Updated: May 24, 2020
Hope everyone is doing well in these crazy times. Stay safe!
Here's a fun little project I've been working on over the past couple of weeks. In short, it's a RGBHV or RGBs to SCART/S-Video/Composite encoder. This was something I really wanted for my MiSTer setup. Hopefully by releasing these designs, someone(s) will run with this and take it to production.
The encoder is based on the AD725. You must use negative going sync pulses for this to work. If your source is RGBs, then either disconnect the vertical sync line (it's pulled high internally) or set the vertical sync line to high. On the MiSTer, don't use composite sync = 1 unless you disconnect the vertical sync line, as it is normally held low.
The board is hardwired to output NTSC. Building a PAL version is possible by switching the crystal to the appropriate frequency and setting the mode pin on the AD725 encoder.
The one thing I'm not 100% happy with the design is the generation of the NTSC encoder clock. Unfortunately, it would require a complex circuit to multiply the horizontal frequency up to produce a phase locked chroma clock. So in this design, the 3.58 MHz carrier is not synchronized to the origin video clock resulting in more color fringe artifacts. However, this does not affect S-Video quality.
The sync line for the SCART is derived from the composite video signal and output as standard 75 ohm video. No TTL CSYNC nonsense here.
There is no scaling - you'll need to feed it 15 kHz sources.
CAD Files and BOM: https://www.dropbox.com/s/ejhmivn1yw0b378/VGA2NTSC%20v11.zip?dl=0
UPDATE: jft was kind enough to convert the entire project to KiCAD, make quite a few quality of life improvements and upload to Github. I suggest using this as the official project repo: https://github.com/jnftech/vga2ntsc-kicad
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