RF / VHF / UHF signals are very tricky to work with, because they are radio frequencies. This range of frequencies (and higher frequencies) were chosen because they could be transmitted long distances through the air.r00t wrote:I don't get the point why is so difficult to convert VHF/UHF to composite, I remember back in the VCR era most has VHF/UHF input and composite output, I am right?
If you do it the traditional way, using analogue circuitry, when trying to design and construct electronics to operate at these frequencies, you have to use expensive, (and increasingly hard to find) transistors and diodes that have specifications that are suitable (low self capacitance and for the transistors, a respectable gain at high frequencies). Then you have to find suitable transformer cores, then wind your own windings (of wire) on the transformer former (core). You have to custom design a PCB. This PCB has to be very carefully designed. If any of the amplified signal gets back to the input, it will amplify that instead of the input signal and goes out of control (producing garbage). It's not easy designing and building good audio amplifiers using transistors. Let alone RF amplifiers and demodulators (where you have a RF amplifier stage, a mixer stage, a detector stage and a video amplifier stage).
If this all sounds like a black art, that's because it is. As digital television has replaced, or is replacing analogue television around the world, so there is less and less demand for products that do what you want. That's why its so hard to find anything suitable.
Add to that, the demand for digital / HDTV, and and as chip based digital processing has fallen in price, so these out compete analogue designs. But in general, the digital / HDTV products are designed for the last versions of the analogue standards using composite signals.
Also, as TV continues to be driven in a quest for better picture quality, keep in mind that analogue RF/VHF/UHF signals are very low quality compared with "standard definition" digital TV picture quality.
The best that a ZX81 / TS1000 can do (after modification), is monochrome composite video (a cleaned up monochrome composite signal is much better than RF/VHF/UHF). Or with a more expensive external interface (the Chroma interface) it can do TV standard RGB (great for TVs used in Europe where RGB SCART sockets are available on most TVs, not so great for elsewhere in the world).