zx81 rf output

wozname

Hi All

I'm not sure if this is an appropriate forum for the zx81, however I'll ask anyway;)

I've connected my zx81 up to a mid 90's color tv. I can tune it in and I can get a fairly clear picture. However it is very dark almost black. The text and K cursor have a bit of whiteness around them that fades off toward the right.

I can read everything but it is pretty ugly. I remember the screen as being white with black text, the last time I used it (literally about 25 years ago).

Is it using some dodgy way of producing the picture that doesn't work with modern tvs?

Cheers

rwap

The ZX80/ ZX81 have their own dedicated forums at:

http://www.rwapservices.co.uk/ZX80_ZX81/forums/

However, the issue you describe is due to the ULA used in the ZX81 - there was a later ULA which fixed this issue, although it is nearly impossible to find now (and most ULAs on the ZX81 are soldered directly to the motherboard, so not an easy fix).

Alien 8

I wonder if the composite video mod could be done on the ZX81, and if so, if that would help.

gtsamour

Theoretically it wouldnt help because the black notch still wont be in the output signal causing a dark screen..
The composite mod is possible and its the same as on the Spectrum, i've done it on ZX81 and Timex1000.

wozname

This auction of a zx81 has a picture of the display that is exhibiting the symptoms I was describing. Mine is a bit worse, I should try and get a photo sometime.

http://www.trademe.co.nz/Computers/Vintage/auction-345961369.htm

I don't know if the composite mod would help, I vaguely remember that the zx81 used the z80 itself to generate the video signal. Thus the reason you could run it in "slow" mode and see a display or in "fast" mode with no display. I suspect I would just get the same image but coming through the composite plug instead of the rf plug. In other words, if the underlying image is screwed, it doesn't matter how you transmit it.

I suspect that the old tvs were just more accepting of screwed up input signals. I wish I still had my old black and white 1980s set so I could prove this theory.

Thanks for all your help

Cheers

wozname

I've just gotten round to checking out the zx81 forums hosted by rwap. There seems to be a lot of info about this problem, something about a "back porch". I have some reading to do. :)

Since I don't want to mod my zx81, I'm hoping maybe there is a postprocessing solution. I will probably have to do the composite mod though, in order to get a signal to postprocess.

Cheers

Jose_Leandro

Hi

I wrote an article where you can read about this problem ( sorry, only in spanish)

http://trastero.speccy.org/cosas/JL/ZX81-ULA/ZX81-video.html

The "blacking level" was not implemeted in first ZX81 ULA ( ULA 2C184). The "good" ULA is the 2C210 model.

There are some solutions (sorry again, only in spanish)

http://trastero.speccy.org/cosas/JL/zx81%282%29/ZX81-video.html

The first circuit add "blacking level" to zx81 video signal.
Other solution is invert the video signal ( black screen and white cursor ). I put some example of invert circuit

Bye

Jos? Leandro

RetroTechie

Alien 8 wrote: »

I wonder if the composite video mod could be done on the ZX81, and if so, if that would help.

Yes it's possible, but beware that the video signal which goes into the modulator isn't transistor-buffered like it is on the Spectrum; IIRC it comes directly from the ULA.

Routing that directly to the outside world is a good way to kill the ULA's video output. It does work though: last time I tried, it gave a sharp but low-contrast (greyish-black) image.

Better is to replace modulator circuitry with a transistor buffer like this: www.zx81.de/english/video_e.htm

My TV also preferred a capacitor in the signal (+ to transistor output, - to center pin of outgoing connector), which may not be necessary. I used 47uF, better is 100uF (or more?). On my ZX81's, image is a bit less sharp this way (compared with straight ULA -> comp. video input on TV), and sometimes there's a little distortion depending on screen contents (big black & white bands causing the border to become non-linear in places), but overall the image is very good and black & white levels are excellent / as they should be.

Above transistor buffer (+ capacitor if used) fits comfortably inside the modulator case.

Pilsener

I don't have a ZX81, but I still wonder: Is the ULA defective in this case ?

gtsamour

Bugged yes, defective no since it was performing like this from the moment it was made.

RetroTechie wrote: »

Better is to replace modulator circuitry with a transistor buffer like this: www.zx81.de/english/video_e.htm

My TV also preferred a capacitor in the signal (+ to transistor output, - to center pin of outgoing connector), which may not be necessary. I used 47uF, better is 100uF (or more?). On my ZX81's, image is a bit less sharp this way (compared with straight ULA -> comp. video input on TV), and sometimes there's a little distortion depending on screen contents (big black & white bands causing the border to become non-linear in places), but overall the image is very good and black & white levels are excellent / as they should be.

Above transistor buffer (+ capacitor if used) fits comfortably inside the modulator case.

I double that....

I used this cirquitry with a 100uF capacitor to the video out. Excellent picture.

RetroTechie

Pilsener wrote: »

I don't have a ZX81, but I still wonder: Is the ULA defective in this case ?

gtsamour wrote: »

Bugged yes, defective no since it was performing like this from the moment it was made.

Neither - the ZX81 ULA's video output pin was never meant to drive a 75 Ohm load (and rightly so, it only needs to drive the modulator input with a few cm. wire in between). Do that anyway, and whatever results is your problem... ;)

gtsamour

No problems so far... and somehow I doubt there is a possibility to encounter any...

I was wondering if I could also use the same cirquit board to get composite signal on a QL, since the QL uses the same modulator kit as the ZX Spectrum and the ZX81.
Yes I know the QL has an RGB out, made a cable for that too but I would like to have the composite out as an easy and fast connection on a tv option.

wozname

Thanks very much for the clear answers to my question. Especially after reading Jose's answer (even translated from spanish), I finally understood the problem.

I found another back porch generator circuit at Paul Farrow's site:
http://www.fruitcake.plus.com/Sinclair/ZX80/VideoGeneration/ZX80_BackPorchGeneration.htm