Which video mod for composite?

Discussions about Sinclair ZX80 and ZX81 Hardware
Dukes909
Posts: 61
Joined: Thu Dec 31, 2020 11:21 am

Re: Which video mod for composite?

Post by Dukes909 »

1024MAK wrote: Sun Oct 03, 2021 8:04 pm Lack of stability indicates that the TV is not happy with the sync part of the signal.

You could try adding one or two silicon signal diodes (such as 1N4148 or1N914) between the input from the ULA and resistor R1. The anode (a) end goes towards the ULA. The cathode (k) goes towards resistor R1. If one does not work, try fitting two in series.

Alternatively try fitting a 100Ω resistor between the phono centre (signal) pin and 0V/GND.

The weird colours are due to the colour decoder getting confused and misinterpreting the signal. I’m not sure what to suggest here, as this is much less common in PAL land.

Mark
Thanks i have a bag of 1N4148's somewhere I will try one or two.
Dukester
Dukes909
Posts: 61
Joined: Thu Dec 31, 2020 11:21 am

Re: Which video mod for composite?

Post by Dukes909 »

An update:
I added one 1N4148 and then 2 in series. First tried this with the 1000 connected to a JVC LCD TV/monitor. Made no difference in stability of the image. Then tried a 100Ω resistor between the RCA (signal) pin and 0V/GND...no dice there either. I then tried connecting with the single diode in place to my even older Gateway FPD2185W - a much better image but the stability was still not there with video blanking out every so often plus a lot of "fluid" motion like watching the screen as if it were underwater.

Then tried an RCA "TruFlat" (CRT) TV that had composite inputs. A stable picture on it compared to the 2 LCD's but much grainier. I would settle for this but this TV is too big to use on my bench with the Timex Sinclair so I will have to find another LCD monitor that fits better or find another solution.

Does anyone know if the TFW8B Atari 2600 mod works well on the Sinclairs?
Screenshot at 2021-10-05 20-47-43.png
Dukester
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Which video mod for composite?

Post by 1024MAK »

Dukes909 wrote: Wed Oct 06, 2021 3:52 am Does anyone know if the TFW8B Atari 2600 mod works well on the Sinclairs?
The TFW8B COMPOSITE VIDEO MOD V2.1 - ATARI 2600/7800 is a similar circuit, so if my design does not work, it’s likely that this will produce similar results.

Part of the problem is that the video signal from the ZX81/TS1000 is not to the broadcast standard. The computers were designed for use with black and white (monochrome) CRT televisions of the day. These being totally analogue designs, they were much more forgiving if the timing or signal levels were not quite right. Further more, most would work if the synchronisation signal included in the video signal was to the old standard. The ZX81/TS1000 video generation design only produces the bare minimum in terms of video signal complexity in order to work with black and white CRT televisions.

LCD televisions and monitors work completely differently. They sample the analogue video input signal, and convert it to a digital signal. Then it’s processed. It’s examined to find the sync part of the signal. The picture information goes to a frame buffer memory, so that the correct information can be supplied to the matrix grid of the LCD pixels.

Generally speaking, they are looking for a video signal that is to the established broadcast standard (the analogue standard that was current before analogue TV was switched off). Different designs of TV vary enormously in which 1980s computers they will work with.

You appear to have been unlucky in that you have two TVs that are not happy and won’t work.

Unfortunately it’s the luck of the draw on what works and what does not.

The grainy picture is possibly noise in the signal. Pokemon’s design fixes this, but he is as far as I know not producing them at the moment. And we don’t know if this will change.

It may be possible to improve the picture by using the brightness and contrast controls.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Lardo Boffin
Posts: 2160
Joined: Sat Nov 26, 2016 2:42 am

Re: Which video mod for composite?

Post by Lardo Boffin »

Dukes909 wrote: Wed Oct 06, 2021 3:52 am
Does anyone know if the TFW8B Atari 2600 mod works well on the Sinclairs?

Dukester
I have tried this a couple of times and it worked ok on one TV but not very well on the other three I tried.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Dukes909
Posts: 61
Joined: Thu Dec 31, 2020 11:21 am

Re: Which video mod for composite?

Post by Dukes909 »

1024MAK wrote: Wed Oct 06, 2021 6:50 am
Dukes909 wrote: Wed Oct 06, 2021 3:52 am Does anyone know if the TFW8B Atari 2600 mod works well on the Sinclairs?
.

You appear to have been unlucky in that you have two TVs that are not happy and won’t work.

Unfortunately it’s the luck of the draw on what works and what does not.

Mark
Well, fiddle faddle. Thanks again for the extra detailed info Mark. It's amazing how little I knew about these machines back when I was learning BASIC and then assembly language on them. I saw a couple of monitors at a thrift store a week or two ago; going back to see if they are still there and have composite inputs.
Dukester
Dukes909
Posts: 61
Joined: Thu Dec 31, 2020 11:21 am

Re: Which video mod for composite?

Post by Dukes909 »

Lardo Boffin wrote: Wed Oct 06, 2021 11:04 am
Dukes909 wrote: Wed Oct 06, 2021 3:52 am
Does anyone know if the TFW8B Atari 2600 mod works well on the Sinclairs?

Dukester
I have tried this a couple of times and it worked ok on one TV but not very well on the other three I tried.
Ok thatnks for that info.
Dukester
User avatar
BarryN
Posts: 151
Joined: Thu Nov 09, 2017 11:34 pm

Re: Which video mod for composite?

Post by BarryN »

1024MAK wrote: Wed Oct 06, 2021 6:50 am Part of the problem is that the video signal from the ZX81/TS1000 is not to the broadcast standard. The computers were designed for use with black and white (monochrome) CRT televisions of the day. These being totally analogue designs, they were much more forgiving if the timing or signal levels were not quite right. Further more, most would work if the synchronisation signal included in the video signal was to the old standard. The ZX81/TS1000 video generation design only produces the bare minimum in terms of video signal complexity in order to work with black and white CRT televisions.
Mark
The 210E version of the ZX81 ULA is pretty compliant with the standards. Is this a UK/PAL ZX81 or a US NTSC unit? Maybe part of the issue on some of the TVs/monitors is a PAL 50hz vertical refresh rate.

50Hz/60Hz Refresh Rate Jumper
Even though the display refresh rate is actually software based, the system determines the local desired rate by reading from I/O port FEh. In the ZX81, the setting depends on whether Pin 22 of the Ferranti chip is shortcut to ground or not.
When it is grounded, by a 0 Ohm "resistor" called R 30, then vertical blanking time is reduced for 60Hz timing - note that this heavily reduces the user-available CPU time in SLOW mode.

The wavy pattern may be due to interference from the computer. Make sure you are using a good quality video cable with with good shielding.
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Which video mod for composite?

Post by 1024MAK »

BarryN wrote: Wed Oct 06, 2021 7:20 pm The 210E version of the ZX81 ULA is pretty compliant with the standards.
It’s better, but I would not describe it as pretty compliant.
BarryN wrote: Wed Oct 06, 2021 7:20 pm 50Hz/60Hz Refresh Rate Jumper
Even though the display refresh rate is actually software based, the system determines the local desired rate by reading from I/O port FEh. In the ZX81, the setting depends on whether Pin 22 of the Ferranti chip is shortcut to ground or not.
When it is grounded, by a 0 Ohm "resistor" called R 30, then vertical blanking time is reduced for 60Hz timing - note that this heavily reduces the user-available CPU time in SLOW mode.
I believe the normal value of R30 is 10Ω when fitted. Although don’t think using a wire link would cause any problems.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Dukes909
Posts: 61
Joined: Thu Dec 31, 2020 11:21 am

Re: Which video mod for composite?

Post by Dukes909 »

BarryN wrote: Wed Oct 06, 2021 7:20 pm
The 210E version of the ZX81 ULA is pretty compliant with the standards. Is this a UK/PAL ZX81 or a US NTSC unit? Maybe part of the issue on some of the TVs/monitors is a PAL 50hz vertical refresh rate.
US NTSC
BarryN wrote: Wed Oct 06, 2021 7:20 pm The wavy pattern may be due to interference from the computer. Make sure you are using a good quality video cable with with good shielding.
Are there shielded RCA cables? The ones I have used in my ham radio stuff for digital modes are unshielded and I wasn't aware of any other kind. (When I think of shielded, I am thinking of plenum type coax or CAT 6 cabling.)
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Which video mod for composite?

Post by 1024MAK »

Shielded or braided screened co-ax cables are available with RCA / phono connectors. Although it’s not possible to tell on most pre-made cables with moulded connectors.

Look for cables specifically described as high quality video cables.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Post Reply