Spectrum 48k Image quality.

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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flaviosr
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Spectrum 48k Image quality.

Post by flaviosr » Sat Aug 11, 2018 12:07 pm

Dear All,

still working on my Sinclair collection I was testing all cassettes to see which are good and which not.
Image quality is not so good (ghosting, pixels not so clear) using my "old" LG CRT but I though the issue is with TV.
Then I switched to my ZX81 and... MIRACLE!!! Better than on a LCD video!!!
I checked another Spectrum (in which I have to replace the membrane, so I only have been able to check the starting screen quality) and it seems the quality is better.
So the question is: is there a way to improve image quality (The cable is the original TV one. And anyway the same cable I also used with the ZX81)? Something to replace on the mobo? Other ideas?

Thank you
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1024MAK
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Re: Spectrum 48k Image quality.

Post by 1024MAK » Sat Aug 11, 2018 5:52 pm

Okay, so before talking about problems with the ZX Spectrum, it's worth pointing out that with a RF (UHF) or composite video signal, the monochrome picture (containing no colour information, as produced by ZX80, ZX81 etc.) will always be sharper and clearer compared to a colour video signal. This is because of the way that colour information is added to the monochrome picture information. The frequency range of the monochrome picture information partly overlaps that of the colour information. So sometimes the television cannot tell which is colour information and which is picture brightness/contrast information. This results in incorrect colours being displayed where there is fine detail (such as the division between different colour pixels). This happens even if the actual picture is in black and white if the source of the video has the colour encoder circuitry enabled...

The 16K/48K/plus ZX Spectrums have a design fault which means that the frequency of the monochrome and synchronisation signals is not locked to the colour encoding frequency. This causes some of the displayed colours to be 'unstable'.

The modulator is a purely analogue design and is unlikely to be producing a RF signal at the exact TV channel frequency. Some TVs don't like this, even if they have a fine tuning control. Modulators may also not be functioning as well as when first manufacturered. 35 years is a long time for such analogue devices.

Issue 1 and issue 2 boards also have to have the colour encoding circuitry manually calibrated (later versions use an automatic system). This results in strange colour shades instead of the correct colours being displayed. There are two ('preset') variable resistors that need adjusting.

In addition, ageing electrolytic capacitors causing more noise on the picture, combined with owners greater expectations (due to improvements in television, and video consumer electronics products) means the video output of a Spectrum does not look very good on modern large screen sets (compared to the relatively small CRT screens of the 1980s).

So here follows the usual suggestions...
Use a SPECTRA so you have RGB SCART output
Use the ZX-HD HDMI adaptor/interface
Convert to a composite video output
Consider renewing the old electrolytic capacitors

Mark
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flaviosr
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Re: Spectrum 48k Image quality.

Post by flaviosr » Sun Aug 12, 2018 12:37 pm

1024MAK wrote:
Sat Aug 11, 2018 5:52 pm
Okay, so before talking about problems with the ZX Spectrum, it's worth pointing out that with a RF (UHF) or composite video signal, the monochrome picture (containing no colour information, as produced by ZX80, ZX81 etc.) will always be sharper and clearer compared to a colour video signal. This is because of the way that colour information is added to the monochrome picture information. The frequency range of the monochrome picture information partly overlaps that of the colour information. So sometimes the television cannot tell which is colour information and which is picture brightness/contrast information. This results in incorrect colours being displayed where there is fine detail (such as the division between different colour pixels). This happens even if the actual picture is in black and white if the source of the video has the colour encoder circuitry enabled...

The 16K/48K/plus ZX Spectrums have a design fault which means that the frequency of the monochrome and synchronisation signals is not locked to the colour encoding frequency. This causes some of the displayed colours to be 'unstable'.

The modulator is a purely analogue design and is unlikely to be producing a RF signal at the exact TV channel frequency. Some TVs don't like this, even if they have a fine tuning control. Modulators may also not be functioning as well as when first manufacturered. 35 years is a long time for such analogue devices.

Issue 1 and issue 2 boards also have to have the colour encoding circuitry manually calibrated (later versions use an automatic system). This results in strange colour shades instead of the correct colours being displayed. There are two ('preset') variable resistors that need adjusting.

In addition, ageing electrolytic capacitors causing more noise on the picture, combined with owners greater expectations (due to improvements in television, and video consumer electronics products) means the video output of a Spectrum does not look very good on modern large screen sets (compared to the relatively small CRT screens of the 1980s).

So here follows the usual suggestions...
Use a SPECTRA so you have RGB SCART output
Use the ZX-HD HDMI adaptor/interface
Convert to a composite video output
Consider renewing the old electrolytic capacitors

Mark
Ok. More than clear. I like Spectra since it is more complete but SCART is older (and HDMI can be easily converted to everything!) and Spectra is missing a case... ZX-HD HDMI is my right choice! And I also discovered the SD reader.
I was thinking about a Spectrum 128k since it has RGB port but they are sooooo expensive, and I do not like so much the +2 and +3 versions since they are missing the label on keys! :(
I want to make two 8bit areas: one Commodore (C128D was my choice with SD1IEC and Tapuino) and one with a Spectrum zone! Also this last is going to be finished! :) I have only to find a way to connect the ZX-HD HDMI to a 1901 monitor! :D
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flaviosr
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Re: Spectrum 48k Image quality.

Post by flaviosr » Sun Aug 19, 2018 7:37 pm

I have discovered today the AV MOD!
It seems a good solution. Some feedback?
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1024MAK
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Re: Spectrum 48k Image quality.

Post by 1024MAK » Mon Aug 20, 2018 6:06 pm

Which one? Link?

Mark
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flaviosr
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Re: Spectrum 48k Image quality.

Post by flaviosr » Mon Aug 20, 2018 6:32 pm

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Ast A. Moore
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Re: Spectrum 48k Image quality.

Post by Ast A. Moore » Mon Aug 20, 2018 6:52 pm

I’d add a decoupling capacitor in series. Something along the lines of 47–100µF.
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Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.

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1024MAK
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Re: Spectrum 48k Image quality.

Post by 1024MAK » Mon Aug 20, 2018 9:51 pm

flaviosr wrote:
Mon Aug 20, 2018 6:32 pm
I have found this very nice page:
http://www.retrogamescollector.com/simp ... osite-mod/
Ahh, that's the cheapest version.

Yes, as Ast A. Moore says, go for the better version, use a 100uF 16V electrolytic capacitor. These are polarised components. The negative lead will be the one on the side that has the "-" (minus) symbols on it. The negative lead should be connected to the centre pin of the phono socket in the modulator case. The other lead/leg of the capacitor (which is the positive one) should be connected to the video signal wire coming from the ZX Spectrum PCB. I also strongly recommend you use insulated sleeving on the bare leads/wires. The capacitor should fit inside the case of the modulator.

Mark
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