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.
Notes
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.
Resources
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
License
The materials are released under the terms below. I don't have time for pedantic nonsense or legal theory and like to keep things simple. I call this the 'Mike Chi' license. *For each project, at my sole discretion, I may release whatever sources, to whatever level of abstraction, that I deem appropriate. Additional information/materials may be released on a scheduled determined at my sole discretion.
*You may modify, use or adapt any or all portions of the released material for any purpose you wish, including sale. *The exception is that the files may not be used for commercial activities by entities, determined at my sole discretion, that have a history of violating intellectual property rights or licenses. *If these materials were helpful and/or of use to you, I only ask that you provide some form of acknowledgment that you feel is fair. *I'd also love to hear about how you're using it! *If you re-distribute the material in a substantially unaltered form, please include this notice. *NO WARRANTY EXPRESSED OR IMPLIED. WE TAKE ABSOLUTELY NO RESPONSIBILITY FOR WHAT MAY OCCUR FROM USAGE OF THE FILES. *WE CANNOT AND WILL NOT OFFER ANY SUPPORT FOR THE USAGE OF THE FILES. IF YOU DO NOT AGREE WITH THESE TERMS THEN DO NOT USE ANY OF THE MATERIALS AND DON'T COMPLAIN.