What gamma curve did ATARI's SC monitors follow?

[pixelpusher]

Hi all (and happy christmas)!

I was recently experimenting with some color image processing and did so - as probably most of us do these days - on monitors that roughly follow the sRGB standard (with a 2.2 gamma). The results transfered quite well to older CRTs (e.g. a 17" Eizo or an Apple iigs Color RGB monitor) that seem to use have a gamma of 2.3 -2.4.

However, two ATARI SC1224 I got my hand on, showed a vastly different behaviour - images are much, much darker - that makes me think those monitors are more in the range of a 3.1 - 3.5 gamma. Of course, two monitors aren't a sufficient amount of samples to be sure as these two might have just aged very bad.

So,I'd like to ask for a bit of help from those members that have ATARI RGB monitors and could make a picture of the palette colors on display (I attach a color control panel with set color palette to use) to determine, if those ATARI monitors really had such a high gamma, or if its jut the two monitors I had access to.

If you open the colorvga.cpx in a 16 color mode (you need XCONTROL.ACC or Cops.acc of course), you should see something like this on a sRGB monitor:

sRGBColors.jpg (53.33 KiB) Viewed 1464 times

If ATARI SC monitor in general behave like mine, you probably will see dark tones (dark grey, dark red, dark green being) nearly indistinguishable from each other - and very dark.

Thx in advance for anyone who can help to get this riddle solved...

P.S: The palette values are utilising the STe's 4 bit palette. So, on ordinary STs with 3 bit palette indices, values will be slightly different - but perceived colours should still appear too dark

COLORCPX.zip

(10.79 KiB) Downloaded 39 times

[sandord]

Right now I can't do any testing but I can assure you that both my SC1224 and my friend's are both distinctively dark in the lower end. That means that colors like #111 are hardly visible (if at all).

[pixelpusher]

Right now I can't do any testing but I can assure you that both my SC1224 and my friend's are both distinctively dark in the lower end. That means that colors like #111 are hardly visible (if at all).

Thx for confirmation. I wouldn't have expected that #111 would be distinguishable as such, but the dark green for example is 3-7-0, dark grey is 6-6-6 and they are - in person - hardly distinguishable.

To illustrate what I'm seeing, here's a photo from one of the SC1224 (the camera adds some artefacts of its own. as well as brighten up the darker colours more than I perceive it in real life):

SC1224.jpg (328.82 KiB) Viewed 1429 times

[Darklord]

Okay, I tried this out here. I've got a JVC brand SC1224 and a Samsung brand SC1224 monitor.

Just for the fun of it, I also included a shot of it running on my Atari STacy with the RGB2HDMI
mod, on a modern LCD replacement screen.

Like other people have experienced, the camera seems to wash out the colors a bit. Also, with
my S22+ Android Galaxy phone, it tends to capture the vertical scan. Sorry about that.

Of the 2, I thought the JVC SC1224 looked the best. The Samsung SC1224 in real-time looked
fairly dark.

Of course the STacy was bright and beautiful.



SC1224-JVC Brand (Mega ST4)

GAMMA-RGB-JVC.JPG (198.88 KiB) Viewed 1387 times

SC1224-Samsung Brand (STacy external color monitor port)

Gamma-RGB-Samsung.JPG (247.27 KiB) Viewed 1387 times

STacy with the RGB2HDMI mod.

Gamma-HDMI-STacy.JPG (159.71 KiB) Viewed 1387 times

[pixelpusher]

Okay, I tried this out here. I've got a JVC brand SC1224 and a Samsung brand SC1224 monitor.

Just for the fun of it, I also included a shot of it running on my Atari STacy with the RGB2HDMI
mod, on a modern LCD replacement screen.

Like other people have experienced, the camera seems to wash out the colors a bit. Also, with
my S22+ Android Galaxy phone, it tends to capture the vertical scan. Sorry about that.

Of the 2, I thought the JVC SC1224 looked the best. The Samsung SC1224 in real-time looked
fairly dark.

Of course the STacy was bright and beautiful.

Thx for the screenshots. I think with these other crt vendors (my screenshot was from a SC1224 with MATSUSHITA CRT) things are getting more complicated than I anticipated (there was probably no real check/strategy about color reproduction @ATARI).

Was the the Mega ST display on the JVC in real life closer to my initial screenshot, or is the red (index 2) really displayed more as an orange than red?

My first experience with color monitors on the ATARI started with a NEC Multisync (I wish I still had...) around 1991/92, so I never experienced this kind of variation ATARI seems to have had with their original monitors (as I only had a SM 124 on the ST).

[Darklord]

Thx for the screenshots. I think with these other crt vendors (my screenshot was from a SC1224 with MATSUSHITA CRT) things are getting more complicated than I anticipated (there was probably no real check/strategy about color reproduction @ATARI).

Was the the Mega ST display on the JVC in real life closer to my initial screenshot, or is the red (index 2) really displayed more as an orange than red?

My first experience with color monitors on the ATARI started with a NEC Multisync (I wish I still had...) around 1991/92, so I never experienced this kind of variation ATARI seems to have had with their original monitors (as I only had a SM 124 on the ST).

I think it was a bit closer to your shot - the camera really did wash it out some. I even tried turning the flash off,
overhead room light off/on, etc but it didn't seem to make much difference.

Of the Atari SC1224 CRT's I've experienced (Goldstar, JVC, and Samsung), the JVC brand consistently gives the
best overall picture, IMHO.

Hope all this helps.

[pixelpusher]

Thx for the screenshots. I think with these other crt vendors (my screenshot was from a SC1224 with MATSUSHITA CRT) things are getting more complicated than I anticipated (there was probably no real check/strategy about color reproduction @ATARI).

Was the the Mega ST display on the JVC in real life closer to my initial screenshot, or is the red (index 2) really displayed more as an orange than red?

My first experience with color monitors on the ATARI started with a NEC Multisync (I wish I still had...) around 1991/92, so I never experienced this kind of variation ATARI seems to have had with their original monitors (as I only had a SM 124 on the ST).

I think it was a bit closer to your shot - the camera really did wash it out some. I even tried turning the flash off,
overhead room light off/on, etc but it didn't seem to make much difference.

Of the Atari SC1224 CRT's I've experienced (Goldstar, JVC, and Samsung), the JVC brand consistently gives the
best overall picture, IMHO.

Hope all this helps.

I recapped the main pcb, neck board and PSU (replacing only the electrolytic capacitors though - nearly 70, which was not a fun job...)

MainPCB.jpg (360.29 KiB) Viewed 1236 times

Afterwards the picture was still too dark for the colors I mentioned in my original post.

Therefore I adjusted Focus and screen brightness on the Flyback Transformer (FBT) and later on the sub brightness (on the main pcb) and now got something that is quite close to what I've seen on Apple RGB monitors (and probably you see on the JVC too):

SC1224Recapped.jpg (440.6 KiB) Viewed 1236 times

So, my conclusion would be
- Recapping alone doesn't solve the issue of a too high gamma value of these Goldstar monitors, but the new components give you some more wiggle room (and also some confidence to exploit it).
- The Goldstar based monitor's display can be improved compared to what ATARI's original setup delivered.

DISCLAIMER: If you don't know how to discharge a CRT (with its potentially deadly voltage) and don't have suitable plastic adjustment tools for the the FBT or mainboard, then don't try this.

[Darklord]

Nice job.

[sandord]

I think it was a bit closer to your shot - the camera really did wash it out some. I even tried turning the flash off,
overhead room light off/on, etc but it didn't seem to make much difference.

Of the Atari SC1224 CRT's I've experienced (Goldstar, JVC, and Samsung), the JVC brand consistently gives the
best overall picture, IMHO.

Hope all this helps.

I recapped the main pcb, neck board and PSU (replacing only the electrolytic capacitors though - nearly 70, which was not a fun job...)

MainPCB.jpg

Afterwards the picture was still too dark for the colors I mentioned in my original post.

Therefore I adjusted Focus and screen brightness on the Flyback Transformer (FBT) and later on the sub brightness (on the main pcb) and now got something that is quite close to what I've seen on Apple RGB monitors (and probably you see on the JVC too):

SC1224Recapped.jpg

So, my conclusion would be
- Recapping alone doesn't solve the issue of a too high gamma value of these Goldstar monitors, but the new components give you some more wiggle room (and also some confidence to exploit it).
- The Goldstar based monitor's display can be improved compared to what ATARI's original setup delivered.

DISCLAIMER: If you don't know how to discharge a CRT (with its potentially deadly voltage) and don't have suitable plastic adjustment tools for the the FBT or mainboard, then don't try this.

What are those plastic adjustment tools you are talking about? I'm very interested in that... I guess you used them to calibrate the monitor, like the technical manual should explain? That should involve using a scope and signal generator as well.

[pixelpusher]

What are those plastic adjustment tools you are talking about? I'm very interested in that... I guess you used them to calibrate the monitor, like the technical manual should explain? That should involve using a scope and signal generator as well.

These are isolated plastic tools of different length - example about 30 cm - used for adjustment (of potentiometers/variable coils) in CRT's - mine being more than 40 years old, so I don't have a link where to find them )

IMG_5075.jpg (88.48 KiB) Viewed 1178 times

I didn't touch the the RGB drive/cut off controls mentioned in the service manual (as at that point you'd need a good test pattern and colorimeter; they give a x,y white point to adjust for as well as instructions for adjusting the convergence, but that wasn't a road I was comfortable to take).

With an Apple IIgs RGB CRT monitor for comparison as well as a sRGB calibrated LCD (with SCART input) and several sample images I knew which brightness I had to achieve (and which colors were much too dark with the original setting of the SC1224).