Update 05-10-21: I will be releasing a video comparing the HDMI converter used in this video with the RetroTINK 2X Pro recommended by so many of you. In the meantime, I want people to be aware that they have other options. The HDMI converter works fine for me but you are displaying a 240p display on a 1080p screen. The RetroTINK doubles the resolution to 480p, and adds features like scanlines, so it’s output is much better but at 4x the cost. If cost is an issue I would stick with the converter at $35, otherwise, the RetroTINK at $130 looks much better.
This post and video are based on a blog post by Ilkka Sjöstedt. I recommend looking at his blog post at Ilesj’s blog.
I made an S-Video cable to connect the Commodore 64 to an HDMI monitor via an inexpensive HDMI Converter box. It works fairly well but I will be posting a followup once the RetroTink 2X Pro I have on order arrives. I will post side by side images from both so you can decide which solution is right for you. The converter is inexpensive and comes with both of the cables needed to make the customer S-Video cable but the RetroTink should have a much better picture.
The DIN connectors I used were pretty awful but I have finally found better ones thanks to Hey Birt on YoutTube! I set up a page where you can get the new ones or the ones I used in the video. I STRONGLY recommend the new ones, I just got a set in the mail and they are much better than the Amazon ones and they have the correct pin configuration. Get them here!
The S-Video and Composite to HDMI converter I used does a fairly good job. A true upscaler would look better but at about 4 times the cost. As an added bonus it also comes with S-Video and Composite cables which can be used to make 2 Commodore 64 S-Video adapters. Amazon Link
In the near future, I will expand this post to include information on the RetroTink 2X Pro along with side by side image comparisons of it and the HDMI converter. In the meantime here is a link to a page with some options for finding one since, as of this writing, they are in short supply.