Greg Andrzejewski wrote: > "David Empson" wrote in message > news:1g07trm.jzu2yw1xxcqb8N%dempson@actrix.gen.nz... > > Juspion2002 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