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I've added color support to the Atari 2600 emulator for the Commodore 64. Here's a screenshot of Gate Crasher Jazz Improv, my 10 line BASIC entry for the 2024 BASIC 10 Liner contest. Features two algorithms for Jazz improv, you have to make it thru the gauntlet to unlock the second algorithm and multicolor mode. I'll be releasing an updated version of the emulator with Atari Flashback BASIC support in addition to Supercharger BASIC prior to the March contest. Hope C64 fans are encouraged to enter the 10 liner contest with this fun BASIC! Download the game and the10 line BASIC program listing here from CSDB: https://csdb.dk/release/?id=238874 Video playthru with Jazz improv soundtracks and amazing color bloom Fx and artifacting on my classic JVC Television and 1982 breadbox with a 6581 R? SID and with RF over NTSC: real video unretouched How do you feel about the screenshot and the video shot on classic hardware and CRT? Is it fair to use classic Television and RF instead of Composite or S-Video the C64 natively supports? Many people had an ordinary color TV connected to their Home Computers in the 80's. Some sceners touch up their retro productions with 256 colors or more on modern hardware. We can do cool stuff with NTSC over RF too, the visuals are retro tricked out like my computer art contest entry from the 80's: That's a black and white image.
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I was reading flashjazzcat's thread about his GUI project and saw his screenshots, and it got me thinking about artifacting (again). Artifacting, for those who aren't familiar with the term, is the effect on NTSC systems with a composite monitor where the LUM signal interferes with the COLOR signal, which causes half-size pixels to display with a colored "fringe". If you draw a whole row of even-numbered or odd-numbered pixels, you get one of two "fake" colors. By setting the background color to black and the foreground to white (or vice-versa), the effect is most pronounced. Anyway what got me thinking about it is, FJC showed (I believe) an 800XL with purple/green artifacting, and a 65XE with blue/orange. My XEGS does purple/green, and my old (long-dead) 800XL did blue/orange. I've also heard of yellow/blue though I've never seen it. So I'm wondering: what actually determines the color of the artifact? I doubt it's the (solely) IC because swapping GTIAs doesn't seem to affect it (with my limited sample set anyway). To answer this mystery I dug into the GTIA (and CGIA) data sheets to figure out exactly how this color timing works. Hope this comes out right: _____________ / LUM ------------------------------------------------------------------------------------------X LUM 0-F |<----------------------------------------------------------------------------------->|\_____________ | T(lum1) = Luma output delay (max 450ns) ___ | ________________________ __________________ \| / \ / OSC \ OSC low 140nS / OSC high 140nS \ / |\________________________/ \________________________/ | | T(inv) = Color output delay (max 190nS) |<-------------------------------->| ______________________________________|_________________________________________________________________ |/ COL / Color Reg = 0 (No color output) _____________________________________/ _________________________________________________________________________________________________ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ COL \ 1 \ 2 \ 3 \ 4 \ 5 \ 6 \ 7 \ 8 \ 9 \ A \ B \ C \ D \ E \ F \ \___\___\___\___\___\___\___\___\___\___\___\___\___\___\___\____ |<->| Δt = 16-25 You can see from the diagram the same thing I realized: the luma output for a given color clock comes much later than the color output! This seeming discrepancy is possibly explained by delay introduced by capacitance in the color output circuitry, which can work as an AC delay (technically [in a perfect capacitor] it causes the signal to be changed into the derivative of its input, for any calculus people out there). Now one thing you may be thinking is "What about the color adjustment pot?", which is a valid question. The CADJ signal is a voltage input to C/GTIA which adjusts the delta between different colors. Because this affects the delta, there is only one setting that is "right": too high or too low and your spectrum goes either above or below 360°. So given that only one setting is "right", it follows that you cannot change artifacting using this input without actually making "normal" colors incorrect. As you can see, every hue value corresponds to a 16-25 nS delay; thus a difference in delay of as little as 10s of nanoseconds in either the luma or color output can, without impacting normal color output, dramatically change the artifacting colors observed. So take this data and combine the fact that just about every Atari model (and, thanks to widespread modification, often multiple systems in the same model) has a different color output circuit, and the conclusion I come to is that the video output circuit used is a primary (if not THE primary) influence on the artifacting output. Interestingly when I added color output to my 600XL, I used the simple XE color circuit, and as a test I created a simple "composite" output by bridging the chroma to the luma via a capacitor, and observed the output - and the output was purple/green just like my XEGS with the identical color circuit. The only problem with my theory is the observations of FJC's demo output. He showed his 65XE with blue/orange and his XL with purple/green! This would seem to poke a big hole in my theory that I cannot explain - so FJC, did you happen to mix up the labels on those two screen grabs?
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Thanks to a generous contribution, I now have an 800 for testing and have finally been able to investigate firsthand the elusive mystery of why the 800 produces such odd artifacting colors. It's been known for a long time that this model is odd in being able to produce green/blue artifacting, which is unlike other models that produce opposite colors. I'd determined a while ago that simulating this with linear filters wasn't possible and had been wanting to track down the cause. However, when I hooked up the 800 to a Commodore 1702, I was a bit disappointed: Brown/blue artifacting, opposite colors. But then on a whim I hauled the machine into the living room and hooked it up to a Samsung TV, and was shocked to see this: A Dell U2711 monitor shows similar: But, then, capturing with an All-in-Wonder USB device produces this: This is similar to what I see on an 800XL. Mind you, this is the exact same machine, just swapping the composite output over to each monitor. I've never seen anything as crazy as this. To double check, I repeated the same test with a 130XE, and all four displays showed similar brown/blue artifacting colors. Anyone familiar with composite video or the output circuit who might have insight into why the displays vary so much in decoding artifacted colors from the 800? The one thing I've been able to determine so far is that the 800 has greater skew between the luminance bits, which manifests as pretty dark bars between luminances 3-4, 7-8, and 11-12 in a color ramp. This causes the artifacting colors to vary slightly at different luma levels. That makes me suspect that there might be something odd about the color burst, like an uneven duty cycle. Still, I can't think of what would cause the artifacting colors to vary by as much as 90 degrees between displays.
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I was writing an instructional thread on the programming forum on programming music and graphics on the VCS without writing any code, like programming Big Trek, and saw this amazing smiley face animation not intended to be part of the code. It appears to be ideosyncratic artifacting of a type never encountered before. The Atari VCS is still full of surprises: You can watch the video with the detailed animation on the instructional thread and download the ROM Explanations for the effect and discussion welcome, also would like to know if anyone is interested in the original topic, Programming Music and Graphics on the Atari with no code. It's currently possible to program the Atari like Big Trek for the Music Tracker, and to draw the background image and the sprites with ASCII art, if there is interest I could add another Big Trek section for the sprites so that they could be programmed to move about like Big Trek too. Right now you would have to program that with BASIC code.
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I've been experimenting with a new type of artifacting using some of the same techniques we leveraged in the 80's to render 4 and 7 color displays with the CoCo's 256x192 monochrome graphics mode. In conjunction with 30 FPS full screen animation I'm getting plasma effects like these: As well as the familiar CoCo artifacting we are used to: And no artifacting as expected dependant on wavelength: The plasma effect is materializing at 30 FPS from a pure 30 hz signal without the alternating field; the Atari has a unique signal that can paint the entire screen at 30 hz instead of half of it which is also part of it. The effects vanish rendering 30 hz over 60 hz objects and are maximized at 30 FPS of animation; the extra phosphor and increased contrast (30 hz objects over nothing, not even a subcarrier) seems to potentiate the plasma effects. The video of the game shows these effects also merging with the traditional CoCo effects and some others that didn't show up as well - gives a good idea but you really have to see it on a tube television. What do programmers who have worked with artifacting think? If anyone wants to check out the source or try rendering other artifacting tricks with the code, it's Tiny BASIC, similar to the original TRS-80. The compiler is included in the download site As a related subtopic, composite mod's break the artifacting on Atari just like they did initially to the CoCo; I wonder if CoCo specialists like Boisey could create a composite mod for the Atari that would bring back the artifacting and really turn it up, like John did for Tandy when he had to revise his composite mod. Not all Atari's are equal here - the Vader has fantastic artifacting capabilities but the Jr is barely able to do CoCo artifacting and cannot do the plasma effects.
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I am hooking up an Atari 130XE to a DLP video projector. Because of the large size of the projection screen, I need the highest quality video possible, which is the RGB output of the VBXE. However, I want to occasionally play games that use artifacting. Instead of using the noisy composite out, can I use the RGB through an RGB to composite converter to get artifacting? I know I could just use the composite out, but I would suspect RGB --> Composite will give a better result.
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