[Penthode]

I am a broadcast engineer and for the last 25 years the studio interconnections have been SDI or the serial digital interface. SDI is a component video interface that better lends itself to rescaling or aspect ratio conversion with no perceptible quality loss. Our vintage TV video sources from DVD are recorded component video and so if you interface from the DVD player to SDI input of a good quality used broadcast quality rescaler and then afterward convert to composite NTSC, you will maintain the best quality.

I just checked eBay and saw a number of used rescalers with SDI input available. There are cheap HDMI to SDI converters and cheap used SDI to NTSC converter encoders on eBay as well.

That is the way to go if you want professional quality conversation for a couple of hundred bucks maximum.

On the other hand, I convert internet streamed YouTube video to my vintage TV from a Roku 1. It has an internal rescaler which ensures the video is correct 4:3 on its output. Only drawback to the Roku 1 is the audio output is high and overmodulates cheapo RF modulators. A simple resistive antenuator fixes that.