How can I boost color saturation on an Atari 800?

[bugbiter]

I have an Atari 800 which has quite low color saturation compared to some other XLs I own.

Especially when displaying APAC images the colours are quite dull due to APAC using color only on every other scanline.

 

How can I boost the saturation in the circuit? surely there must be a resistor on the Chroma line somewhere that could be modified?

 

Any hints?

[+Larry]

Can you post comparative pictures?  PAL or NTSC? 

[Rybags]

I'd guess PAL.  APAC doesn't work real well on NTSC.

 

Fairly sure the Atari generates a constant level saturation (as in output level).  You can see that in full colour/luma range displays.  Supposedly to get higher saturation the colour amplitude has to rise as the luma rises which is why brighter luma levels get washed out.

 

Unsure if 800 is different to XL since the power supplies are different.  Later machines supposedly use a voltage doubler on Phi2 clock (?) to generate the higher voltage needed for the colour signal.  Since earlier machines have 12V DC internally they might use that instead.  In my experience the older computers seem to generate better looking colour than the newer ones.

Possibly some passive component is worn to the point where insufficient voltage is being supplied so giving weak saturation as a result.

Edited November 11, 2022 by Rybags

[bugbiter]

I'll try to make some images of a side by side comparison of a 600xl to the said 800 with lower saturation.

[a8isa1]

2 hours ago, Rybags said:

APAC doesn't work real well on NTSC.

 

 

I'm trying to keep on topic (at least a little) but I have a question regarding this.  

 

When I once again owned a CRT TV (NTSC) I discovered APAC pictures, movies (in Phaeron's format), and TIP animations looked terrible.  For APAC and the movies the only colors to appear were shades of blue and faded pinks. Just hints of other colors.  Green is completely absent.  For TIP animations all colors are nearly muted to grey.

 

I understand the TV exploit used in TIP does not exist on NTSC TVs/monitors.

 

Here's the odd thing.  I previously used a consumer grade scan converter (Viewsonic VB50HRTV, model number may be wrong).  APAC pictures and the movies worked with all 128 colors, slightly low saturation, but still quite colorful.   TIP animations kind of worked.  Lines of color alternating with lines of greyscale but my eyes saw the images in all the colors.

 

Can anyone theorize why all three formats work as well as they do on my scan converter?

 

Thanks.

 

-SteveS

 

Edited November 11, 2022 by a8isa1

[ivop]

2 hours ago, Rybags said:

Later machines supposedly use a voltage doubler on Phi2 clock (?) to generate the higher voltage needed for the colour signal.

It's indeed a voltage doubler, but it uses Phi1. I have once simulated it in LTSpice. Maybe I can find it again (or do it again). IIRC it was not a particularly clean 10V

 

 

2 hours ago, Rybags said:

Since earlier machines have 12V DC internally they might use that instead. 

They do. You can see it here on the CPU board:

+12V going directly to a small filter and the color adjustment pot, and then to pin 17 of CTIA/GTIA.

[+mytek]

4 hours ago, Rybags said:

Since earlier machines have 12V DC internally they might use that instead.

And so did I for the 576NUC+, but I down regulated it to 10V with a precision temperature compensated zener diode to produce a stable color hue with any PSU.

I kinda hopped in here, so I didn't read the entire topic, but this particular adjustment won't change your color saturation, and only affects the hue (tint).

 

EDIT: This schematic is not exactly the 576NUC+ video, but a more bare bones representation of what it could have been like without the VGATE circuit involved.

 

BTW, there is also a stand-alone UAV quality/VGATE board design in the public domain HERE that piggybacks the GTIA chip.

[bugbiter]

Here is a side by side comparison of the 800 on the left - and a 130XE on the right. The difference shows only slightly on the pictures. Its quite hard to capture APAC with interlacing on Foto.

But you can see Alfred's face is more rosy with the 130XE.

 

[ivop]

What output do you use? RF, Composite or Y/C?

 

The 130XE is known for its simple but very effective chroma output amplifier with a single transistor:

Compare that to the 800XL circuit that's coming from the COLOR pin

[bugbiter]

I use Y/C (separate luma and chroma input)

 

Does that mean there is no easy way to improve color strength on the old 800?

[+bob1200xl]

I would try making R203 (now 270 ohms) smaller and/or R204 (now 75 ohms) larger.

 

Bob

 

 

[oky2000]

Looks like a gamma and saturation issue combined, the third picture on both is the closest, the first picture makes me think the saturation is low and gamma high on the 800.

 

Not sure about the 800 modulator but on the C64 early models (AKA breadbin) there are two pots you can adjust to adjust gamma and chroma even for RF output. 

[bugbiter]

On 11/15/2022 at 3:43 AM, bob1200xl said:

I would try making R203 (now 270 ohms) smaller and/or R204 (now 75 ohms) larger.

 

Bob

 

 

Thanks Bob, I'll try that!