Sinclair ZX80 / ZX81 / Z88 Forums

I've recently acquired a zx81 (rebranded as ts1000) with a 210E ULA
After doing the composite video mod it works well with my Samsung TV, however I'd like it to work with a smaller screen (https://www.amazon.com/gp/product/B01BE ... UTF8&psc=1)

It gets signal, but doesn't resolve to a good picture like the Samsung.
Instead I get diagonal lines, blinking, and blank screens.

I've used my oscilloscope to take a look - but I'm very new to reading composite video signals.
It certainly has a back porch. But seems to be missing the front porch.
The ratios seem to be off. And of biggest concern, the VSync signal seems to be one lone (about 375us) low rather than pulses.

Is this "normal" for a zx81 ?

Yes, this is normal for a ZX81/TS1000.

Sinclair designed these to work with monochrome (black and white) CRT television sets. CRT sets were completely analogue, so you can get away with a video signal that does not meet the broadcast television standard. Further, the vertical (field) synchronisation standard changed during the lifetime of the CRT technology.

CRT sets were designed to be as tolerant as possible in part because signal fade and signal reflections otherwise would screw up the picture.

LCD displays are often far less tolerant of non compliant video signals. And the ZX81/TS1000 do not produce a video signal that meets the broadcast specification.

For a start, they only output one field, then the exact same field again. Whereas a broadcast television signal is made up of two different interlaced fields (sometimes called the even and odd fields), that make up one frame.

And as you have noticed, there is no front porch. In the first two ULA versions, there was no back porch either. And the simplest vertical (field) synchronisation signal is generated rather than the type found in a broadcast specification signal.

It should also be noted that a lot of these 7” LCD sets intended for use in vehicles don’t actually display the full vertical resolution. I tried one on an Acorn BBC Micro and the displayed text looked horrible, as the 7” LCD set missed out some of the lines.

I laughed at the so call technical detail in that Amazon listing. Yeah, what technical details

In the linked to pdf it says:

Technical Specs:
TFT/ LCD Active Matrix Display Screen Aspect Ratio: 16:9
Video System: PAL/NTSC
Resolution: 800x480
Power Supply: DC 12V

Compare that with the original 4:3 analogue systems:

The U.K. TV system is known as ITU “system I” (I being the uppercase letter i), this is a 625 line, 15625 Hz line rate, 50 Hz field rate (25 frames a second) video standard. This is for a monochrome video signal. This system usually has 576 visible lines.

To add colour, a colour encoding system adds colour information. This can be PAL or the NTSC colour system. Most countries that used 625 line systems chose PAL. Hence in the U.K. to fully identify it, use PAL-I.

The USA 525 line, 15734 Hz line rate, 60 Hz field rate (29.97 [30] frames a second) video standard [ITU “system M”] uses the NTSC colour encoding system (NTSC-M). This system usually has 486 visible lines.
But it is possible to use no colour encoding, or PAL colour encoding (Brazil uses/used PAL-M for example).

Mark

Having refurbished quite a few ZX81s I have had mixed results with various TVs. One TV will produce a perfect picture and another won’t. I guess some are more picky about signal quality than others. I’m sure someone will come along with a scientific explanation of that!
I use these composite mods, the ZX8-CCBs: https://www.sellmyretro.com/offer/detai ... -zx81-3712
Currently not for sale (not a surprise) but well worth it when they are - the output is really crisp and good but some adjustment tends to be required.
There is also the ZXVid:
https://www.sellmyretro.com/offer/detai ... it%29-2338
More expensive but no adjustment required. It has a slightly more mellow output.

Lardo - see my edited post above

It's good to know that the signals look "normal"
I was really worried about the lack of pulses in the vsync


I've taken a look at the schematics for the CCB and was wondering if it would help or not. On my Samsung TV the signal is already clearer than my RF childhood memories. But that may be due to post processing. Maybe a little clean up will help the LCD work.

I've got the spare parts around to make one and try it out.
Since I'm already using a modified composite signal it would be easy enough to breadboard and see.

I have a Samsung giving me headaches.. I see the picture shifting from quite stable, to noisy, then suddend interruptions, and so on I wonder if it is just the thin cable or the SCART adapter (direct compoisite feed).
I tuned the CCP on a SONY Bravia and it worked well (I'll try on it again in the next days)

Partial loss of sync may be the cause. If you are using a ZX8-CCP you may have to adjust it slightly.

Or am I misunderstanding you?

Mark

Correct, I have a CCP circuit, I just tested on the SONY and it is much better on it
I'm not very fond on tuning though because I have the original membrane with very short ribbons .. and I'm not sure i can open it again.

I must add: thankyou "ZX80 Paul" for the circuit!
..and for having pushed me in adding a bit of ZX80 related code to Z88DK

finally I think I found the ptoblem.. in the video cable