To explain further: The internal composite signal of the ZX81 (before it hits the RF modulator) has a poor "back porch"[1] signal, which prevents a lot of digit

To explain further: The internal composite signal of the ZX81 (before it hits the RF modulator) has a poor "back porch"[1] signal, which prevents a lot of digital TVs from showing a steady image of the ZX81s screen. The composite mod for the ZX81 rectifies this, so the composite output can be feed into either a modern TV that still has a composite input, or an inexpensive composite-to-HDMI converter. --- [1] https://en.wikipedia.org/wiki/Horizontal_blanking_interval https://en.wikipedia.org/wiki/Horizontal_blanking_interval