Proper Adjustment of VR1 (Color Potentiometer) using Atari SALT 2.05 "Colorbar" screen

[ACML]

The Figure 3.3 has been updated in color as the Field Service Manual was B&W.  This manual describes how to set the VR1 (1200XL) color pot correctly.  This should be valid for all Ataris, not just the 1200XL.  I simply run SALT 2.05, select the color bar screen and finely adjust the VR1 color pot until the two bars that bracket the gray reference bar are both the same color which is gold. I've been able to do this without too much fuss, but I have a question.  When I run Altirra and run SALT 2.05, why are the color bars both not gold?  The one above the gray reference line is gold to maybe orange and the bottom one locks more yellow.

 

COLOR BAR TEST

This test verifies and allows for adjustment to the color circuitry. Enter the command letter C

and press RETURN to activate this test. Figure 3-3 is a black and white representation of what

your television display screen should look like.

A 15-color rainbow scale is displayed above the reference bar with a single color bar below. The

color, bars directly above and below the reference bar should be the same color (golden rod). If

not, proper adjustment of VR1 makes the color bars above and below the reference bar

identical, thus adjusting the color frequency of the console to the proper setting. See Figure 4-1

for VR1 loćation. Minor glitches on the edges of the color bars are acceptable. Leave this test on

for at least 60 seconds in order to catch any intermittent problems, such as a bar momentarily

changing colors or blanking out. 

-------------------------------------------------------------------------------------------------------------------

 

 

Color Bar screen from SALT 2.05 using Altirra

Edited August 27, 2023 by ACML

[Peri Noid]

Does it work the same way for both NTSC and PAL machines?

[ACML]

49 minutes ago, Peri Noid said:

Does it work the same way for both NTSC and PAL machines?

I do not know.  All of my machines are NTSC.

 

[phaeron]

You can adjust this in Altirra, in View > Adjust Colors > Hue Step. A value of 25.7° makes hues 1 and 15 the same. But this will tend to make the GR.0 screen slightly purplish, and in general tends to cause issues with "sour" colors in at least one characteristic game somewhere.

 

The best that we've been able to figure out about this is that it appears that contemporary displays rendered colors with a bit more of a reddish tint than they do now. One example of this is Lucasfilm's startup logo on its games, which is rendered in hue 1. People who have worked on and played these games have a recollection of this screen being more gold-colored than it appears now. This is also implied by the PAL GTIA, which produces a hue 1 that is closer to gold color.

 

Problem is, hue 1 also doubles as the reference color burst, so it is unaffected by the color adjustment pot. In the NTSC broadcast standard and on modern displays, the color burst color is more of a pale, slightly greenish yellow. The only standard way to reach a more gold-colored hue 1 is to adjust the display tint away from standard, which in Altirra is done by adjusting Hue Start from -57° to around -40°. But this isn't what is seen on modern displays, which is why Altirra doesn't ship with it as the default. The default hue step is overtuned past 25.7° because that's where I've often seen the color pot adjusted to get colors rendering reasonably with the standard burst color on modern displays -- particularly with difficult cases such as Epyx Summer Games, which renders the red stripes of the U.S. flag with hue 15.

 

As for what Atari intended, they unfortunately aren't consistent in their own literature either. The Hardware Manual, for instance, doesn't imply that hues 1 and 15 should necessarily match:

That having been said, the BASIC manual agrees with SALT, and the PAL GTIA is hardwired to make hue 1 and 15 the same, so that was more likely the predominant belief at Atari than otherwise.

 

[DragonFire]

Many NTSC CRTs intentionally exaggerated red colors. The common term for this is "red push" and modern CRT enthusiasts frequently discuss ways to mitigate it. The most popular theory is that this was done to make skin tones more lifelike but that's just a guess. All that's known for certain is that it's a common feature of many consumer CRTs. Most JVC sets and nearly all Sony Trinitron sets (outside of their professional monitors) do this.

This may be one reason that people recall yellow and gold colors being more red than they appear on modern displays.

The fact that modern displays usually ship with a cooler (more blue) color temperature might also be an issue. And NTSC color decoding standards also changed in 1987, so TVs made before and after that year can be expected to behave slightly differently, which will also have an impact on what people see and remember.

I looked into this a few years ago and flipped through several old manuals and reference docs to see if they agreed on what the colors should be. Like phaeron said, contemporary documents are inconsistent but lean towards agreeing with the SALT test. Most Atari documents suggest hue 1 and 15 should match. On the other hand, most modern Atari users seem to lean away from the SALT standard (at least based on what you see in forums and emulators) so it's likely that most homebrew is designed with the assumption that hue 1 and 15 are different.

I actually kept track of all the documents I looked at, including specific page numbers where they referenced colors. If I can find those notes I'll share them here later in case they might be helpful to somebody.

 

 

[_The Doctor__]

there were 2 different ntsc specs even before HDTV and all of this,

they are not the same,

 

LCD displays had another, and it gets worse.

 

todays HDTV have 2 different color spaces as well, and this isn't taking modern lcd and oled monitors into account either.

Check your display and toggle you color space between native etc and whatever else it's choice are, DO NOT confuse that with Color temperature which can throw your colors of as well

 

Altira does a great job of making a modern display close, and there are tweaks in it that you can change.

[ACML]

Another observation I've had over the years is that when I check the color bars using SALT 2.05 on stock machines (i.e. not worked on since it left the factory), the colors bars are usually spot on set so both bars are gold.  So, even with modern LCD TVs like my Samsung, these machines are factory set to the two gold bars just like the service manual says.

 

[_The Doctor__]

The PAL hard-wiring was mistake from a mistake, but it suited the system.

NTSC should produce a slightly different hue between the two.

The mistake is a carryover from the 2600's color adjustment procedure.

GTIA had it right then you end up with conflicting internal Atari documentation. Close enough became good enough.

.

For todays display NTSC they still shouldn't be either

 

They shouldn't be, as the colors-pace is wrong, if they are your other colors will be off.

Didn't we do this before? It isn't one or the other it's multiple issues.

Edited August 28, 2023 by _The Doctor__

[DragonFire]

Here’s a summary of the contemporary sources I found when I looked into this a few years ago. I’ll start with sources that say that hues 1 and 15 should match and end with sources that say they shouldn’t. I've included links to screenshots of relevant pages to avoid this post being too massive, and full documents can be found on Atarimania for anyone interested. All documents are official Atari publications unless otherwise noted.

 

The Supersalt Technical Manual matches what OP found, saying you should calibrate machines so that hues 1 and 15 match.

 

The Atari Field Service Manuals (400-800, 600XL, OP posted 1200XL already) mirror this information.

 

The Atari 800 Technical Reference Notes calls them both Light Orange, meaning they should match. The third-party Analog Pocket Reference Card does the same on page 8.

 

The Atari Basic Reference Manual (Rev C, p58) calls hue 15 “Light Orange” and hue 1 “Light Orange (Gold)” indicating that both terms (Light Orange and Gold) refer to the same colors in that document. I found two third-party publications, The APX Atari Program Text Editor (p33) and The Creative Atari (p31), that do the same thing.

 

The Atari 400-800 Hardware Manual calls hue 1 Gold and hue 15 Light Orange without indicating they are the same color. The third-party book Compute’s First Book of Atari Graphics does the same on page 14. I believe the previous sources make this somewhat ambiguous, though.

 

The (non-Atari-published) magazine Analog Computing included a color-bar program in issue 30, and their screenshot shows hue 1 and 15 looking extremely similar, but maybe not identical.

 

The (unofficial) Atari XL User’s Handbook calls hue 1 Gold and hue 15 Orange on p181... but later calls hue 15 Light Orange on p237. Meanwhile, another (unofficial) guide, Your Atari Computer XL Edition, calls hue 1 brown-to-gold and hue 15 khaki-to-yellow on p389, which sounds more similar (and less red) than the other non-identical descriptions.

 

On the other hand, the Atari CGIA Chipset Documentation p48-51 includes specific phase angles for each color, and color 1 (0 degrees) and 15/F (336 degrees) are explicitly different. This is the only official Atari document I can find that unambiguously says the two hues should be different, but it is very clear on that matter.

Short version: Most sources I encountered indicate hues 1 and 15 should be identical (many referencing the SALT test) but several do not or are ambiguous, and at least one official source (the GTIA technical documentation) says they should be slightly different and explains exactly how different they should be.

Edited August 28, 2023 by DragonFire

[phaeron]

Adding to the confusion is that the documentation isn't always correct. The CGIA documentation, for instance, has the most precise information about cycle timing of any official document, but it's wrong for P/M graphics (it should be 240 and 960 cycles per field for missiles and players, respectively). The color information in it may also have reflected what the hardware folks thought was best instead of actual practice in the field -- adjusting hue 1 = 15 is the easiest procedure, but a waste of a hue. Other sources like the BASIC manual probably just copied from other docs (and missed a hue!).

 

There is a similar phenomenon with the PAL GTIA. It appears to have been designed for a 22.5° color delay, as the assigned phase angles align best at that setting -- but no real-world picture of a PAL color palette has been adjusted to this. Most are around 18-19° per hue instead, with the resulting striped hues 3 and 10 and uneven color bands.

 

[_The Doctor__]

If you waste the hue, it compounds in wasting color/hue in nifty extended graphics goodies we have enjoyed as well. The missing hue is missed not only twice in such operation, but is not there to mixed in other combinations as well. We have always stuck with the engineering specs. The slight imperfections and drift that could occur at the end users display never seemed to affect things to a degree that was bad enough to be a visual problem unless the display or re-rendering device itself needed calibration.

 

Thank you DragonFire an phaeron for cutting to bone so quickly in the last few posts. It was engineered to be different and there is a bit to be gained by calibrating it as such.

 

Modern displays are fast losing the ability to adjust color space or calibrate to any of the older standards. It requires additional equipment for those to look remotely correct.

[+Stephen]

1 hour ago, _The Doctor__ said:

Thank you DragonFire an phaeron for cutting to bone so quickly in the last few posts. It was engineered to be different and there is a bit to be gained by calibrating it as such.

Well, the machine was always (from GTIA onwards) as a 256 colour machine, not a 240 colour one.  So I never understood (nor adjusted) mine to make the two hues match.

[ACML]

9 minutes ago, Stephen said:

Well, the machine was always (from GTIA onwards) as a 256 colour machine, not a 240 colour one.  So I never understood (nor adjusted) mine to make the two hues match.

Agree.  So what does/should "right" look like (SALT 2.05 color bars)?

 

[_The Doctor__]

1 is GOLD

15 is Gold with an ever so slight Green tinge is one way to describe it

make sure the machine has been on and doing something for at least 15 minutes then adjust

 

did you look at the color wheel?

Edited August 28, 2023 by _The Doctor__

[ACML]

16 minutes ago, _The Doctor__ said:

1 is GOLD

15 is Gold with an ever so slight Green tinge is one way to describe it

make sure the machine has been on and doing something for at least 15 minutes then adjust

 

did you look at the color wheel?

Can you provide a visual (screen shot) of the 2.05 SALT color bars set to your description?  If I can see it, I can follow it.  Thanks!

 

 

[+Stephen]

1 hour ago, ACML said:

Can you provide a visual (screen shot) of the 2.05 SALT color bars set to your description?  If I can see it, I can follow it.  Thanks!

Only problem is, every screen looks a little different, and cameras almost always ruin the colours.  Add all this up and then add on top of it, the frustration of being colour blind.  It's a losing battle for me, for sure.

[ACML]

I've run across four factory set 1200XL mainboards that have never been installed.  All four have the two color bars set to Gold.  There is a minor shade difference between the two, the one above the grey reference bar slightly darker gold, but they are still very close.  I'm viewing this on a 46" Samsung TV.

 

[_The Doctor__]

the slight difference in shade is more than likely correct, your samsung is a modern display with a different color space, do the other colors come close to being correct?

[DragonFire]

Boards that still have their factory calibration are probably the best reference point for how VR1 was originally set. I wish there was an easy way to record the color output from multiple "factory-set" machines so we could have a good data set to draw from.

Since we're talking analog video, the other half of the equation (literally) is how 70s and 80s TVs would have processed those signals. Unfortunately that varied from set to set and rarely followed the NTSC standards perfectly. I think that's why some people suggest adjusting the hue on your display rather than VR1 on your Atari. Whether you should attempt to emulate the output of specific old tv sets or stick to a specific technical standard is a matter of preference.

For example, most NES emulators include several color palettes. One common palette is called "Consumer" (or CXA2025AS) which is based on the colors produced by Sony consumer CRTs that used CXA2025AS (or similar) chips to decode composite video. There are also palettes that are based on the output of professional CRTs or based on pure math and measurements, and those have a few noticeable differences compared to the Consumer palette, indicating that the Sony sets were doing something non-standard... but if you grew up playing an NES on a Sony TV, the "consumer" colors might closest to what you remember.

[ClausB]

On 8/28/2023 at 4:26 PM, _The Doctor__ said:

1 is GOLD

15 is Gold with an ever so slight Green tinge is one way to describe it

make sure the machine has been on and doing something for at least 15 minutes then adjust

Agreed. I recently adjusted a 400, left it on for an hour, and had to readjust.