Generating Sync Signals
Sync information must always be sent with the signal, or else the TV will not be able to display a stable picture. The TV needs separate horizontal and vertical sync pulses, but they can be combined with logic gates to make a single pulse train called 'Composite Sync'. 



This can then be added to the bottom of the video signal to make 'Composite Video'. Composite video is used for rf transmission because it is a single waveform that can be sent over one rf channel, whereas if the sync and video were kept separate they would need a channel each. 

Composite video typically has an amplitude of 1V, with the top 0.7V being video and the bottom 0.3V being sync pulses. The TV extracts the sync pulses with a comparator or 'sync separator' which chops off the video leaving only the sync pulses below it. Color is encoded onto the video with phase modulation, using a carrier frequency of 3.58MHz (NTSC) or 4.43MHz (PAL). This requires a reference phase, which is the high frequency burst seen just after the horizontal sync pulse in the waveform below.           



Early TV sets used crude circuits that needed a continuous stream of pulses to lock onto. To improve picture stability a series of 'equalizing' pulses were added during the vertical blanking time, making it easier for the horizontal oscillator to stay in sync while permitting vertical sync extraction with a simple low-pass filter. This is further complicated by the need to split each each frame into odd and even fields for interlacing (one half of the lines are displayed in the first field, then the other half are displayed - with half a line between them so the lines of one field will interleave the other).     



With a wired connection there is no need to combine signals because the cable can have a separate wire for each signal. VGA monitors use a separate wire for Hsync and Vsync, plus Red, Green and Blue video. Some monitors can accept Composite Sync on the Hsync input, while others may work with 'Sync on Green'. Component inputs on modern TVs have sync on the Y (luminance) input. SCART connectors have separate inputs for RGB and composite video/sync (which supplies composite sync in RGB mode and composite video in CVBS mode).