“Component video (YPbPr) output for the ZX Spectrum computer
General
The ZX Spectrum in its original form produces a radio frequency signal that can be fed into an analog TV. The quality of this signal is very poor, but this can be improved by a simple modification that will make the ZX Spectrum output a composite video signal instead. While this is actually better, it is nowhere near good. To bring the video quality to the next level, this modification will produce a component video signal (YPbPr) that should be as good as an analog signal can possible get. This video signal can be used either directly with a TV that has a YPbPr input, or it can be passed through a dedicated upscaler device (“Framemeister”, “OSSC”, etc.) before displaying on an HDMI screen.
This hardware modification is a more compact version of the solution that uses an A-Video board with seperate level shifters. It tries to solve various shortcomings of https://github.com/c0pperdragon/A-VideoBoard/tree/master/zxspectrummod :
Smaller and cheaper to make
No need to modify the case of the ZX Spectrum
No need to remove any components besides the RF modulator.
Theory of operation
The mod board passively listens to various digital data lines through which the ULA communicates with the rest of the main board - specifically the signals CAS,RW,IOREQ and the data bus. This information is processed by the FPGA to figure out what image the ULA intends to display. The FPGA drives a simple 3-channel digital-to-analog converter built from resistors to produce the 3 output voltages of the YPbPr signal. Because the main board runs with 5 volt logic levels and the FPGA can only work with 3.3 volts, a voltage regulator and two input level shifter ICs are necessary (on the bottom side of the mod board).
Video output possibilities
288p (default) or 576p
YPbPr (default) or RGsB
Output modes can be selected by jumpers or optional switches.
The color palette I am using has slightly softened colors. The original monochromatic colors are just too hard on my eyes especially when viewed on a modern display. I took the platte from [https://lospec.com/palette-list/zx-spectrum] and in my opinion this really looks great.”