The Sinclair ZX Spectrum at Thirty (Part 5)

It’s Time to Get Technical!

The end of the week is getting closer and today in part five, I am going to look at the hardware of the Sinclair ZX Spectrum. I’m going to try and not to get to technical as I don’t want this post to confuse people but in places I won’t be able to avoid the technical stuff.

I’m not massively into the technical side of the ZX Spectrum anyway so the research for writing this particular blog entry was quite fascinating. Hopefully some of my readers might also be intrigued by what actually went into the humble rubber keyed ZX Spectrum that was the brainchild of Sir Clive Sinclair but it was designed by Rick Dickinson and engineer Richard Altwasser.

Thirty years ago they created what would become the UK’s Number One home computer.

Together we will discover the technical secrets of the ZX Spectrum and what made it a great machine and I promise we are going to have fun doing it to!*

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Technical Overview

• Processor: Zilog Z80A CPU @ 3.5mhz
• Memory: 16, 48 or 128 kilobytes
• Display Resolution: 256×192 pixels
• Sound (16k & 48k Model )  Single Channel, 10 Octave Beeper
• Sound (128k Model) Three channel General Instruments AY-3-8912 PSG

There is also an expansion edge connector and 3.5mm audio line in/out.
The 128k model added MIDI compatibility, RS-232 port and an RGB monitor port.

Firmware: Sinclair BASIC (stored in ROM)

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I did say that we wouldn’t get to too technical and I’m not going to as really want this to be a fun post that will perhaps open the eyes and ears of a few people, Commodore owners in particular are going to be surprised by one item that is coming up!

Full Colour Graphics and Colour Clash

Colour Clash in action!

First of all lets have a look at the graphics capabilities of the humble ZX Spectrum. As I noted in the technical overview the display resolution is 256×192 pixels. What is common knowledge, and the source of much derision by owners of other computers, is the way that they ZX Spectrum handles colour – In any 8×8 pixel block there can be only two colours at once.

In games that had lots of different coloured sprites or multicoloured backgrounds this lead to what became known as Colour Clash. As you can see in the screenshot to the right there is a lot of colour clash going on as the trails from the missiles and the explosions are animated in full colour so when they overlap the colours merge and it looks nasty. In Cybernoid this isn’t too bad as the explosions and missile trails are quite quick but in other games the colour mashing is awful!

So we’ve established that you can’t get full colour graphics on the ZX Spectrum without risking colour clash due to the limitation imposed – two colours in any 8×8 pixel block. While the common workaround was to just use monochromatic graphics many programmers, such as Don Priestly, used some clever methods to avoid colour clash. Don would draw all his graphics based on 8×8 pixel blocks and would ensure that the graphics filled, as much as possible these 8×8 blocks. This would avoid the problem colour clash.

So, is it therefore possible to do something like the picture below on a ZX Spectrum?

Well my friends the answer would be a yes because the above screenshot is taken from a ZX Spectrum.
Mind Blown yet? Seriously, If you have a ZX Spectrum at home and have the ability to playback a tape file into your Spectrum then download the file from here and run it on a normal home CRT based television and be very surprised.

The technique used is only practical for single screen images and not for in game graphics, regardless though it’s a very impressive thing to do. It is done by using two pictures that are very rapidly flicked between to produce one picture. It’s a cheat, but it does produce an amazing result.

Here’s a fun YouTube video that shows off some very impressive ZX Spectrum graphics and it has a decent AY soundtrack too!

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