Spectrum Computing

Or....

Go and have fun with it

Mark

Hello.

Just thought I'd update. The spectrum has been working great and I've been having lots of fun with it. I did the composite mod the other day, opting for the transistor based mod (base connected to composite, collector to 5v and emitter to the phono socket centre pin of the modulator). This works great.

I was curious though. the instructions on Retroleum say that this mod may not work on some TV's. I wondered what that meant?

I have noticed that when I hit the reset button on my Smart Card V2 and it returns to the SnapLoader I occasionally get screen wobble. It's not severe and switching the TV off and on cures it.

Thanks

Most of the composite modifications are inexpensive solutions. As such they are not exactly professional quality video circuits. The AC video signal level, the DC component of the signal and the impedance matching are not fully to the correct composite video signal specifications. But the output is near enough for most CRT TVs and some LCD TVs.

The other factor is the non-standard video signal that the ZX Spectrum produces in terms of timing and the actual video waveform only includes one field (the same field is constantly repeated) instead of two interlaced fields (two interlaced fields make up one video frame). Some LCD TVs don’t like this non-standard signal.

Because of the above reasons, sometimes you will get warnings saying that a composite modification / video lead may not work for all TVs.

If you have problems with a 1980s/1990s computer and a LCD TV, the normal recommendation is to try another TV. Better yet, if you have one, or can beg, borrow or steal one, try an old 14” or similar size CRT TV...

Mark

Thanks Mark.

To be honest, the picture is really good apart from a bit of moire here and there on dithered patterns. I'm super impressed. Just this occasional wobble which is easy to resolve, I assumed it might be a slight sync issue.

I'll have to try it on my little CCTV monitor at some point, but its wired for BNC (for the Beeb) and I've lost my phono adapter Can't have gone far.

With the 16K/48K/+ there is a design limitation with the video circuitry, in that two separate oscillators are used (the ULA oscillator/clock and the colour encoding chip oscillator). Whereas there all the video circuitry should all be referenced from a single oscillator.

The problem is that the two different oscillators are not synchronised. This causes colour bleed problems where you have a sharp vertical edge and different colours abutting.

Mark