UHF Circuit

The Screen signal Output from the ULA (Uncommitted Logic Array device) is literally an RGB output. Therefore, the included interface is in itself a seperate entity. To this end, the UHF circuit converts an RGB signal into a UHF signal. This may sound simple, until one considers that the four ULA outputs; Red, Green, Blue and Sync lines must be converted into one single UHF line.

In order to accomplish this, the three Colours are used to index a colour intensity array ROM. The output is then modulated with the sync output (Synchronising pulses to indicate End of line and End of Frame) before finally being sent to the UHF modulator.

All this converting takes its toll on the final output by losing a little of its original clarity.

The Aforementioned Sync line indicates two actions.

A short Negative pulse indicates the end to a line. The raster (Actual positional formation of the screen) scans across the screen and once it reaches the end, a short sync pulse is generated by the ULA. This triggers the TV to "Fly back" to the next line down. This repeats for the 625 lines until a long low pulse is generated by the ULA.

This longer pulse lasts for almost the amount of time to generate a whole line (40 micro-seconds) and triggers (tells) the TV to end the frame (Fly back to the top of the screen again). All this occurs within a fraction of a second.

The sync line is an intersting line to play around with. With Fabrice Frances help, a simple link can be established between the Sync output of the RGB port and the Cassette input of the Cassette port. It is found that by doing so, the line fly back trigger is not enough to generate a change in the cassette input line, but a Frame trigger is. Therefore, with this simple link, one can generate a screen refresh signal. Very useful for games to prevent sprite, scrolling and screen forming flicker.

Another use of the sync line would be to slightly delay the pulses. By doing so, the screen may be shifted to the left or right. With a set of 6 delays, one might even achieve Hardware smooth Scrolling!