Anyone using an RGB SCART cable with a +2A/+3 and LCD TV?

stephenw32768 · August 2013

In a recent thread, I mused:

Can +2As be composite modded? If so, I'd definitely like that doing. I know that the +2A's RGB output can take a SCART conversion cable, but I've read mixed reports about whether this works with modern flat-panel TVs.

Does anyone here use a +2A or +3 using an RGB SCART cable and an LCD TV? This is the sort of cable I'm talking about; notice the big disclaimer on the page saying that it's not guaranteed to work with modern tellies.

My television is a Sony Bravia KDL-26V4500, a few years old (can't remember exactly how old, it has a built-in Freeview tuner, but not Freeview HD).

If there's a reasonable chance of such a cable working, I'll take a punt. RGB will of course look a little better than composite, and there's no sense in getting the +2A composite modded if it doesn't need to be.

-Stephen

1024MAK · August 2013

Yes! I use a Sharp Aquos LC-19D1E-BK LCD TV.

I have a Spectrum +2 grey and a RGB-SCART cable (suitable for the grey) from retrocomputershack (but see note below). It now works fine :p

I also have a Spectrum +2B (black) and a Spectrum +3, both of which work just fine with another RGB-SCART cable (suitable for the +2A, +2B and +3), also from retrocomputershack :smile:

The Sharp Aquos also works with my other 8 bit computers and Atari ST computers.

Not had any problems with the Sharp with the Sinclair computers, but had to change a colour system setting from "AUTO" to "PAL" for the Acorn Electron and BBC Micro computers.

About the Spectrum +2 grey. Some (as in most) have a problem with three transistors being fitted incorrectly (see here) so if you have a machine with this problem, a seemingly correctly wired RGB-SCART cable may not work. Either a differently wired cable is needed, or the transistors need to be desoldered and replaced or turned around. Please talk to the supplier before ordering.

I hope this helps :D

Mark

stephenw32768 · August 2013

That's really helpful, thank you :-) Looks like one of these cables might be worth a go.

Oh, one more question: do you get all 15/16 colours with these? The information on this page suggests that if the electronics in the cable are not correct, the bright colours don't work. I take it that yours is fine though?

-Stephen

joefish · August 2013

The cable is OK for brightness levels. The problem is that between the 12V power supply input and the BLANKING pin in the RGB socket is a 1000 Ohm resistor. Coupled with the resistor in the TV this line produces too low a voltage to tell the TV to accept RGB data. Old CRT tellies didn't care, but modern digitally-controlled LCD tellies are very picky about getting the right voltage. If this is the problem, what you get is a black screen with just ghostly edges showing up between pixels of different colours.

There are two solutions to this problem:
1 - Take 12V from either the PSU, the RS232 or the Aux port, fit 620 (470+150) Ohms of reistance in-line, and supply that to the blanking pin of the telly.
2 - (What I did) - Open up the Speccy and fit another 1000 Ohm resistor in PARALLEL with the one inside, thus halving its resistance, then use that cable, which (if I remember rightly) has another small resistor in-line to make it right.

The second problem is that some of these TVs simply don't like the Speccy's half-PAL output. The PAL signal is supposed to alternate between two interleaved frames, and the Speccy output doesn't. Tellies which are trying to capture both frames to then stretch and re-display at 100Hz or whatever may get confused, and you get a jumping or rolling picture like the vertical hold has gone. So far I've found this with old widescreen LCDs, and am looking for a 4:3 instead to try it.

1024MAK · August 2013

Here are some photos that I took with my smartphone.

The computer used is my Spectrum +3
The lead is a "+3" RGB-SCART cable from retrocomputershack :smile:

First, the Sharp Aquos LC-19D1E-BK LCD TV, this is a widescreen TV:-

Second, a Mikomi Model LCD15796BF LCD TV that came from Argos (their own brand). This was the cheapest 4:3 aspect LCD TV that they had at the time:-

Note that how good the colours in picture are, and how much difference there is between BRIGHT and non-BRIGHT colours is partly determined by the contrast and brightness settings on the TV's.

The photos do not really show the image that well, but you get the idea :-P. May I should have charged up the battery for my camera...

Mark

XorA · August 2013

My LG TV hates the output from a +2a/+3 never renders it correctly, it will however render the 128k/+2 perfectly.

My Samsung TV displays it perfectly.

So its really pot luck with your TV.

joefish · August 2013

I thought LG TVs were supposed to have good renderers, but maybe not old ones. It's a widescreen LG one that I have that makes the picture roll over.

guesser · August 2013

The usual problem people have is with switching the TV to display from the RGB input. This is because SCART requires voltages within certain ranges present on switching pins to select input, aspect ratio, etc.

It's helpful I think to understand how displaying RGB signals works with SCART. There isn't really an RGB mode - it's really a bit of a hack. The RGB inputs and blanking pin were originally intended for displaying overlays such as subtitling over a composite video signal. Putting between 1 and 3 volts on pin 16 made the TV stop displaying the RGB signal it was getting from the decoded composite signal, and instead allow you to provide the RGB signals externally. This is why there is no vertical and horizontal sync inputs like on a VDU - the synchronisation etc is still coming from the composite video input and the TV is simply suppressing the picture information and displaying the external RGB signal instead.
Hence To use SCART as an RGB input you just hold that pin between 1 and 3 volts permanently so that the decoded composite video is always suppressed.

As joefish mentioned you can't derive these voltages from the +3 RGB socket without modifying the spectrum internally. If you don't want to do this you must obtain the switching voltages from elsewhere.

Some TVs provide the ability to manually suppress the composite input and display the RGB inputs without needing the blanking voltage present. I suspect that some older analogue TVs have the RGB connected constantly and so work as RGB displays so long as the composite video input is a black picture (as is the case when feeding the TV composite sync instead).

"Spectrum SCART cables" are often really not SCART at all, they're an analogue RGB cable with a euroconnector on the end that may or may not work with any particular television pretty much randomly. If you buy one online you're taking a gamble that the person who produced it had some knowledge of video interfacing and didn't just blindly follow a random circuit diagram off the internet...

stephenw32768 · August 2013

Thanks for the replies, everyone. Boy, what a minefield this all is!

@1024MAK: those pictures look wonderful. The sound quality must be a lot better than over RF as well?

My TV has a SCART input that can be explicitly configured for composite, RGB or S-video. If I connect my composite-only VCR to that input and configure it for RGB, the screen does go black, so the telly is doing what it's told, even if it's "wrong". Now, this is only supposition based on that evidence, but I reckon that if it's set to RGB, the strength of the signal on the BLANKING pin won't matter; the telly will just display what it's told to display.

That does leave us with the possibility of the "vertical hold" problem that joefish mentioned. I'll contact the seller first to make sure that I can return the cable if it doesn't work with my telly.

Thanks for your help, folks :-)

-Stephen

guesser · August 2013

stephenw32768 wrote: »

Now, this is only supposition based on that evidence, but I reckon that if it's set to RGB, the strength of the signal on the BLANKING pin won't matter; the telly will just display what it's told to display.

Worth a try at least.

The issue of what the TV will do with a 288p signal remains, but again, this is going to depend on your TV. If it works via composite then I see no reason it shouldn't sync in "RGB mode" too though.

1024MAK · August 2013

The problem with TV's and SCART connectors is the same as computer interface standards where the "standard" comes from the industry.

That is it takes time for all the details to come out, then they mess with it, so the "standard" becomes blurred. Take both the serial RS232 and the Centronics parallel printer ports. Both are suppose to be "standards", but there are many variations.

It is not helped by different designers interpreting the so called "standard" in different ways. As far as SCART inputs are concerned, Sony TV's are known to be really fussy, see here. Some other makes/models are not as fussy.

guesser wrote: »

It's helpful I think to understand how displaying RGB signals works with SCART. There isn't really an RGB mode - it's really a bit of a hack.

This is not that helpful. Many, many years ago, yes, but now, no. SCART has evolved. The RGB bit, as used for displaying a full picture has been around for at least 18 years. I know, I've been using it with all kinds of televisions to display RGB satellite receiver pictures since 1994.

And talking of things being a bit of a hack, a lot of the tricks Sinclair used, could also be described as "a bit of a hack"... :p

So if you put the correct voltage levels in the correct control pins, put analogue RGB video signals (of the correct level, and correct video system) in the RGB input pins and put composite sync signals (of the correct level, and correct video system) in the composite video input pin, all TV's that I have used and which have RGB inputs will display a picture.

Heck, I even fed a single monochrome signal in the green channel of a SCART socket once.

guesser wrote: »

This is why there is no vertical and horizontal sync inputs like on a VDU.

Err, a lot of old CRT VDU's / monitors only had composite sync inputs... Again, not that helpful. Maybe I should waffle on about sync on green CRT VDU's / monitors that had no separate sync input at all...

guesser wrote: »

"Spectrum SCART cables" are often really not SCART at all, they're an analogue RGB cable with a euroconnector on the end that may or may not work with any particular television pretty much randomly.

Like trying to connect a lot of computer or TV or audio cables then, where you are playing with the unknown... and it may, or may not work... If you don't try, it is certain that it won't work...

The only problem with the Spectrum computers (that have RGB video output sockets) is that the +12V pin (on the RGB video output socket) is obtained from the +12V line via a 1k (1000 ohm) resistor. Why such a high value of resistor was chosen, I don't know. Maybe it was a typo and it was suppose to be a 100 ohm resistor (in which case there would not have been any of these problems...).

Anyway, to get a TV to switch properly to a RGB input (via a SCART connector), supply it with 9.5 to 12V on pin 8 AND 1 to 3V at 13 to 40mA on pin 16. Some TV's are less fussy and the 0.8V you get from a Spectrum's +12V pin when it is connected to the SCART pin 16 will work.

</Rant>

Mark

PS if you TV has settings in the menus, you may not need to supply the correct voltages to SCART pin 8 and/or pin 16.

1024MAK · August 2013

stephenw32768 wrote: »

The sound quality must be a lot better than over RF as well?

Err, well, yes. But!!! Yes, there is a but.
Sinclair/Amstrad made a bit of a mess with the sound circuits. The level of the bleeper is different to the AY chip level. And on the +3, Amstrad made another mess by missing out a resistor, so that a transistor amplifier stage works more like a digital gate than an audio amplifier.
See this thread and this one
For more info, search the forum...

(And this has nothing to do with the SCART socket / interface / lead)

Mark

1024MAK · August 2013

And so, what is the result if the TV takes a dislike to the Speccy?

Well, here you see my modern Samsung LE46D550 46 inch LCD manually switched to the SCART input with a Spectrum+3 connected via the same SCART lead as in my earlier post:-

As you can see, the resulting image is a mix of the RGB picture and the noise on the composite video pin :mad:

If I want to solve this, I need to "steal" a +5V or +12V supply from elsewhere and feed it to pin 16 via a current limiting resistor.
Or I can modify the Spectrum by adding an extra resistor on top of the existing one.
(See joefish's post)

But as this is the main TV in the house, it very rarely gets used with my 8 bit computers.

And yes, a lot of the other 8 bit computers have the same problem.

Mark

1024MAK · August 2013

If you want to know more about the various different arrangements for the RGB/video output sockets on the 128 / +2 / +2A / +2B /+3 and how best to wire them up to a SCART socket, see Paul's excellent site here :p

Mark

orange · August 2013

yeah, pin 16 is usually the culprit. at least on modern TVs.
my good old Sony Trinitron CRT could be set to RGB/CVBS via remote (and picture quality was way better than plasma/lcd).

btw, my Panasonic plasma works with that +12V output on pin16 without modifying the 1K resistor, so its worth trying. ( i think that resistor is used to prevent short circuit)

guesser · August 2013

1024MAK wrote: »

It is not helped by different designers interpreting the so called "standard" in different ways. As far as SCART inputs are concerned, Sony TV's are known to be really fussy, see here. Some other makes/models are not as fussy.

Fussy in what sense?

1024MAK wrote: »

This is not that helpful. Many, many years ago, yes, but now, no. SCART has evolved. The RGB bit, as used for displaying a full picture has been around for at least 18 years. I know, I've been using it with all kinds of televisions to display RGB satellite receiver pictures since 1994.

I'm not saying it shouldn't be used! I was just writing a bit of the background of why it behaves the way it does.
A plain RGB input would have been much simpler to use for our purposes (and games consoles, set top boxes, etc that just want to use the TV as a dumb monitor). This wasn't the problem they were trying to solve though or they'd have used a big switch instead :)

1024MAK wrote: »

And talking of things being a bit of a hack, a lot of the tricks Sinclair used, could also be described as "a bit of a hack"... :p

Indeed...

1024MAK wrote: »

So if you put the correct voltage levels in the correct control pins, put analogue RGB video signals (of the correct level, and correct video system) in the RGB input pins and put composite sync signals (of the correct level, and correct video system) in the composite video input pin, all TV's that I have used and which have RGB inputs will display a picture.

Yes, if you input signals all as per the standard everything works... What's your point exactly?

1024MAK wrote: »

Err, a lot of old CRT VDU's / monitors only had composite sync inputs... Again, not that helpful. Maybe I should waffle on about sync on green CRT VDU's / monitors that had no separate sync input at all...

Yes, it's not common on "modern" systems though. I was just pointing out the reason SCART has no sync input for component video.

1024MAK wrote: »

The only problem with the Spectrum computers (that have RGB video output sockets) is that the +12V pin (on the RGB video output socket) is obtained from the +12V line via a 1k (1000 ohm) resistor. Why such a high value of resistor was chosen, I don't know. Maybe it was a typo and it was suppose to be a 100 ohm resistor (in which case there would not have been any of these problems...).

That certainly makes sense, it wouldn't be Amstrad's first typo in a schematic. :D

1024MAK wrote: »

Anyway, to get a TV to switch properly to a RGB input (via a SCART connector), supply it with 9.5 to 12V on pin 8 AND 1 to 3V at 13 to 40mA on pin 16. Some TV's are less fussy and the 0.8V you get from a Spectrum's +12V pin when it is connected to the SCART pin 16 will work.

</Rant>

Mark

PS if you TV has settings in the menus, you may not need to supply the correct voltages to SCART pin 8 and/or pin 16.

In other words, feed it the voltages the spec demands and you should get the desired outcome ;)

stephenw32768 · August 2013

1024MAK wrote: »

Sinclair/Amstrad made a bit of a mess with the sound circuits. The level of the bleeper is different to the AY chip level. And on the +3, Amstrad made another mess by missing out a resistor, so that a transistor amplifier stage works more like a digital gate than an audio amplifier.
See this thread and this one

I'd already read about that, and don't know if my +2A is affected or not. In other words, I don't know whether what I have is actually a +2B. The blurb on the underside of the case says "128K Spectrum +2A Made in Taiwan", but I gather that doesn't necessarily mean it's a +2A motherboard.

I was playing Target Renegade the other evening and the sound was pretty horrendous, it sounded very clipped, not clean like on an emulator. But I then stuck the original Renegade in and the music (well, what there is of it on the +2A) sounded fine, as did the SFX.

I'll add it to the list of things to get checked out when I have the machine refurbished.

-Stephen

guesser · August 2013

stephenw32768 wrote: »

I'd already read about that, and don't know if my +2A is affected or not. In other words, I don't know whether what I have is actually a +2B. The blurb on the underside of the case says "128K Spectrum +2A Made in Taiwan", but I gather that doesn't necessarily mean it's a +2A motherboard.

It's easy enough to tell if you open the case, but if you don't want to do that, just plug in a pair of headphones or stereo speakers into the sound socket.

If the sound is in both channels it's a +2B, but if it's only in the left channel then it's a +2A.

stephenw32768 · August 2013

guesser wrote: »

just plug in a pair of headphones or stereo speakers into the sound socket.

If the sound is in both channels it's a +2B, but if it's only in the left channel then it's a +2A.

Both channels. +2B it is, then. So the sound socket must be output only; another thing to get modded since I'd like to use playtzx to inject games directly into the +2A without having to make a tape.

-Stephen

balford · August 2013

For what it's worth, I built the cable described on Paul Farrow's site here tonight and tried it with my usual monitor, an LG M2232 (about a year old).

Couldn't get RGB out of my +3 for love nor money.

The same monitor is fine with RF and composite from almost any source - it even detected and auto tuned the dodgy RF signal from my ZX81 pre-CCB mod :D

I'll be sticking with RF or composite where possible, I think...

B

stephenw32768 · August 2013

Ack, that's a shame.

Composite is a perfectly fair compromise from the looks of the high-quality photos I've seen people posting here. RF is nasty though, at least on my telly: unsteady picture, interference; though some of that could be due to the loose connections that my Spectrum is suffering from. (Not that it stopped me getting a personal best on Arkanoid this evening; I seem to be playing better on the real Speccy than I do on the emulator...)

-Stephen

guesser · August 2013

The trouble with composite video on the speccy is poor separation of the chrominance from the luminance which leads to annoying colour artefacts on pretty much anything with graphics that use dithering.

On the plus side you can play my game :D

balford · August 2013

That's thoroughly evil :-D

orange · August 2013

@guesser
awesome..

@balford

I had similar problem with CPC and not just enough voltage on pin16. solved it by adding capacitor:

http://www.cpcwiki.eu/imgs/a/a8/Alt_Scart_Connect.PNG
http://www.cpcwiki.eu/index.php/TV_SCART_cable

perhaps that might help you somehow.
otherwise, you can always add some external battery.
pity TV has no dcvoltage on scart?

balford · August 2013

Thanks - I'll give that a try next time I get some free time :smile:

stephenw32768 · August 2013

My cable (from retrogamingcables.co.uk)arrived yesterday. Picture looks superb.

:-D

-Stephen

Alessandro Grussu · September 2013

Me.

EDIT: Actually the first picture is of a modified 48K sending the composite signal through a SCART cable.

balford · September 2013

Stephen, do you ever play anything else apart from Arkanoid :D

B

orange · September 2013

I like Arkanoid too, its one of the rare (if not the only one) ZX games that I finished. it came bundled with +3.

stephenw32768 · September 2013

balford wrote: »

Stephen, do you ever play anything else apart from Arkanoid :D

:o ...there are games apart from Arkanoid? Wow, who knew?

-Stephen :-D

orange · November 2013

um, sorry, seems its a bad idea to use csync for pin16:

http://www.worldofspectrum.org/forums/showthread.php?t=45427

Anyone using an RGB SCART cable with a +2A/+3 and LCD TV?

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