Connecting a CPC+ to a 15Khz VGA monitor

[adolfo.pa]

Hi all,

Recently, I got a BenQ BL720A monitor, which I've been using with my Amiga for a couple of weeks now with no issues at all. Now, I'd like to connect my Amstrad Plus 6128 directly to it.

As my soldering skills are nil -- and I also lack the proper equipment -- my intention was to reuse the RGB->SCART cable that I have (which works fine), and build a SCART(female)->VGA cable. To work around the no soldering requirement, I got a VGA connector similar to this one:

https://www.ebay.co.uk/itm/DB15-DSUB-15-pin-HD-Male-Adapter-VGA-Breakout-Board-Connector-D3-/152757864761

So the idea is to take a SCART-SCART cable, remove one of the SCART connectors, and wire in the correct inputs into this VGA connector.

My first question is, will this actually work? Or would it be necessary to add some extra components that would require some soldering?

Also, does anyone have a diagram with the correct wiring? I googled for it, but I couldn't find one that matches exactly what I want to do. By checking the RGB->SCART diagrams in the wiki and using quite a bit of guessing I came out with the following mapping, but it is missing the GND connections (in parens the corresponding VGA pin):

Code Select Expand


Composite Sync (13)
   |   
   |RGB Switch (14)
   |   |
---|---|---------------
\  0 O 0 O O O O O O O |

\ O O 0 O 0 O 0 O O O |
  -----|---|---|-------
       |   |   |
       |   |   Blue (3)
       |   Green (2)
       Red (1)


I just got my M4 board, so I'm pretty anxious to get this thing working :-)

Thanks a lot!

[Poliander]

Even if the monitor is capable of 15 kHz, it still doesn't mean it can use the sync signal... probably some HSYNC/VSYNC circuit might still be required?

[adolfo.pa]

Then, I was very lucky because it kind of works :-)

I connected the SCART GND to VGA pin 10 and I can see a picture. It's stable and colors are nice, but there's some noticeable noise; but it's a good start anyway. I read somewhere (sorry, I don't have the link) that I should connect the SCART chasis to the VGA one; do you know if I'm missing some other connection that could help me reduce the noise? Or could it be caused by bad connections/insulation?

The thing looks like this right now (X means not connected, the number inside the brackets is the VGA pin):

Code Select Expand


Composite Sync (13)
   | GND (10) 
   | | RGB Switch (14)
   | | |
---|-|-|---------------
\  0 0 0 X X X X X X X |
`                     |
\ X X 0 X 0 X 0 X X X |
  -----|---|---|-------
       |   |   |
       |   |   Blue (3)
       |   Green (2)
       Red (1)


Thanks!

[roudoudou]

Will the VGA monitor still need a V tension in equivalent of scart pin 16?

[Bryce]

@roudoudou: No VGA doesn't have switch signals like SCART does. VGA uses ID Bits (a four bit identification word), but they are ignored if not present.

@adolfo.pa: Make sure all GND pins (5 - 10) on the VGA connector are connected to the CPC GND. At the breakout connector, keep the single wires as short as possible. To reduce the noise further you need to remove the SCART connector from the cable completely and wire directly from the CPC to the breakout connector.

Bryce.

[adolfo.pa]

After connecting VGA pins 5 & 10 to GND almost all the noise went away. Thanks a lot Bryce.

There are still some vertical lines, but I think that not related to do with the connector itself. I had the same issue with the Amiga, and I was able to fix it by changing the monitor settings (sadly, that didn't work that well with the CPC).

Anyway, the lines are very faint (you have to pay attention to see them) and the quality is more than enough for my purposes.

In the end, the connections looks like this. The only change wrt the previous one is that I rewired VGA pins 10 and 5 to SCART 4 & 5.

Code Select Expand


Composite Sync (13)
   |
   |   RGB Switch (14)
   |   |           GND (5)
---|---|-----------|---
\  0 X 0 X X X X X 0 X |
`                     |
\ X X 0 X 0 X 0 0 X X |
  -----|---|---|-|-----
       |   |   |  GND (10)
       |   |   Blue (3)
       |   Green (2)
       Red (1)


[Bryce]

It's not 5 AND 10 it's 5 TO 10, ie: Pins 5,6,7,8,9 and 10 of the VGA connector should be connected to GND. That may help remove some more noise.

Bryce.

[Joseman]

hi!

can you post some pics of the cpc with this monitor?

do demos/games work ok with this monitor?

what about split-screen or smooth scrolling?



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[adolfo.pa]

hi!

can you post some pics of the cpc with this monitor?

do demos/games work ok with this monitor?

what about split-screen or smooth scrolling?

Sure, I'll post some pics this evening.

I played a couple of games with no issues, but I guess they don't use any tricks that may push the monitor too far; I played Zynaps (CPCWiki version), Burning Rubber, Baba's Palace and Alien Storm. The epilepsy inducing effect in Zynaps looked exactly as I remembered it 

Any suggestions on what to try?

[adolfo.pa]

It's not 5 AND 10 it's 5 TO 10, ie: Pins 5,6,7,8,9 and 10 of the VGA connector should be connected to GND. That may help remove some more noise.

Bryce.

Oops. I'll do it and report back. Thanks a lot!

[Joseman]

Sure, I'll post some pics this evening.

I played a couple of games with no issues, but I guess they don't use any tricks that may push the monitor too far; I played Zynaps (CPCWiki version), Burning Rubber, Baba's Palace and Alien Storm. The epilepsy inducing effect in Zynaps looked exactly as I remembered it 

Any suggestions on what to try?

The batman forever demo  uses a mode 2 screen with very tiny letters called bat-system 5.9, is a good screen to see if the text is correctly displayed and clean to read...

The "pinball dreams" game demo is good to see how it scrolls and the split screen for the scoreboard

Striker In The Crypts Of Trogan uses a lot of raster to achieve more colors on mode 1

Edge grinder uses R3 register

Super cauldron uses R3/R5 CRTC register

this two have a very smooth scroll... it's interesting to see how a lcd monitor handles this...

[adolfo.pa]

Hey @Joseman,

I ran the games/demos you suggested and here are the results :-) I took some pictures and videos, I'll update the post later when I upload them somewhere. I couldn't get Super Cauldron to run; I have my HxC as drive B, and the game refuses to load from there.

On the winners side we have:
  - Batman Forever: The whole thing looks very good, and the high resolution text is readable, although a bit blurry. There are noticeable pauses when the demo changes between video modes, and in some parts there are some minor glitches on the top/bottom of the screen. Once (almost at the end), the monitor lost the signal and then it came back. But overall, it looks really good.
  - Pinball Dreams: Except for a little glitch at the bottom of the screen, everything looks and moves fine (i.e. amazing)
  - Striker: Good. Background effect looks ok. No issues noticed.

The other side of the coin is Super Edge Grinder. The game loading screen (+ slide effect) and start menu look good. But the screen goes blank when the game starts :-(

I took the pics and vids with a relatively cheap smartphone, so lower your expectations :-) In particular, the colors look much better on the monitor, and there are no ghosting effects (I noticed some while recording the videos).

[adolfo.pa]

Oh, btw, I ran these tests without wiring the VGA connector completelly, it's still missing some GND connections. Tomorrow I'll get some extra cable and repeat some of these tests to see if it improves things a little.

Here is a picture of Pinball Dreams (click to see larger image). At the bottom you can see the little glitch.



In this video you can see the scrolling in action (I haven't uploaded it to YT to avoid framerate issues):

https://drive.google.com/open?id=1SX_4H6wOg9lPYkUEq1gjI3gLELoTNHHc

Striker, with the nice background gradient:



and a short video showing the scrolling:

https://drive.google.com/open?id=11ZD8U9ga_NOkZc7I9qSAqBbT_Z4w5vaA

I recorded the whole Batman Forever demo (zooming in when the hires text shows up). When it has finished uploading I'll link to it here. Here it is:

https://drive.google.com/open?id=1KMmf0nBLjLEf5olAJ-8C5HGfIBNkrIeY

[Skunkfish]

Looks like pretty impressive results, the slow switching on Batman Forever would be a bit annoying though but not an issue for most software...

[adolfo.pa]

Ok, so I finally connected all VGA ground pins; I don't see any big improvement, so I suppose this is as far as I can get with cheap components and no soldering.

I also tried to copy Super Cauldron into a real 3" disk to see if it worked, but it gives me read errors on drive B while copying (drive B is the HxC, so it is a little weird). Sorry Joseman, but I give up :-)

Anyway, I'm pretty happy with the results. I can finally put my CPC+ to good use.

Thanks a lot for all your comments and suggestions.

[Joseman]

Hi

I think that there is a 19" version of this monitor.

maybe i'll buy one to try...

Enviado desde mi GT-N7100 mediante Tapatalk

[adolfo.pa]

@Joseman, if you feel adventurous the BenQ bl912 has also been reported as being 15khz friendly. It is in the same price range (although a bit more expensive) but has an additional DVI input.

Other than the pauses when changing video modes (which is normal and expected), there's a little problem that I believe has nothing to do with my crappy cable. If you look closely to the Striker screenshot I posted above, you'll notice some vertical bands in the background gradient. It's no big deal for me, and most of the time I don't even notice them, but you should be aware of it before spending any money. Also, there's a slight blurriness which is not obvious in the videos/pics I posted; again, nothing too crazy (text is perfectly legible) but you should know it.

Having said that, I'm very happy with it, and if you're not going to use it exclusively with the CPC I think you'll be satisfied. I'm using it with an Amiga 500 and a Mist and the image quality is pretty good. For these two, I was able to remove the vertical bands by adjusting the monitor settings.

[VincentGR]

Same happens to a MultiSync LCD1970NXp, bars are there too.
Not a big problem except when I plug my Amiga there I have problem with PAL modes as they are cropped.

[adolfo.pa]

Not sure if this helps you in any way @VincentGR , but on the Amiga I was able to get rid of them completely by setting the pixel and phase clock to 90 and 30 respectively.

[pelrun]

I was about to say, the vertical lines are almost certainly a clock/phase issue. Practically every LCD VGA monitor I've ever used lets you adjust this, if the provided "Auto" function doesn't get it right.

[VincentGR]

Not sure if this helps you in any way @VincentGR , but on the Amiga I was able to get rid of them completely by setting the pixel and phase clock to 90 and 30 respectively.

Thanks, but unfortunately you can't do much on this monitor even in factory mode.

[||C|-|E||]

The jailbars are always present in the Plus range, but are very difficult to see in a CRT screen. The video output of the machine lacks a few caps that should be there (and are certainly there in the normal range). The best option to reduce them is to mod the motherboard and add the caps... but this is not easy if you lack soldering skills. The alternative would be to incorporate them into the cable, but you would need to solder anyway  . With the caps in place you won't completely get rid of the problem but almost 

[adolfo.pa]

Thanks @||C|-|E||. Really valuable info here 

Do you have any extra info about these missing capacitors? I'd like to build a proper cable in the future, so I'll definitely try to add these caps.

Just in case, if I mess up something in the cable, could there be some damage to the CPC+? or is this relatively safe to perform? I'm very sentimental about this machine, so I want to avoid any risk of damaging it.

Thanks!

[Bryce]

I posted the information about these caps a while back:

http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/limited-batch-of-new-s-video-adapters/msg61860/#msg61860

Bryce.

[Gryzor]

The 4:3 monitor feels so good... Unfortunately, big CRTs are so BIG. Were there shortneck monitors?