🚀 go-pugleaf

In article <9359@nsc.nsc.com> woolsey@nsc.nsc.com (Jeff Woolsey) writes:
>I'm sitting here listening to my CD player being listened to by someone 
>else, and find that the noise it makes (sounds like the disc sliding
>on something) is not present between tracks, yet the disk still spins.

What you're hearing is white noise, more or less.

The pattern of pits on the disk modulates the light that reaches an
array of 4 or 6 photodiodes in the laser pickup.  The output of some
of these is summed to produce a data signal, from which the digital
data is extracted.  The difference between pairs of these photodiodes
is used to provide focus and tracking error signals.  The data bits
is still present in the "difference" signals to some extent, and
for most intents and purposes is similar to random bits, which is
white noise.

This noise is then fed through the servo amplifiers
and to the mechanical actuators that actually move the little lens.
The frequency response of these actuators extends into the audio range,
and so they reproduce the digital information as a hissing noise that
you can hear.

Between tracks, the digital data is mostly zeros in a very regular pattern,
so the "white noise" sound goes away.  As soon as the music starts, it
comes back.

If you don't believe me, follow the tracking or focus servo circuits
with an oscilloscope.

>It occurs to me that this noise is also present in the FPA on Vaxen.
>It's a sort of hissing sound.

You're probably hearing a switching voltage regulator's noise change
as the load on it changes.  (But on our VAX, all I can hear is the fans).