[NJARC] RCA Selectavision Help

[Dave Sica]

On Fri, 12 Oct 2007 06:49:33 -0700 (PDT) john ruccolo <jr6v6gt at yahoo.com>
writes:
> What's amazing about CED's is that 1) they were not
> digital, and 2) they were not optical. It was
> basically a very high-speed phonograph record!

Not digital, not optical and not exactly the same as a phonograph record
either! Maybe you could call them a very "high tech" phonograph record
that works the same in some ways and quite a bit differently in others.
In a CED, the stylus rides v-e-r-y lightly in the groove and is merely
guided by the sides of the groove to stay in place. The signal is stored
in the varying depth of the groove (an "analog" of the composite video
and audio signal.) The needle does not actually ride up and down on these
vertical variations (as it does when playing, for example, Edison
"vertical cut" records). It just "flies" above the bottom of the groove
at a constant height and doesn't touch the bottom of the groove at all.
The varying distances between the peaks and the valleys of the conductive
material from which the record is made result in a variation of the
capacitance between the surface of the groove and the needle. This
changing capacitance is an analog of the composite video signal embossed
in the groove and, similar to a condenser microphone, that capacitance is
used to generate a voltage containing the signal. 

My understanding is that wear experienced by CED needles would be
primarily on the sides from brushing against the wall of the groove.

--Dave