Re: Audio Noise Problems (IIgs ROM 01)

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

Tempest <tempest@atariprotos.com> wrote:

> Yesterday while using my IIgs I noticed a strange noise coming out my
> speakers.  It seems that whenever I was in System 6 there was a quiet
> poping noise coming out the speakers.  The pops were about a second
> apart and it was constant. Opening menus or moving the mouse around
> didn't seem to change anything.  I don't recall ever having this
> problem before, so I'm wondering if something I've setup recently could
> cause this.  The problem doesn't happen during games, so I think it
> must have something to do with System 6.

Known hardware issue with the audio circuitry in a ROM 00/01 IIgs,
possibly only affecting some machines (mine was affected).

Whenever the IIgs polls the 3.5" drive, there is a burst of noise which
gets into the audio circuitry through its connection to the power
supply. In some cases you can actually hear the 3.5" drive motor
stepping through the IIgs speakers.

A similar problem I've encountered is that in some applications,
selecting a menu item would produced a high pitched whine in the
speakers, and the tone changes as you move up and down the menu. This
was probably related to the speed at which a loop was executing on the
CPU.

It basically means that anything which causes induced noise on the power
supply will be amplified by the audio circuit and be audible on the
speakers.

The ROM 3 has better filtering on its audio circuitry, so it has much
better audio output quality.

-- 
David Empson
dempson@actrix.gen.nz