https://www.aliexpress.us/item/3256806016451623.html?
It appealed to me particularly because it makes use of the last iteration of the Sony CXA video encoder chip. (CXA2075) The S-Video output seems fine though colors might be a little saturated, but with the composite output there is a consistent fuzzy/shimmery effect that's best pointed out here in this demo video I took:
https://www.youtube.com/watch?v=yJ_laGvsgwo
It's really noticeable around the blue-on-white Sega logo at 0:27 and in the yellow emblem surrounding Sonic in the title screen. Digging around I found what looks to be some notes by someone involved in designing the chip, note the NTSC application PDF but the datasheet's good reference too:
http://www.videoi.com/~pietro/cxa2075/
I've tried a number of things to pin down why composite is shimmering:
• My first suspicion was noise from the 5v USB power jack- I bypassed this with a dedicated benchtop power supply. No difference.
• Board's output caps were 100uf, swapped them with 220uf which the example circuit recommended, no difference. (Wasn't expecting this to work.)
• This board takes SCART input and runs the RGB lines through a THS7314 to bring .7vpp signals up to 1.2vpp. The CXA datasheet states the RGB input expects 1vpp with a threshold up to 1.3vpp so on a whim I removed the amp entirely, bridged the gaps and the resulting image was predictably darker/dimmer, but I could still make out the shimmering.
• I noticed the amplified RGB lines ran -under- the CXA chip before they reached its inputs, and this happened to route close under pin 20 (composite out,) so I tried bypassing this to see if there'd be any difference. Nope.
• CXA pin 17 is an internal switch for internal or external luma trap to make the Y/C mix for composite output- the board is set to use the default internal trap, but I tried disabling this with an external trap circuit as described in page 14 of the CXA datasheet, testing with varying levels of capacitance. Composite still shimmered.
The only thing I can think of at this point, but haven't tried, is that CXA pins 1 and 24 are indicated as being separate grounds and on the board they're sharing the same ground plane. Pin 24 is technically supposed to be the isolated ground for all of the video output. I could try lifting that pin and dedicating it to composite output but wouldn't it be messing with S-Video's output too if that were the case? Is there anything else I might be missing here?
