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Posted by Tim Smith on 01/05/07 09:23
In article <gparp2t4rbmfg2952ucgiqcq4853263cc5@4ax.com>,
JoeBloe <joebloe@thebarattheendoftheuniverse.org> wrote:
> >Too bad that the market didn't go for the "needle and groove" video
> >disk that RCA tried to make work, eh?
> >
>
> That's not what it was.
>
> It was a capacitive pick up stylus. No needle at all. No groove.
>
> http://en.wikipedia.org/wiki/SelectaVision
From that page (underlining added):
======
SelectaVision used a special medium known as a Capacitance Electronic
Disc (CED). The VideoDisc was a 12 in (305 mm) platter housed in a
special caddy. The video and audio signal is stored on the Videodiscs
via peaks and valleys in the grooves, similar but not exactly like a
==============
phonograph record, of both sides of the discs. To play a Videodisc, you
inserted the caddy into the player and the platter would be extracted.
A keel-shaped needle with a titanium electrode layer would ride in the
================== ====
groove with extremely light tracking force, reading the electrical
======
signal from the groove where it is decoded back into its FM state.
Unlike a phonograph record, where physical movement (vibration) of the
stylus in the groove of the platter led to an audio signal, the stylus
in a SelectaVision player slid along the crests of the groove, at a
===================================
constant rotational speed of 450 rpm. The varying undulations of the
peaks and valleys in the groove provided differing amounts of
capacitance between the stylus and the conductive carbon loaded PVC
disc. This varying capacitance was measured by the player circuitry,
providing an audio/video signal.
======
--
--Tim Smith
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