Re: [ARSCLIST] 78rpm archaeology project

["Steven C. Barr(x)" <stevenc@xxxxxxxxxxxxxx>]

Now...a message about the original subject here...!

Note that a 78rpm phonorecord has to complete each rotation in 60/78
of a second...and occupies the full 360 degrees needed for that task
(before being broken)...! On the outside grooves, this amounts to
very close to pi * record diameter...or about 31.4 inches of travel;
on the innermost groove, around pi * 4 (inches), or about 12.6 in.
If we select (for argument) a piece which came from around the
middle of the spiral...and is roughly 2" x 1"...well, a 3-minute
recording has to make 3*78, or 234, rotations in its duration.

Record grooves are about .005" in width...and there has to be
space between each pair of spiral turns as well. Our 234 
rotations will need 234 concentric spiral turns...so, if we
assume 9.8" outer and, say, 3.8" inner groove diameters, we
have 3" of radial-measure space to be occupied by our 234
concentric "grooves" (being a spiral, there is really only
ONE groove, of course....?!). This works out to the turns
being .01282" apart (inclusive). Note that the .003" diameter
of the needle tip does NOT mean it has that size "all the 
way up"...only that the cone that is defined by the needle's
tip has a maximum width of .003.

What one gets is more or less:

 |<-x->|
__    __    __
  \  /  \  /
   \/    \/
(NOT to scale!)
where x=> .01282...so we have plenty of space there...!

So, our vaguely-trapezoidal fragment would have groove segments
c. 2" long...and about 78 or so of those. The middle groove in
the record would have a nominal diameter of 8" or so, and an
approximate total length of around 25 inches. Since we only
have 2 of those 25 inches, we have 2/25 of a rotation which
lasts 60/78 of a minute...or, about 78 3.7-second-long parts
of the recording...! It would appear that one could hear a
few words (for a vocal recording)...or quite a few notes in
succession...possibly enough to identify a popular song?

Also, note that my hypothetical record...containing exactly
3 minutes of music...plays one concentric groove (see note!)
each 60/78-minute (.76923 minutes, or a little over 46
seconds. This means we wouldn't hear the same part of the
recording each rotation (like we would on a 60rpm disc).
In fact, the next segment would start 18/78 minute, or
just under 14 seconds, later into the song. In fact, it
might be possible to calculate how much of the song we
hear, using 3.7 second segments spaced c.14 seconds
from one another...?!

Steven C. Barr