Video problems when running games and... I am new

[bola_dor]

RGB SCART uses three channels to carry colour with no separate monochrome signal. The channels are called red, green and blue to correspond with the three election guns in a colour CRT TV.

It is possible to convert from RGB SCART to composite video, but it takes rather a lot of electronics, so in your case is not a realistic option.

Video component is normally colour difference signals along with a monochrome video channel. So very different from RGB SCART.

VGA is a RGB system, but unless you have one of the rare monitors that can work at TV frequencies, it’s not a lot of help. Nearly all VGA monitors will only work with VGA specification video signals.

Mark

What about this kind off adapter, won't this work?
https://www.retrogamingcables.co.uk/swi ... eo-adapter

Thanks again!!

[1024MAK]

What about this kind off adapter, won't this work?
https://www.retrogamingcables.co.uk/swi ... eo-adapter

Those and similar devices just break out the appropriate SCART connections into separate connectors. They contain no electronics and are only useful if the SCART connection is actually carrying composite video or S-Video signals.

SCART can carry either:

• composite video
• S-Video
• RGB Video
• Component video

The following conditions apply:

• Both devices have to support the type of signal. The default is composite video.
• The SCART cable and connectors need to have the appropriate wires, connections and pins present (so for anything better than composite video, that normally means a ‘fully wired’ cable).
• It cannot carry RGB and S-Video at the same time. It can’t carry RGB and component video at the same time. It can’t carry S-Video and component video at the same time.
• Some devices (mainly home computers) don’t provide a composite signal on the same connector as the RGB video signals, hence the RGB SCART lead will ONLY contain RGB signals.

The primary purpose of the Chroma interface is to produce a colour RGB video signal. I can’t remember if it provides a monochrome composite video signal. But it does NOT have PAL or NTSC colour encoder circuitry, so it will NOT produce a colour composite video signal on its SCART socket.

I hope that helps.

Mark

[1024MAK]

If your machine does not produce a video signal that includes the ‘back porch’ part of the video signal, my recommendation would be to try one of PokeMon’s ZX8-CCB devices. They are available here. These also have the advantage of producing a nice clear clean and sharp video output. I use them myself.

Otherwise, a ZX81 Video Transistor Buffer Amp is a good low cost option. It does not clean up the signal. It just electrically buffers the signal.

The ‘back porch’ part of the video signal is not needed for most monochrome / black and white CRT TVs. But it is something that most colour televisions require. This part of the video signal allows the TV to work out the correct reference signal level for showing black on the screen. But the first two versions of the Sinclair ULA did not generate this part of the video signal. The result of this part of the signal not being present is a very dark picture and the user having to turn up the brightness and contrast controls to near maximum.

Mark

[1024MAK]

Loss of synchronisation on the the ZX80, ZX81, TS1000 and the various other clones can be down to a number of reasons.

These machines use the CPU (with help from the glue logic or ULA) to generate the correct signal for the video output. In simple terms, the video is software generated. When running BASIC (or using the ROM code to generate the picture), on most machines, BASIC (the ROM code) checks to see if the machine is configured for a 50Hz TV system or a 60Hz TV system. It then generates the appropriate timing when producing the signal for the display. Not all games do this. Some are coded only for 50Hz. Maybe some of our USA members can help with information on any games that they know of that have this problem.

Some TVs sold in areas of the world that use 60Hz television will actually work at 50Hz. But not all.

The second problem, is that using a simple transistor buffer to provide composite video does not work on every TV / monitor. The voltage levels are not near enough to the normal composite video levels (as expected by the TV). There are varying levels of leeway between different designs of TVs. Some some will work, while others are not happy.

So my advice is: do you have any other TVs you can try your machine on?

Mark

[bola_dor]

What about this kind off adapter, won't this work?
https://www.retrogamingcables.co.uk/swi ... eo-adapter

Those and similar devices just break out the appropriate SCART connections into separate connectors. They contain no electronics and are only useful if the SCART connection is actually carrying composite video or S-Video signals.

SCART can carry either:

• composite video
• S-Video
• RGB Video
• Component video

The following conditions apply:

• Both devices have to support the type of signal. The default is composite video.
• The SCART cable and connectors need to have the appropriate wires, connections and pins present (so for anything better than composite video, that normally means a ‘fully wired’ cable).
• It cannot carry RGB and S-Video at the same time. It can’t carry RGB and component video at the same time. It can’t carry S-Video and component video at the same time.
• Some devices (mainly home computers) don’t provide a composite signal on the same connector as the RGB video signals, hence the RGB SCART lead will ONLY contain RGB signals.

The primary purpose of the Chroma interface is to produce a colour RGB video signal. I can’t remember if it provides a monochrome composite video signal. But it does NOT have PAL or NTSC colour encoder circuitry, so it will NOT produce a colour composite video signal on its SCART socket.

I hope that helps.

Mark

You made it very clear to me.. thanks!!

[bola_dor]

If your machine does not produce a video signal that includes the ‘back porch’ part of the video signal, my recommendation would be to try one of PokeMon’s ZX8-CCB devices. They are available here. These also have the advantage of producing a nice clear clean and sharp video output. I use them myself.

Otherwise, a ZX81 Video Transistor Buffer Amp is a good low cost option. It does not clean up the signal. It just electrically buffers the signal.

The ‘back porch’ part of the video signal is not needed for most monochrome / black and white CRT TVs. But it is something that most colour televisions require. This part of the video signal allows the TV to work out the correct reference signal level for showing black on the screen. But the first two versions of the Sinclair ULA did not generate this part of the video signal. The result of this part of the signal not being present is a very dark picture and the user having to turn up the brightness and contrast controls to near maximum.

Mark

I don't think back porch is missing.. actually I had a clear image with the RF modulator and now I kind of regret about modifying it to the single transistor + resistor arrangement.. image now is sharper but I have this problem I think is something about Sync..
I saw your circuit schematic, as I don't know much about electronics (just copy and solder) would any 100uf 16v capacitor work?
Thanks!!!

[bola_dor]

Loss of synchronisation on the the ZX80, ZX81, TS1000 and the various other clones can be down to a number of reasons.

These machines use the CPU (with help from the glue logic or ULA) to generate the correct signal for the video output. In simple terms, the video is software generated. When running BASIC (or using the ROM code to generate the picture), on most machines, BASIC (the ROM code) checks to see if the machine is configured for a 50Hz TV system or a 60Hz TV system. It then generates the appropriate timing when producing the signal for the display. Not all games do this. Some are coded only for 50Hz. Maybe some of our USA members can help with information on any games that they know of that have this problem.

Some TVs sold in areas of the world that use 60Hz television will actually work at 50Hz. But not all.

The second problem, is that using a simple transistor buffer to provide composite video does not work on every TV / monitor. The voltage levels are not near enough to the normal composite video levels (as expected by the TV). There are varying levels of leeway between different designs of TVs. Some some will work, while others are not happy.

So my advice is: do you have any other TVs you can try your machine on?

Mark

Yes.. I 'very tried on other two LCD TV but only got a " no signal" message.. ..
In my country air TV is/was PAL-N (60i) but any other input as DVD players or consoles are NTSC.. so TV SETS usually support both 50 and 60 Hz signals.. my CZ1000 is indeed a TS1000 with a sticker over the timex original one ...
The two games I tried and did have the problem with
composite tv and LED TV , worked well with the original hardware on an old BW CRT TV..(I don't have it any more)
Any suggestions are welcome.. thanks!!!

[Lardo Boffin]

What about a HDMI upscaler? I have an Atari ST and on one of my tellies the picture rolls up the screen endlessly when using SCART. When I plug the SCART into a SCART to HDMI upscaler the picture is fine.

[1024MAK]

I don't think back porch is missing.. actually I had a clear image with the RF modulator and now I kind of regret about modifying it to the single transistor + resistor arrangement.. image now is sharper but I have this problem I think is something about Sync..
I saw your circuit schematic, as I don't know much about electronics (just copy and solder) would any 100uf 16v capacitor work?

I also I don't think the back porch is missing. As far as I am aware, most TS1000 machines use the later ULA that produces a back porch. But I thought it was worth mentioning just in case.

Any 100uF 16V electrolytic capacitor should be fine. But it is important to connect it with the correct polarity. The value is not critical, the circuit should work with a 47uF 16V type as well.

As these computers don’t produce colour, PAL and NTSC standards are in theory irrelevant, as they are colour TV encoding standards. CCIR 625/50 and EIA 525/60 (line count and field rate) are the proper names for the monochrome baseband (composite) video standards. Alternatively the ITU standards can be used, for example the monochrome U.K. TV standard is System I, that’s 625 line, 15625Hz/50Hz. However, the options shown on screen (if relevant) and in the user manual / instructions of TV and other consumer video equipment often use PAL, PAL 60, NTSC or similar. Sometimes they use PAL I, PAL B/G etc. More information here and here.

And of course, most people outside of TV engineering often just use PAL to mean any 625/50 system and NTSC to mean any 525/60 system.

As far as using these computers is concerned, the USA models of ZX80, ZX81, TS1000 etc should work with TVs capable of showing 525/60 (which is normally NTSC) video.

The European (U.K., Germany etc) versions (ZX80, ZX81) should work with TVs capable of showing 625/50 (which is normally PAL) video.

With the ZX81 / TS1000 / CZ1000, the only differences on the board that is relevant to the video system are:

• R30 (10Ω) is fitted to select 525/60 (and hence this resistor is often called “USA”) but left off for European boards (the ULA pin is called USA/UK). Where a TS1000 has found its way into Europe, you often find someone has removed R30...
• Different modulators are used, and the connections are different
• French ZX81 models have some extra components as they use a different video system.

So if you can get a stable display when using BASIC, I find it strange that your TV loses lock when playing games that you are sure worked fine when you used the RF output. One point that may be worthwhile mentioning is this: some TVs support more TV standards when receiving via RF than they do when connected via a direct composite video connection.

I do think it is worthwhile trying a ZX8-CCB. The output from this should be a better signal (closer to the correct voltage levels) than you get from a simple transistor buffer.

Mark

[bola_dor]

What about a HDMI upscaler? I have an Atari ST and on one of my tellies the picture rolls up the screen endlessly when using SCART. When I plug the SCART into a SCART to HDMI upscaler the picture is fine.

I'm going to try that.. thanks