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SCART image quality

Posted: Thu Jul 19, 2018 8:36 am
by Andy1979
This weekend I soldered up an RGB SCART cable for my Master Compact using the wiring diagram shared in numerous threads (I think the Retroclinic design is similar). I've used 220ohm resistors, except for sync where I subsequently changed this to 270ohm (as per the diagram), which had no impact on image quality. The cable works, but I've had variable results.

On my Samsung 40inch LCD TV (2010 vintage) the picture is fantastic.

On an LG 42inch LCD TV (from 2015) the picuture is almost good, but in certain modes I get a blank image (regardless of interlace settings) e.g. when I load Prince of Persia, the image vanishes.

The main screen I want to use this with is an "old" (2006?) Samsung 940MW (it's a 19 inch LCD TV with 1440x900 resolution, and all manner of legacy inputs), which is said to work well with a lot of different retro computers/consoles. This is where the picture isn't as good. I'm getting some colour bleed, and mode 0 isn't particularly sharp: (photos are mode 7)
Any suggestions for improving the quality (e.g. tweaking resistor values), or is this just a case of certain modern TVs not implementing the SCART specifications properly?

I've already tried various combinations of *co. TV255,0 / 255,1 / 0,1 / 0,0 and changed all of the settings available to me through the TV menus (sharpness, etc).

Prior to all this I've been using my old Acorn AKF30 and the picture on that was fine.

Re: SCART image quality

Posted: Thu Jul 19, 2018 9:03 am
by hoglet
I'm nor sure how technical an answer you are looking for here.

The simple answer is it's largely down to the scaling in the TV/monitor and the panel resolution. Generally a TV/monitor with a "Full HD" Panel (1920x1080) will look much better than one with a lower resolution.

The more complicated answer is that the TV/monitor has to do two things well

a) Accurately sample each Beeb pixel, ideally just once.

The big problem here is that the Beeb uses non-standard pixel clocks. Mode 7 is 12MHz and Mode 0..6 is 16 MHz. Most TVs will sample a SCART signal at 13.5MHz, so you can already see how information is lost here.

b) Scale the resulting image to the panel resolution with the minimum of artefacts.

The higher the resolution of the panel, the less noticeable any scaling artefacts will be.


Re: SCART image quality

Posted: Thu Jul 19, 2018 10:07 am
by Andy1979
hoglet wrote:
Thu Jul 19, 2018 9:03 am
I'm nor sure how technical an answer you are looking for here.
Thanks Dave. Pixel clocks point is interesting - could explain the disappearing image on the LG TV. Could it also explain the colour bleed? Know my photos aren't that clear, but don't think they are scaling artefacts.

To be honest I was mostly wondering if anyone had overcome similar issues by modifying either the cable, or changing settings at the Beeb end of things.

Have seen cheap SCART to HDMI adapters recommended on here, so could always try one of those as a last resort (with added advantage of being able to use it on my main 1080p monitor).

Re: SCART image quality

Posted: Thu Jul 19, 2018 10:13 am
by Elminster
Andy1979 wrote:
Thu Jul 19, 2018 10:07 am

Have seen cheap SCART to HDMI adapters recommended on here, so could always try one of those as a last resort (with added advantage of being able to use it on my main 1080p monitor).
You might want to hold off getting one of those until Dave's RPI SCART/HDMI gadget is debugged and get one of them. viewtopic.php?f=3&t=14430

Re: SCART image quality

Posted: Thu Jul 19, 2018 10:25 am
by 1024MAK
Dave has hit the nail on the head.

Please disregard the over simplified 'digital' is better perception that gets pushed by marketing people and others who jump on bandwagons. All LCDs are digital.

It does not matter if the signal is analogue or digital, conveying information electronically always has limitations. With LCD televisions and monitors, they have to sample the three analogue red, green and blue video signals that form the SCART RGB video.

When the Beeb was released, all televisions and monitors were analogue CRT types. Transmitted television signals have a bandwidth of around 6MHz (for the monochrome signal). But direct video connections could actually achieve a greater bandwidth. As a monochrome CRT display is completely analogue, the electron beam can illuminate or not illuminate any part of the scan line. There are no pixels as far as the display is concerned. With colour, there is a shadow mask (metal sheet with tiny holes punched in it) and a pattern of the three phosphors needed for colour. So then there are discrete elements. But there is no calibration of any circuitry to the display, so they are not pixels. The finer the shadow mask and the pattern of the three phosphors, the better the display is at rendering the received image.

The point being, the actual method used to generate the content of video signal does not matter to the CRT display.

Now, with LCD displays, the system inside the television or monitor is completely different. The display has to convert the analogue signal to a digital representation of the analogue signal. Some televisions are only designed with broadcast quality television pictures in mind. It is also more expensive to sample the analogue signals at a greater resolution than can be displayed on the native resolution of the actual LCD panel. Hence the sampling rate on the analogue inputs (SCART RGB) on some LCDs are not actually able to sample the analogue signal from the Beeb at a sufficient resolution. Note that for a digital system to accurately represent an analogue signal, the digital system has to sample at a multiple many times greater than the "analogue resolution". So although the Beeb resolution is low compared to modern standards, it's still able to show up the limitations of LCD technology!

In addition, the LCD television has to carry out various image processing in order to make the displayed picture similar to what people expect when compared to a CRT television. But what is good for normal TV pictures, is not good for a computer display.

So turn off any and all picture enhancements/processing.

With regards to your television that blanks it's display, keep in mind that some modern televisions auto switch their SCART signal inputs between composite and RGB modes. If the TV does not receive the correct voltage on the function switching inputs on the SCART socket, it will switch from RGB to composite. With no composite picture information present, you will get a blank / black screen. See if there is a on screen menu option, or a button on the remote control to force SCART RGB mode. If no user/manual control, then you need a 12V DC signal to be wired to the SCART plug.

I hope this helps.


Re: SCART image quality

Posted: Thu Jul 19, 2018 3:01 pm
by colonel32
Good answer, Mark.

Just to add that cheap SCART/HDMI converters are not a magic bullet either. I have this one, and while it's useful as a quick way to hook up lots of retro RGB devices, it includes a pixel smoothing algorithm it doesn't seem to be possible to disable.

Random screenshot I had lying around. Look at the curves on the S and the crossbar of the T in the word START. The crossbar doesn't have any serifs, it's supposed to be a straight line!


It's OK I suppose, if you don't look too carefully, but I have several modern 8-bit remakes that output perfect crisp pixels over VGA or HDMI, and it's really jarring to switch from those to this.

Re: SCART image quality

Posted: Thu Jul 19, 2018 5:54 pm
by Andy1979
Thanks everyone for the responses.

In conclusion then it seems two of my screens are not well suited for BBC use. On the plus side I must have soldered my SCART lead correctly, and the picture on my big Samsung TV is great :D
The 19 inch screen isn't totally unusable, so will make do for now and stick to the CRT when the loft's not so hot!

That SCART/HDMI widget has some of the same artefacts I'm seeing (pinched H's, etc) so I'll eagerly await the release version of Dave's RPI gadget (I've been following that thread, but it's currently well above my skill level!) There are no options to turn off smoothing on the Samsung monitor either.

Other alternative is a Benq BL912 which seems to be well behaved with Beeb signals.

Re: SCART image quality

Posted: Thu Jul 19, 2018 7:39 pm
by tricky
Andy1979 wrote:
Thu Jul 19, 2018 5:54 pm
...There are no options to turn off smoothing on the Samsung monitor either...
Are there any options for sharpness or sometimes contrast enhancements?
I have an LCD that doesn't like my beeb's signal much, the picture slides from side to side and blanks completely sometimes.
If I increase the h-sync pulse with by one, it it fine, but every mode change resets it, so not much use really.

Re: SCART image quality

Posted: Thu Jul 19, 2018 8:18 pm
by Lardo Boffin
I can’t add anything technical to this but I have ZX81s, various Spectrums, Beebs, C64s and now an Atari ST.
I have three different TVs I use, a 14” portable (just about) CRT screen, a 17” low resolution (normal TV resolution) LCD and a hi res 19” LG LCD TV. All of the various computers perform differently on all three of the tellys. I use UHF, composite, SCART and HDMI (via a SCART upscaler).
Some of the ZX81s refuse to show a picture on the LG using a composite mod (ZX8-CCB) but work fine on all others (including the big telly in the living room).
The Atari ST which arrived today has a perfect picture on the LG but it vanishes every 20 to 30 seconds for just a fraction of a second. On the other tellys it is rock solid.
I guess some tellys are just more tolerant than others.

Re: SCART image quality

Posted: Thu Jul 19, 2018 9:31 pm
by colonel32
The Spectrum and ZX81, as Mark will confirm, output a non-standard video signal (missing or nonconforming front porch and back porch) for reasons best known to the costcutting bods at Sinclair. It's possible to do extra mods to make them play nicer with modern TVs.