The ZX Spectrum Reverse Engineering and Clone Desgin Blog


A site dedicated to the reverse engineering of the ZX Spectrum and related projects.

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The first TV test - Hooking up

Mar 29, 2007

The implementation at present could do nothing other than generate sync pulses. What I really wanted to do was plug it into my TV and get a picture!

So I wired up a SCART lead to provide RGB and CompositeSync signals, plus an RGB mode select signal (1-3v on SCART pin 16: as pin 16 has a 75R load, connecting +5v through a 100R resistor to pin 16 creates a suitable voltage).

As for the picture itself, I decided to activate the RED signal of TV RGB input whenever the video display signal is active (HC8 low) and Blank is inactive. We also need a composite sync signal which is easy to produce as (HSync + VSyncEn) • (VSync + VSyncEn), so whenever VSyncEn is low VSync is chosen, otherwise HSync is chosen.

NANDing HBlank and VSyncEn gives us an overall Blank signal (active high). This can be ORed with HC8 to create a signal which is low only during the main video display rectangle and not during the border or any sync signal. Blanking the video output during horizontal or vertical retrace may or may not be important depending on your TV - some get worried if you don't....

To hook up to the TV I inverted the active low display signal and connected it to the RED line through a 470R resistor to give at most a 0.7v signal, and the composite sync through a 1K resistor to give a 0.3v signal.

Nervously I plugged it into the TV and turned it on.....