One of the downsides of connecting a retro console to your high defenition TV is the low quality video output. Nowadays our consoles are connected via a HDMI connection to our TV, but a console like the CD-i have to do it with composite (RCA, SCART, S-Video, depending on your CD-i player type), RF output or in the best case a RGB (although that's not available as standard on every type of CD-i player). Now CD-i member WillowFox has upgraded his CD-i 910 with a HDMI output. He uses an upscaler to get the best picture quality. It's a first attempt so it's a bit dodgy, but the potential is real nice.
You need to make a hole for the HDMI output, which you can cover with a fancy front of course.
WillowFox: "From the underside, rear off of RCA outs. RCAs soldered onto the bottom (All 4 share common ground)"
He continues: "The circuit is designed to take RCA in and put it to HDMI out, so whatever the machine was giving for RCA is the same. There is some brightness loss and a bit higher contrast with the HDMI output of the machine, and less noise, but that's only because of the HDMI upscaler's built-in filters. I used an up-scaler in this modification, It works but it works weird. Like, the picture is different than normal video, in that it is a little more harsh, almost like the line tripler is trying too hard to clean up the video. It is tolerable but it is still, well kinda crappy. And you can only upscale 240x480 lines so much. If you bring it up to 4k it will look terrible, you can tell the up-scaler to bring it up to 1080p, and it's not super bad, i mean it still looks like CD-I at normal resolution, but you will see it fine on the TV, and your 4k TV should auto adjust to the 1080p just fine as most stuff still uses it so it has to accept both 1080p and 4k. That being said, I mean it will work, but don't expect it to look like an x-box or something. "
[Thanks, WillowFox]