[Mike Rivers[_2_]]

On 4/17/2018 9:39 AM, wrote:
Scott Dorsey wrote: "...it's not total excursion that limits your
cutting levels. "

Ok, you lost me with the above!

It's not the total amount of swing side to side that determines the
output level of the pickup, it's velocity. This is a fundamental
principle of electromagnetism - something has to move to cut across
magnetic lines of force in order to generate current, and the faster it
moves, the more current it generates. Think about the alternator in your
car - it doesn't generate any current when the engine is stopped, and as
the engine drives it faster, it generates more current (which then has
to be regulated to keep the lights and battery from blowing up).

Let's say that we have a special cutter that can swing the stylus half
an inch to either side of the center, and we want to get maximum level
from it. The stylus has to move a total of one inch between the positive
and negative peaks of the groove you're cutting. Since it needs to move
the width of the groove and back in order to cut a full cycle of
whatever frequency you're cutting, if you were cutting a 50 Hz bass
note, it would have to move two inches, fifty times in once second, or
100 inches per second (that's _velocity_). That's pretty hard to do, so
think of what kind of velocity you'd need to cut a groove that wide at
20 kHz!

Thinking back the other way, if you know the maximum velocity that the
stylus can achieve, you can figure out what's the widest groove you can
cut at the highest frequency you want to record. Try to move it any
further at that frequency and you'll get distortion, and, if you try
hard enough, smoke. On playback, the stylus will fly out of the groove
because it can't slow down fast enough to turn around at the peak of the
cycle.

You could, however, cut a wider groove at low frequencies. And to make
things more difficult, there's the RIAA equalization curve that boosts
the highs and cuts the lows when cutting, and does the opposite on
playback. That serves to keep the groove width relatively constant for a
constant level, and, as well, whacks off some high end on playback,
which makes record scratch less audible.

When you're using a laser rather than an electrical transducer, you're
not bound by the inertia and mass of the stylus and what moves it (or
what it moves). So you could cut a crazy groove with a laser and play it
back with a laser (every hear of a CD?), but a conventional phono pickup
still has the velocity and excursion limitations that only let it go so
far.


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