Pattern Generator | Everygamegoing


Pattern Generator

Published in EUG #64

It has been some years since I've contributed to EUG. In fact, I missed out much of the 'Gus era'. During the intervening time, I have played with many different computers from Spectrums and Amstrads to Amigas and PCs but the Electron is the one machine I always return to. Just last summer I turned to the Electron to help me test a Closed Circuit TV installation. I needed some test equipment that would allow me to inject a known good signal at field end of the system and 30 miles away check the quality of the received picture.

Apart from buying a Pattern Generator, the Electron was the obvious answer - it could be easily programmed to provide some simple test patterns similar to that of the Pattern Generator. The transmission system required a composite video input, which is precisely what the Electron can provide with its Video Out socket. The CCTV System will be using monochrome cameras so the Electron's Video Out socket is ideal.

The program provides the following screens (waveforms):

This waveform shows one line of the Grey Scale, Black to White – sometimes known as the ‘staircase’
  1. Cross Hatch - used for convergence tests on a monitor.
  2. Grey Scale - White to Black, used to set size and position on a monitor and to set the contrast and brilliance.
  3. Grey Scale - Black to White, again used to set size and position on a monitor and to set the contrast and brilliance.
  4. Black Level - produces a blank screen.
  5. Peak White - used to check colour purity on a monitor.
  6. Linearity - Five circles, a large one in the centre and one in each corner, all with crosshairs.
  7. Pulse and Bar - used to test frequency response in the transmission system.
  8. All - cycles through all of the above screens until a key is pressed.
This waveform shows one line of the Pulse and Bar screen – the Pulse is the short spike on the waveform and is used to monitor the high frequencies in the signal path and the Bar is the longer duration spike on the waveform and is used to monitor the lower frequencies

The first attempt at programming was in BASIC. Although all the procedures worked OK, some were very slow at drawing on the screen, namely the Cross Hatch, Grey Scales and the Pulse and Bar so they were converted to machine code to speed them up. Funnily enough, converting the Linearity procedure to machine code actually slowed it down!!

With the program written, it was converted into a ROM image and blown into an EPROM to make it practical to leave on site - no disc drives required and very easy to restart the program - *PG.

I have tried to make it as hard as possible to break the program. Its design and development was a combined effort by a colleague, Colin O'Brien and myself.

Derek Walker, EUG #64

Derek Walker