From a “theory” perspective, here’s a diagram that may be helpful (Principles of Television Engineering by Donald G. Fink, 1947).
For a monochrome image, you have a voltage signal corresponding to brightness. With the original technology used in analog TV, this voltage signal would directly modulate the intensity of an electron beam. The TV would contain two sawtooth oscillators which drive solenoids (electrically controlled magnets), the horizontal and vertical deflection solenoids. These solenoids would generate controlled magnetic fields that deflect the electron beam to hit a specific point on the (phosphorescently treated) screen.
The “horizontal” sawtooth oscillator controlling the horizontal deflection solenoid is set to a high frequency, scanning from left to right across the screen thousands of times per second – each such pass is called a “scanline”. The vertical deflection oscillator runs more slowly, at 50 or 60 Hz depending on what television standard is used in the country the equipment was made for. Each pass of the vertical deflection oscillator is called a “field”.
When you feed in some signal to modulate the intensity of the beam (the brightness), that signal needs to be synchronized with the cycles of the horizontal and vertical deflection oscillators. This is where sync comes in – horizontal and vertical sync pulses are transmitted along with the intensity modulation signal, and these sync pulses are used to reset the phase of the deflection oscillators, exactly as a sync input works on many oscillators used for audio synthesis. When the signal you are using to modulate the beam intensity is also being generated by an electronic oscillator (LZX Prismatic Ray, Cadet IX, other “video oscillator” modules), you may wish to also reset the phase of that oscillator so that your Prismatic Ray or whatever gets its phase reset at the same time as the deflection oscillators do. If you don’t reset the phase of your “video synthesis oscillator” with the horizontal or vertical sync pulses being fed to the display, then your oscillator will not display as a still image: for instance if you don’t sync the video synthesis oscillator to the vertical sync pulses, the oscillator will appear to scroll vertically. Without horizontal sync, an oscillator running at horizontal scan frequencies (a few kHz up to 10 MHz) will generally have an unrecognizable waveform.
This is how it all basically works for monochrome images. For color images, “how color is encoded” depends some on the display format (composite vs YPbPr vs RGB) but video synthesis systems are designed to let you patch with the individual color channels as though they were independent monochrome signals. (In the “glitch” domain, there are devices which deliberately interfere with the color encoding and/or sync to produce interesting visual effects.) And of course non-analog displays like LCDs don’t work like this at all, but may include some circuitry to translate from a standard analog format like a composite input to a digital representation appropriate for driving the display.
