Most pre-NES consoles offered only RF output, and can be difficult to modify. Almost every post-NES system had a wealth of video options, most offering at least composite video output, and many easily hacked to allow RGB output, and sometimes S-video. On this page you'll find the GameSX library of video hacks - some useful, some trivial, and some downright evil. All, however, will grant you better quality images than you're seeing now.
When Lawrence started GameSX.com back in ~1996 there were no sources for this information online. Gopher was still vying with the infant WWW as the most useful resource, and GameSX.com - originally intended to be a website for Game Station X (Lawrence's store) was instead a repository of knowledge gleaned from obscure sources and trial and error (mostly the latter).