Re: IIGS Drive Questions

[dempson@actrix.gen.nz (David Empson)]

Greg Andrzejewski <pressyourluck85@yahoo.com> wrote:

> "David Empson" <dempson@actrix.gen.nz> wrote in message
> 1g07trm.jzu2yw1xxcqb8N%dempson@actrix.gen.nz">news:1g07trm.jzu2yw1xxcqb8N%dempson@actrix.gen.nz...
> > Juspion2002 <juspion2002@aol.com> wrote:
> >
> >
> > The sound you are hearing is due to electrical noise on the +12V rail
> > (or possibly the +5V rail) leaking into the sound circuitry.  The ROM
> > 00/01 IIgs is most affected by this: the ROM 3 has improved sound
> > circuitry which is much less susceptible to noise.
> 
> Without actually knowing what "+12V rail" means

The "+12V rail" is the output from the computer's power supply which
supplies positive 12 volts (DC) to some parts of the computer.  The main
system components which use it are the drive motors (on both 5.25" and
3.5" drives) and the sound circuitry.  The drives need a lot of power to
spin the motor, so this power supply rail tends to be used for sudden
heavy loads, while it is normally not supplying much power.

By comparison, the "+5V rail" (detailed description left as an exercise
:-)) is used by almost every part of the computer - nearly all the chips
on the logic board require a 5 volt power supply.  The 5V output from
the power supply is able to supply more current than any other, and the
amount of current drawn doesn't vary much over time.

The power supply also has -12V and -5V outputs, which don't supply much
current and are only used by a few parts of the computer.

> I will point out that the clicking only happens when using the finder and
> maybe with some other GS/OS apps (most which have an "OPEN" window where
> you can choose a disk).

This is quite likely, as 8-bit software doesn't usually poll the 3.5"
drive.  The Finder and open file dialog boxes are the only parts of
GS/OS which actively poll the 3.5" drive to detect whether a disk has
been inserted or removed.

> I'd think the +12V rail would have something to do with the power supply,
> and therefore, if causing the clicking, have it happen at all times when
> the machine is powered on.

The 12V rail isn't the cause of the noise - it is the means by which
electronic noise is transported from one part of the computer (the
floppy drive) to another part of the computer (the sound circuitry).

What is happening is something like this:

Under normal conditions with the drives idle, the computer isn't
supplying very much power on the +12V rail.  As soon as the computer
enables one of the drive motors (which happens very briefly if you have
a 5.25" drive connected and the computer polls the 3.5" drive) there is
a sudden increase in the amount of power needed on the 12V rail.

The surge in current causes the voltage to drop a little until the power
supply stabilises again, and the sound circuitry is affected by this,
causing one or more of the amplification stages to generate audible
noise.

Apart from the issue of a temporary voltage drop, it is also possible
that there is electronic noise being induced by the drive onto the one
of the power rails (e.g. a short "spike" which causes the voltage to
rise or drop for a few microseconds, or a series of fluctuations in the
signal), and that is feeding back into the sound circuitry where it is
being amplified and is then audible through the speaker.

There are other areas in which the ROM 00/01 IIgs sound circuitry can
pick up noise from elsewhere in the computer.

One of my favourites (which doesn't affect all machines but did affect
mine) is the "menu item piano" effect: in some applications, you can
hear a tone from the speaker while a menu is held open.  As you move the
mouse up and down the menu items, the pitch of the tone changes, kind of
like moving up and down keys on a piano.

In this case, the sound circuitry is picking up some side effect of the
frequency of a loop being executed by the CPU - the further down the
menu you go, the longer it takes the CPU to get around the loop again,
and the pitch of the tone is correspondingly lower.
-- 
David Empson
dempson@actrix.gen.nz