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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Driving The Display

Apr 17, 2007

The previous testing hack of connecting the red RGB line to the TV through a resistor worked, but doesn't give very much voltage control, current control or protection.

as well as sending on/off RGB signals of a suitable voltage to the TV we also have a Bright mode which is common and affects all RGB signals simultaneously when active.

When Bright is enabled, the RGB voltage levels rise a small amount, but only if they are active. For instance, when generating bright red, only the red RGB line increases in voltage, the green and blue remain at 0v.

This cannot be done with a resistor network alone as we do not want to apply a 'bright' voltage offset to a line that should be at 0v. We would instead have to create Red-Bright, Blue-Bright and Green-Bright switching signals and use these individually. Also, this does not address current or protection issues.

Another approach is to use a transistor to drive the output. I favour this as we can control the drive current and protect our logic from stray voltages on the TV connection cable.

As we are not interested in any voltage amplification, but do want current amplification, we can use a common-collector/emitter-follower configuration.

We need to tune the resistors in the output of this circuit to set an appropriate drive current, and on the input to set an appropriate RGB drive voltage.

That is the next task...