There is several threads on the topic here in Tech talk.
Here is the tiers for analog video quality, going from worst to best
RF or Radio Frequency - Worst image quality, most visual artifacts, heavily prone to interference.
Composite video - Usually uses a single yellow RCA plug for the picture (with separate RCA audio plugs)
S-video - Image is separated into luma (brightness) and chroma (color) normallyuses a mini din connector
Component video (YPBPR or YUV) Component separates the video signals to Luma, the diffrence between Chroma and Blue and the Difference between Chroma and red (green info is extraplated from the other Chroma info). Composite is not RGB video. usually uses 3 RCA plus for video or BNC connectors for commercial grade displays. Supports resolution up to 1080.
RGB or Red, Blue, Green Video. There are two categories here, 15 khz and 31 khz (technically there also a format in the middle but that only used by certain arcade boards).
15 khz RGB video is used by older Arcade Monitors, CGA/EGA Computer Monitors, older Broadcast industry displays (prior to HD) and some Television sets (mostly scart enabled European TV sets and Some Japanese TV sets). RGB Video is the cleanest, purest uncompressed video format capable up to 570i. Although Component can support HD resolutions it can and does supper from some minor image artifacts and is not a clear as RGB.
15 Khz connectors do vary wildly, RCA, BNC, DB9, Din/mini Din, HD15, DB15, DB25 and Scart connectors has been used.
31 khz RGB, most commonly found as ‘VGA’, unlike 15khz 31khz can support HD/ high resolutions. Usually found as a HD 15 connector.
What is scart ? Scart is not a video format, its a multi format cable that can carry multiple signals including Composite, RGB, Stereo Audio, S-Video and Component. Note a Scart cable can not carry Component (YPBPR) and RGB video at the same time, and repossesses the lines for one signal or another. Scart can also be used for a bi-directional Composite video, which is great for European VCRs.
I see you are thinking of a RCA/s-video to hdmi converter, don’t.
Alot like making a photo copy, if you start with a inferior image you going to get just as bad if not worst quality image as your final product.
All the flaws in the low quality RCA (Composite) video is going to get magnified on your HD Monitor.
Your options as of follows
Find a older CRT Display, Something that takes one of the following. Component (YPBPR), 15Khz RGB, or VGA.
In the US as Scart is almost nonexistent. You have to find a old broadcast Display such as a Sony PVM moniotor, a working and good order older Computer monitor like a Amiga Monitor or a older Non-VGA Apple monitor.
Another good CRT display is the Sony WEGA HD TV, yes its a CRT HD TV that can accept up to 1080i via the Component (YPBPR) input. Warring the wega Tv is very heavy.
You have to get a Scart cable for your system and an adapter. Such as Sacrt to BNC for the Sony PVM. There are also Scart to Component (YPBPR) and Scart to VGA adapters.
Keep your HD LCD Monitor. Use a 15hz RGB to VGA (if your monitor supports it) or 15hz RGB to HDMI.
You Need a Scart cable and Scart to VGA or Scart to HDMI. Converters run from $50 to $400 for some of the better Japanese image up scallers (the XRGB series)
@hawkespur what is your budget for this?