Retro Gaming vs Modern Televisions
Video standards aspect ratios and refresh rates.
Now we've got the transportation of the video signal covered, on to the video signal itself.
Modern parlance in video signals is usually XXXp. For example 1080p or 1080i. The 1080 in this instance refers to the vertical resolution, or put simply, the number of lines. The P and the I refer to progressive scan, where all lines are written in a single pass. Or interlaced, where an image consists of two fields, one pass containing the odd numbered lines and the other the even. This can result in visible flicker and odd motion artifacts.
What the above 1080p doesn't cover is the screen's aspect ratio. Most old consoles were designed before the proliferation of widescreen displays and are in a format called 4:3.
There are only really 3 mainly used formats on domestic equipment, 4:3 mainly used by old TVs and consoles. 16:9, commonly known as widescreen TV and used by the majority of modern equipment, and 16:10 (no idea why this isn't called 8:5) which is a compromise standard between the two most often found in computer monitors.
When displaying an old console on a modern widescreen TV, you're going to get borders down the sides of the screen as it reframes the 4:3 image. Also in the UK there are other issues that can result as borders on the top and bottom of the screen.
This comes down to the PAL (UK) video standard being higher resolution than NTSC (US & Japan). In theory PAL has an advantage in that it's both higher resolution (625 lines vs NTSC at 525 lines) and has better colour reproduction (NTSC is jokingly referred to as Never Twice the Same Colour). However when games were ported to the UK, they tended not to take into account PAL's increased resolution, and instead the extra 100 lines became black borders top and bottom. Also there was the issue of refresh rate, PAL being 50Hz and NTSC being 60Hz (how many times the screen is redrawn in a second). Because of this, ported PAL games tend to run slightly slower and they just altered the speed of the music to compensate.
This can create an amusing effect when you have a switch that allows you to run PAL-60 as I do on my Super Nintendo, that results in the PAL games running full screen and speed, along with slightly more urgent music ;)