This discussion is taking me back to the '60s when I worked in Hollywood 
as an assistant film editor.

Tom Fine wrote:

> Hi Doug:
> Thanks for the further info. I had never heard of this oxide-scraping 
> technique until today.

The oxide-scraping technique was also used in 35mm film editing when 
mag-stripe sound tracks replaced optical sound.   We, of course, edited 
our separate sound track with the 35mm work print picture.   There were 
times when a mag head reader or a butt splicer could become magnetized 
and create pops at splices.   You could order reprints of the mag 
soundtrack, but if there were a lot of pops, the assistant or apprentice 
would have to try to fix them, and the easiest way was to either scrap 
the splice or use acetone to wipe off enough of the oxide to get rid of 
the offending modulation.  Also, I saw music editors physically wipe 
(acetone) the beginning or end of a music cue to make a smoother 
transition to a music edit.   You could even make a pretty smooth fade 
in or fade out the technique.   Needless to say, working with sound 
tracks came with its own bag of tricks which you added to as you worked 
around the pros.

The wonderful thing about digital today is the ease and the use of trial 
and error with multiple levels of undo.   I have used CoolEdit Pro for a 
number of years, and when I did remastering from 16" transcription disks 
to digital, I had to fix a lot of clicks and pops.   In fixing pops, I 
found that Pro had an excellent pop eliminator capability:

> The Click/Pop Eliminator works by searching for anomalies in the audio 
> data that could be construed as clicks or pops (Detection), and then 
> replacing or repairing the damaged location (Correction). Using the 
> Click/Pop Eliminator is more accurate than just cutting out the click, 
> or replacing the data with a straight line.

It would seem to analyze the surround modulation and replace the spike 
with modulation that closely matched the surrounding peaks or valleys.   
Sometimes, the pop was so strong that it had a lot of low frequency in 
it, and would have to use a low frequency FFT filter at the pop site.   
It usually smoothed it out, but I wasn't above in replacing the area 
with other nearby modulation (copy) that closely matched the doctored 
section.   If it was in a quiet section,  you could easily grab some 
"fill" elsewhere.   It was much the same technique as in film when sound 
effects editors had libraries of 35mm reels of different kinds of 
backgrounds which they called "fill" to fix every occasion.   The "old 
timers" all had their tricks to make the sound tracks work and to hide 
any bad artifact that might show up on the dubbing stages with those big 
"Voice of the Theater" sound systems running at high decibels.

I would think that the audio engineers who did the three track mastering 
on 35mm probably used similar techniques.

Rod Stephens