Ray Maidstone
No! The Engineer's picture hasn't had a bitmap corruption. He's suffering interference!
So what's the Engineer going on about? Well, if we wish to add this gadget, that hard drive or the other hardware component - we just plug it in and off it goes!
Or at least that's what we hope. Just because it all works most of the time doesn't mean all is well. This article's point is to clarify what is really happening inside our computer. And, to make the point, let's start with an empty computer box.
Put a power supply into it. No interference so far - or is there? Yes, of course there's interference, because a high current, high voltage field effect transistor (FET) is chopping up rectified mains (about 420 Volts D.C.) into square waves and shoving it into a transformer (the very operation of which relies on the fact that one set of coils interferes with another set of coils) and so we have the first of our radiating components inside our case.
Now, in the course of repairing and upgrading people's equipment, all sorts of peculiarities are brought before me. Some of them are component failures, some of them are simple effects of age, but some of them are caused by interference, or 'crosstalk' - that's the technical term used when one thing puts an unwanted signal into another.
Allow me to list the areas of interference:
The power supply
The processor
The I/O bus (where you plug in your expansion cards)
The interconnecting cables
The mains cable
The mains supply quality
The point here is that the entire nature of your computer is digital, i.e. everything is switching from 'on' to 'off' and vice versa. Your computer is, in fact, a vast collection of little transistor switches, all switching on and off like crazy. This all happens at very high frequency, and if you want to see just how noisy your computer is, find yourself an inexpensive transistor radio, use it on AM (preferably on headphones) with the volume low at first - and just move it around near your computer.
If your computer has a metal case, you will notice less 'row' being given off, but put it near the plastic front panel. Now, just for a laugh, wave it around near the front of your monitor (assuming these are switched on - of course). What a noise!
If for some reason, any of you haven't found any noise then there must be something wrong with your radio. (No! Repair Zone doesn't repair radios - well not ones that cost less than £100!)
'So why tell us all this stuff, Mr Engineer?'
I'm telling you this because several other things are listening to all this noise - not just your radio.
For example, when pushing any data down a multiway cable, whether it's an internal cable like a 50-way SCSI ribbon cable, a 40-way IDE ribbon cable or an external cable, all these individual wires have a small but collective effect on signals in other wires near them. Some manufacturers use alternate cable wires to carry 'earth' in an attempt to reduce the noise. Others even wrap each wire in a metal sheath, also connected to earth - but we don't like paying for items as good as this because they are very expensive - but then how do you define expensive?
I have had several occasions where people have tried to use too simple a cable to run too long a distance - so data gets corrupted - and other situations where an over-long ribbon cable has been wrapped up in multiple folds to cram it into a small space. This is very unhealthy because at the high frequencies used in these cables, crosstalk starts shoving error data all over the place.
Protect those cables
Let's think about this. If you have a hard drive in your machine - which is 99.9% likely - then this will apply to you. Have a look. Where does the ribbon cable run? Don't even think about putting your transistor radio near this cable when your machine is running, as the noise is deafening.
To help your computer's noise situation is quite simple, if a little 'Blue Peter-ish'.
You can use cooking foil in suitable strips, with sticky-back plastic covering it, and put his along the back of the cable to give an earth screening cover. The diagram shows some shielded cable with the end of the foil twisted into a tail and put into a terminal block. A wire must then be taken from this terminal block to the black wire or a terminal associated with the black wire from your power supply. (Yes, I know there are many black wires, but they are all at 0 volts.)
One very important detail here if you want to take this advice is that this aluminium foil screening must be fully covered with insulation in order not to touch anything that might have a voltage on it.
This interference situation also relates to printer cables, serial cables and other external links, and can also be assisted by things called ferrite rings. The nature of the help that ferrite rings offer is that because square waves have a habit of 'ringing' or 'bouncing' when put down a length of wire (copper wire having a slight transformer effect) then the square wave becomes more noisy, with the rising edges overshooting. This overshoot is what gets across multiple cables (crosstalk) and the ferrite ring acts as a damper or 'choke' at high frequencies.
ferrite ring around cable
Mains-borne interference
Finally, to protect your computer from incoming 'flak' along the mains cable, devices known as R.F. Suppressors or 'flak catchers' can be obtained from many electrical supply sources. Well worth investing in. If you get to see what the mains supply looks like on a decent oscilloscope screen, you will see that it is far from pure - not, in fact, a nice clean sine wave at all. All sorts of things can put spikes onto the mains, e.g. kettles, drills, microwave ovens, thermostats, time clock devices, mobile phones - just having the mains cable too long, or having a mains cable of too low a current rating can also add to these problems.
Sorry if this has all got a bit wordy, but it is a difficult subject to put into few words. Anyway, I hope it's given pause for thought.