[78-L] Removal of hiss on 78s

[Jeff Lichtman]

>Each scan is only identifying what parts of the groove wall at that 
>single point on the record surface correspond to the shape of the 
>reference sample, (either the shape of the cutting stylus, if known, 
>or an estimation taken from the record itself).  If there is any 
>wear, dirt or groove damage it will cause the scan to deviate from 
>the reference sample and that part of the scan will be discarded.
>
>db

This reminds me of a panel cartoon I once saw. Two scientists are at 
a chalkboard filled with equations. Somewhere near the end of the 
equations is written in bold letters, "AND THEN A MIRACLE OCCURS." 
One of the scientists is saying to the other, "I think you need more 
detail in this step."

Figuring out what part of the groove corresponds to the original 
cutting stylus is worse than non-trivial. Really, if the groove shape 
deviates from the shape of the cutting stylus, how is one to figure 
out where the groove is supposed to be? Knowing the shape isn't 
enough - you need to be able to figure out the lateral displacement.

Here's a reduction ad absurdum argument. Suppose you have a record 
that is completely wiped out - the original signal has been worn from 
the grooves, so all that's left is noise. It seems obvious that no 
playback system could figure out the original signal - that 
information is gone. It doesn't help to know what the groove shape 
should be because the groove is worn so badly that the groove path 
could be literally *anything* (within the range of the recording 
medium). I maintain that, in the system you're proposing, the 
difference between a completely worn record and one that's only 
slightly worn is one of degree, not kind. Knowing what the groove 
shape should be doesn't necessarily tell you what the signal is, 
because every bit of wear removes some information that may not be 
deducible from the remaining groove.

I could make the same argument about noise from sources other than 
wear. How is one to deduce the signal from knowing the shape of the 
cutting stylus? You come across a place where the groove shape is 
different from the cutting stylus - now what? It's not enough to say 
that you should keep the part that matches the stylus and discard the 
rest - you have to be able to figure out which parts of the groove 
match and which don't. There are special cases where your proposal 
could work. For example, a pit or protrusion that doesn't extend the 
length of the groove wall is obviously noise and can be ignored. But 
it's not clear to me that this problem is solvable in the general case.


                        -        Jeff Lichtman
                                 jeff at swazoo.com
                                 Check out Swazoo Koolak's Web Jukebox at
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