Uploaded May 2013 | Updated September 2026, 1 week ago
My father made this ZX81 himself, from scratch, in the early 80s. It was my first computer. I recently tried to hook it up to my LCD TV, but I could not get the TV tuned in to the modulator signal properly (the ZX81 only outputs RF-modulated video).
I could see the output of the computer, it would just not sync properly. I thought that after all this time, the thing may just be somewhat broken, although it looked like the computer itself still worked.
Thrashbarg told me that it may be related to the 'back porch' part of the signal, which is used to re-set the black values at each scanline. Because of cost-saving, the ZX81 does not generate a proper back porch signal. On old TVs (especially black and white) this was not a problem, but modern TVs are more sensitive.
There are some modifications available which insert a proper back porch into the signal. However, when looking around, I also saw the tip of using an old VCR between the computer and the TV. The VCR would demodulate the signal, and have no problem dealing with the back porch issue. Then it would send a reconstructed signal to the TV, with proper back porch, so the TV can display it.
Since I still had an old VCR around, I figured I'd give it a try. And indeed, it works!
My father made this ZX81 himself, from scratch, in the early 80s. It was my first computer. I recently tried to hook it up to my LCD TV, but I could not get the TV tuned in to the modulator signal properly (the ZX81 only outputs RF-modulated video).
I could see the output of the computer, it would just not sync properly. I thought that after all this time, the thing may just be somewhat broken, although it looked like the computer itself still worked.
Thrashbarg told me that it may be related to the 'back porch' part of the signal, which is used to re-set the black values at each scanline. Because of cost-saving, the ZX81 does not generate a proper back porch signal. On old TVs (especially black and white) this was not a problem, but modern TVs are more sensitive.
There are some modifications available which insert a proper back porch into the signal. However, when looking around, I also saw the tip of using an old VCR between the computer and the TV. The VCR would demodulate the signal, and have no problem dealing with the back porch issue. Then it would send a reconstructed signal to the TV, with proper back porch, so the TV can display it.
Since I still had an old VCR around, I figured I'd give it a try. And indeed, it works!
