wrote:

"This won't be much help to you with your particular problem of the
moment,..."


You are absolutely right about the linear tracking, though realize it is
not perfect either. Nothing is friction free enough...

The un-servoed parallel tracker was of dubious value, even in the days
of coarse-grooved 78s.

... so therefore there
is a motor and an optical pickup that tells the little motor when to
turn. It jogs along as the record plays which thus means there is
tracking error oscillating between negative and positive angle to the
groove. It is still a hell of alot better.

If the servo is correctly designed, the error can be very small indeed -
probably smaller than the error in the original cutting facet.

The trick is to have a high loop gain with a very long time constant,
lurking in the background of an ordinary much faster loop with less
gain. Velocity-proportional feedback is also needed for damping the
loop. Finally, there needs to be an over-ride system which disables the
time constants and puts the motor on full power when the error exceeds a
certain range. Otherwise, when the user lifts the pickup to return it
to its stand, he will have to wait all day for the servo to catch up.

Not every disc was cut with a correctly-aligned cutter, so it is helpful
to have the cartridge on a swivel mounting if you intend doing archive
work; an X-Y scope on the two channels is the most accurate way of
aligning it.

Among the worse of ideas was the aesthetically attractive yet a
practical nightmare known as the S shaped arm. First of all the thing
has more mass. The best arm is a straight line because it will have the
lowest mass....

Also, there could be worse torsional resonances in an 'S'-shaped arm
than in a straight arm of similar construction.


And of course make it as long as possible.

If the arm is radial, that makes sense because it minimises the angular
changes; but there are advantages to a short arm, which can be exploited
if a parallel tracker is used with the carriage track running partly
above the turntable.


--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk