Joyce Brite had requested info on removing scratch sounds from vinyl
and LP recordings.  Back in the "Good old days" (1970s, I guess) there
was a company which produced an audio device which sampled incoming
signals (phono record signals, e.g.) on a constant basis (many times
per second).  When the sample showed a variation from the preceding
sample, it checked the next sample.  If the third sample matched the
first sample, the second sample was eliminated.

If I remember correctly, only certain frequencies, which corresponded
to frequencies produced by a scratch, were those examined and/or
eliminated.  A highly scratched record could come out sounding
wonderful.  There was also built in a switch to hear what had been
removed by the filter.  It did not remove constant hissing sounds,
though.

This device cost between $200-$300 at the time, a bit expensive.
I have no idea of its manufacturer or its actual name, but was
impressed with the way that it operated.  There should be a few of
these still floating about, or some other company should have the
same type of filtering device available.

Best wishes,  Mike Walter