Vinyl Recording Characteristic
Up to 500 Hz the groove is cut with constant amplitude modulation. Above
500 Hz it is is cut at constant velocity :–
Constant amplitude means that the 'wiggles' of the record groove are the
same size (side-to-side excursion) at all bass frequencies up to 500 Hz, assuming
that the signal level remains the same. However the pick-up stylus moves faster
and faster as frequency increases, because the distance it travels is actually getting
longer as the 'wiggles' become more compacted together, hence a corresponding increase
in output voltage. It's like a piece of string seems shorter only because it has
lots of bends in it. Pull it out straight to get rid of the bends and it reveals
its true length. That's why the voltage output of a magnetic pick-up cartridge is
specified as being so many millivolts per centimetre (of distance travelled in a
constant time-frame).
Constant velocity means the 'wiggles' get smaller as the frequency increases.
This means that above 500 Hz the stylus always moves at the same speed, regardless
of frequency, because the distance it actually travels remains the same. Hence the
output voltage is also constant (ignoring, of course, the added Decca standard treble
pre-emphasis). This prevents both premature wear of the groove, especially at the
treble end, and minimizes friction heating of the stylus tip. Clever, innit?