Composite Mod LCD vs. CRT

[MrClump]

I've composite modded several Speccies to then connect to modern LCDs and I've always been a little disappointed with the results TBH. Smeary colours, very visible dot crawl, jail bars (even with the jailbar mod).

I picked up a small 14" CRT portable TV for £0 on Facebook Marketplace yesterday. Wow! The composite image on the CRT is sooo much better than with an LCD! I'd tried a couple of 16-bit micros (Amiga and ST) and really didn't like them on the CRT, I think my pin-sharp OSSC/LCD combo suits them better, but the Speccy just looks spectacular! Jailbars gone, dot crawl almost invisible and almost no colour smearing.

It got me wondering what others are generally using with their composite modded Spectrums?

[spider]

I'd guess the 'result' will vary a bit on the display itself (not just the display type) as you might find "brand x" works a bit better/worse than "brand y"

When I had a CRT I was lucky enough to be able to use the RGB so did not bother with composite. But now it is composite on a (old) Alba TV.

I want to mention though its probably worth checking what type of composite mod has been done:

> Straight through as in input to modulator fed to output socket
> 100uf Cap in-line
> Transistor version

FWIW I've always done the capacitor version myself, if you get an axial one you don't need any wiring either as the mod case has an insulated port you can use for the lead.

I did try my 48+ on that flat panel and it was reasonable but it has the 'jaundice' being an Iss2, but there was not much dot crawl in general.

[p13z]

I've always stuck with RF and old CRTs.
Using a period CRT, the picture looks loads better.
The only annoyance/disadvantage with using RF, is that the tuning wanders slightly when it is cold - so either adjust it slightly at power-up, or live with a tiny bit of fuzz for 5 minutes. I tend to think this is worth it to keep my old computers and consoles in original condition.
Occasionally I swap displays around, depending on what I'm doing, and it is very noticeable how crap the Speccy display is on modern screens.

[1024MAK]

Yes, it depends on the TV/monitor. CRT displays, especially those with smaller screen sizes are more forgiving, and hence often give a better picture.

I normally use RGB SCART (128k models) or a Spectra and RGB SCART cable. But do use composite video or RF/UHF when fault finding or testing of a 16k/48k/plus.

Do keep in mind that most of the quick and simple composite modifications are not exactly broadcast quality or even at the correct video level. And modern TV/displays are designed for broadcast quality video.

Mark

[akeley]

I'd tried a couple of 16-bit micros (Amiga and ST) and really didn't like them on the CRT, I think my pin-sharp OSSC/LCD combo suits them better, but the Speccy just looks spectacular! Jailbars gone, dot crawl almost invisible and almost no colour smearing.

Amiga/ST OS might benefit from sharpness, but for games I'll always go with CRT look. Besides, when using RGB & a decent, well calibrated CRT even Workbench looks pretty sharp.

The one 48Kb ZX I have came already modded for composite, and I thought the quality was pretty bad (comparing to other machines with composite). Really heavy dot crawl. I even compared it against my +2A's RF output and the RF came out on top! So maybe you got lucky with your Spectrum's internals or have a better quality mod.

[Ast A. Moore]

128K machines don’t have any dot crawl to begin with. They use a single crystal oscillator.

[Hedge1970]

I have a few spectrum 48k machines and one or two are actually quite good visually through the comp out. I use a small 21” LCD. You can easily play games on the best one but yeah still suffers from stress crawl.

I mainly use Spectra through SCART but I’ve recently picked up a ZX-HD peripheral. Not done very much with it yet so can’t comment on compatibility but picture wise just like Spectra it’s pin sharp.

[Sparky]

I use a small CRT, and even on my +2b, it looks loads better than any modern LCD I've tried. Especially for games, where the picture is always changing, you can't beat a CRT.

[1024MAK]

The dot crawl and colour bleeding on the 16k/48k/plus models is due to a design error. This was corrected in the 128k machines. The problem is caused by the design using one crystal oscillator for the ULA (which does all the video timing) and a separate crystal oscillator for the colour carrier. But both should come from either a single crystal oscillator, or be phase locked some other way.

A modification to overcome this problem means replacing the ULA and rather a lot of changes to the board.

The other thing to keep in mind, is that CRT and LCD technology work completely differently. With a CRT, the pitch of the dots that make up the display can help to average out some display artefacts. Modern LCD have a much higher resolution, so display the image including all the limitations. And of course, the bigger the screen size, the more noticeable the problems are…

Mark

[akeley]

There is no dot crawl per se, but RF on my +2A is still far from ideal, of course. There's the usual wavy interference, some crazy jitter at various places and psychedelic colours which appear in odd places. The latter is actually quite amusing, the first two, not so much. Still, I think it looks overall better than the image via the composite mod on my 48k (though it's subjective of course).

My Philips MSX displays via composite and the dot crawl and colour bleed are present, but on a minimal scale.

[spider]

The other thing to keep in mind, is that CRT and LCD technology work completely differently. With a CRT, the pitch of the dots that make up the display can help to average out some display artefacts. Modern LCD have a much higher resolution, so display the image including all the limitations. And of course, the bigger the screen size, the more noticeable the problems are…

Mark

Although (afaik) its completely incorrect I always in my mind view a CRT as analogue and a TFT/LCD as digital , in that the slight erm 'blur' you can get with a CRT does make the experience and visual appeal nicer. A pin sharp LCD/TFT pic on a large screen does show up any oddness I think. Probably phrased that quite badly. I suppose the way our eyes work may explain it , persistence of vision etc otherwise we'd never see a picture at all on a CRT just a very fast dot lol.

I don't mean the risk of a CRT being out of focus just that the way the piccy is erm "drawn" by the electron gun vs zillions of bits of silicon in a panel.

Although a different 'kettle o fish' I think this is why a pic seems 'sharper' on a mono (BW usually) CRT compared to a colour one , not because there's just one colour displayed more because there's no grid (shadow mask?) splitting it up. I guess I mean you are probably (in theory, correct me here if I'm wrong) more likely to get a sharper picture on a BW TV/Monitor if connected via composite video from the Speccy compared to a colour TV/Monitor of the same size and spec etc connected the same way.

[1024MAK]

Although (afaik) its completely incorrect I always in my mind view a CRT as analogue and a TFT/LCD as digital , in that the slight erm 'blur' you can get with a CRT does make the experience and visual appeal nicer. A pin sharp LCD/TFT pic on a large screen does show up any oddness I think. Probably phrased that quite badly. I suppose the way our eyes work may explain it , persistence of vision etc otherwise we'd never see a picture at all on a CRT just a very fast dot lol.

I don't mean the risk of a CRT being out of focus just that the way the piccy is erm "drawn" by the electron gun vs zillions of bits of silicon in a panel.

Although a different 'kettle o fish' I think this is why a pic seems 'sharper' on a mono (BW usually) CRT compared to a colour one , not because there's just one colour displayed more because there's no grid (shadow mask?) splitting it up. I guess I mean you are probably (in theory, correct me here if I'm wrong) more likely to get a sharper picture on a BW TV/Monitor if connected via composite video from the Speccy compared to a colour TV/Monitor of the same size and spec etc connected the same way.

Old style CRT displays are completely analogue. Some later CRT displays (high end TVs for example) did use analogue to digital converters to store the image in a frame buffer (for special effects or 100Hz scanning).

Modern LCDs always convert an analogue input into digital data via analogue to digital converters. The actual liquid crystal display panel is a matrix driven by digital circuitry. Hence you always need a digital frame buffer.

With a monochrome CRT, the entire visible part of the tube is coated on the inside with phosphor coating, all the same colour obviously.

With a colour CRT, there are three colours of phosphors, red, green and blue. Arranged as either dots or stripes. And yes, the shadow mask has a matching matrix of pin holes. Arranged so that the three election guns can fire at the correct colour dot or stripe of phosphor.

Hence the colour display has individual pixels. But note that there is no direct relationship between these and the pixels used in the computer. As there is no way to line them up. The resolution of the colour display is determined by the pitch of the phosphor and of the holes in the shadow mask. The higher resolution monitors give a much better picture compared to say as standard 14” colour TV.

No such problems with a monochrome CRT, as there are no pixels, or shadow mask. And there is only one electron gun, so no alignment problems. Just focus.

In addition to the above, when using composite video or RF/VHF/UHF, the colour information is mixed in with the luminance (brightness and contrast) information. The display circuitry has to separate the two sets of signals. As some of colour information overlaps and frequency filters have practical limitations, some luminance picture information is lost. With a monochrome monitor fed with a monochrome video signal, the bandwidth can be higher and hence less picture information is lost.

An RGB signal is really three monochrome video signals.

On a 16k/48k/plus ZX Spectrum, on the board, or on the edge-connector, there is actually a monochrome video signal. It does need a transistor to invert the polarity, but it can be used for a higher quality monochrome grey scale video signal.

Mark

[spider]

The vid on the connector I was aware of. I had a vague idea at one point in time to add a phono or similar socket to a Kempston interface and connect it to it. Had not realised polarity change was needed, I don't recall trying it use said output, I had assumed it was on the same track as the modulator feed but I'd never looked!

Does make logical sense too when you say colour is 3x monochrome, yes because they are made of R G and B only.

I'm still phrasing it badly but I think what i meant to say/mean with the BW monitor vs colour with a composite video input ( as per the original posters topic post itself ) I'd expect it to look 'sharper' or 'better' on a mono display CRT because you're not hampered by that shadow mask but at the same time you'd not get the 'digital' of an LCD/TFT type display where any colour bleed/dot crawl would be obvious.

^ I sort of know what I mean , ideally you'd want the same machine driving a mono CRT monitor then a colour CRT monitor (again via composite in) and finally at the same time a modern LCD/TFT flat panel too.

[1024MAK]

Assuming the relevant links are fitted on early issue boards, 16k/48k/plus ZX Spectrums have the following video signals at the edge connector:

Code: Select all

Pin  Signal  Notes
———————————————————
15L  video   This is a composite video signal, but not at the normal video voltage levels
16L  /Y      This is an inverted monochrome video signal.
17L  V       Colour difference signal
18L  U       Colour difference signal

L after the pin number means lower side.

See the Wiki for more information

Mark

[Sparky]

Modern LCDs always convert an analogue input into digital data via analogue to digital converters. The actual liquid crystal display panel is a matrix driven by digital circuitry. Hence you always need a digital frame buffer.

ome video signal. It does need a transistor to invert the polarity, but it can be used for a higher quality monochrome grey scale video signal.

Could that be the reason my LCD monitor shows the Spectrum colours but the screen is out of whack, it is not centered, same with the vertical part?
I am using an RGB to SCART cable, wouldn't not show any colours if it didn't support RGB?

I forgot to add, I'm using a +2b

[1024MAK]

Modern LCDs always convert an analogue input into digital data via analogue to digital converters. The actual liquid crystal display panel is a matrix driven by digital circuitry. Hence you always need a digital frame buffer.

ome video signal. It does need a transistor to invert the polarity, but it can be used for a higher quality monochrome grey scale video signal.

Could that be the reason my LCD monitor shows the Spectrum colours but the screen is out of whack, it is not centered, same with the vertical part?
I am using an RGB to SCART cable, wouldn't not show any colours if it didn't support RGB?

I forgot to add, I'm using a +2b

I’m not entirely clear on what you are trying to say…

It’s the timing in the respective signals that determines the position of the picture information compared to the synchronisation signal. The synchronisation signal includes horizontal timing pulses and vertical timing pulses. The TV/monitor/display should use these to correctly position the picture on the screen. However, some models of ZX Spectrum don’t produce a video output with the paper area perfectly centred on screen.

With a RGB connection, you have a red video channel, a green video channel, a blue video channel plus the separate synchronisation signal (this is also known as combined sync. or composite sync.). In a colour CRT, each colour channel feeds one of the three electron guns. The sync. signal is split into control and timing signals to control where the beams from the electron guns hit the screen.

A LCD monitor/TV converts the signals into a digital format. It then extracts the relevant information. Converts the display data into row and column information and then displays the result. Most monitors and some TVs have menu or on screen display settings to move the image on screen left/right, up/down and the picture size. There may also be other advanced menu settings for adjusting the monitors ‘pixel clock’ reduce vertical line noise and ‘picture phase’ to reduce horizontal line noise.

If the above does not help, maybe post up a photo to show what you mean.

Mark