analog single-channel droputs often leave a very noticeable noise, even when the dropout duration is between 10-100 ms., but if you carefully cut to mono (try to match levels and slopes at the cut) the human ear "integrates" right though the brief mono and does not detect it.

Severe line noise can be removed by carefully constructing narrow filters at 60 Hz. and its harmonics (120 Hz, 240 Hz...possibly higher). It helps to have a frequency domain sample of the noise without any music to determine the filter parameters, and inevitably you are removing some musical content, so be cautious.

Broadband noise is the real challenge. Tape hiss, wind in the mics, vinyl crackle and rumble, and the guy with whooping cough sitting next to the taper are all examples. Hiss removal tools usually use dynamic filters that set a amplitude trigger based on a noise floor, and use attack and decay settings to try and filter from the top frequencies down, using less filtering when the music is "on" and more in the quiet spots. If way overused, you will hear this strange background, like crazy windchimes, in this case back way off on your settings.
In my experience, acoustic music is the most difficult to remove hiss from, you get an oscillation in the decay of the guitar notes ("waveriness"?) that is not unlike a bad MP3.

In any event, use your ears to compare "before and after", and see if you can find a good spectrograph tool to compare before and after to make sure you are not taking too much off of the high end and/or leaving any other "blocky" artifacts.

(EAC has a spectrograph tool, but watch out for its artifacts (false content from aliasing) in the high end, also there is Sound Software Spectrograph 1.0, not particularly versatile but at least it is free. Still available at:
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