How did old video players, game consoles, etc. know what scanline the CRT was on? Did TVs adjust themselves based on the signal they received?
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The video player/game console knew because it was generating the timing signal. The TV interpreted that signal to find the start of frame and the start of each line.
There is a signal know as the horizontal blanking signal, it told the input when it was at the bottom of the screen and when to start from the top again, as the time between top and bottom was a fixed amount it only takes some basic maths to work out where the electron beam is on at any given point.
An NTSC (and PAL) tv signals contain two important periods, a vertical blanking period and a horizontal blanking period. There’s a horizontal blanking period after each line (further divided in to a ‘front porch’ and a ‘back porch’, so conceptually you can view the back porch half as taking place at the end of a line and the front porch half as taking place before the next line, although it’s a contiguous chunk of time). The vertical blanking period comes after a field (tv signals are interlaced, so a field is half the lines in a frame). Using these two blanking periods is all you need to know where you are.
Not directly related, but if the video signal is being driven by the CPU, these blanking periods are the only time you can do actual CPU stuff that’s not related to driving the image to the screen. One of the most well known books on the Atari 2600 is called ‘Chasing the Beam’ because that’s what it had to do, a constant mad dash to stay ahead of the electron beam.
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