I've been working on a new upgrade that can be installed in any A8, 5200, & even the 2600.
I know there's a million upgrades out there, but the good ones are mostly based on tweaks to the original Atari circuit, and the bad ones are haphazardly designed.
1st, the real problems with the Atari circuit(s):
The video circuits in most A8's are not sufficiently decoupled from the digital noise in the system. Video power and ground are often shared with very noisy components like DRAM and this causes vertical bars to appear in the picture. For example, you can often see the refresh cycles on the left side of the screen. Crude DACs like the one made from the CD4050 have no ability to reject power supply noise and will superimpose it on the picture. No tweaking of the video buffers will remove it. Atari actually put components in some XLs to blur the image in an attempt to clear up the lines.
That's not to say the buffers didn't need tweaking. They frequently deviated from the 75-ohm impedance standard that that meant the picture quality could be unreliable (shadows, smearing & ghosting), especially with longer cables. At video speeds, you only get maximum transfer when everything is the right impedance.
In addition, some of the chroma circuits produced highly distorted sine-waves which contributed to noise in the image.
So to solve these problems, my board:
1. Has an on-board regulator to create a clean video power source.
2. Has a 3-channel video amplifier designed for 75 ohm loads.
3. Has a pixel re-clocking circuit to remove skew and better align the 4 luminance signals into a perfect pixel edge.
4. Has a carefully designed chroma-shaping circuit.
In addition, there's an adjustable pot on the board which controls the phase between the chroma and luma signals. A nice side-effect of this that you can change the artifact colors. There's also a jumper to invert the chroma which swaps the positions of the artifact colors.
The board plugs into the CD4050 socket for easy installation on most machines. It can be used in place of the original circuit or along side it.
I'm working on the final board layout and I hope to have them available in the AtariAge store early next year. Here are some pictures of the prototype. Any patterns in the picture are due to the camera picking up the CRT mask, but you'll notice the absence of vertical bars (I'll try to get better pictures...). One of the pictures is taken off my LCD (and even the LCD is hard to photograph). The split screen pictures show the effect of artifact tuning. The board is currently installed in a very noisy 130XE. I'll try to get some before pictures.
I'm calling it UAV for Ultimate Atari Video. More to come!