The color information signal C that, together with Y, forms the composite video signal is a modulated signal. The modulation type depends on the video format.
For all PAL and NTSC formats, the coding is based on the Quadrature Amplitude Modulation (QAM) concept, where 2-color components are amplitude modulated in quadrature and then combined. The modulation must be decoded, so to keep track of the absolute phase needed to decode the color information, a reference signal, called the color burst, is inserted at the beginning of each line, right after the horizontal synchronization pulse. The insertion signal is shown in the Complete NTSC Frame Scan image.
For the SECAM format, the 2-color components are frequency modulated using two different subcarrier frequencies and are sequentially distributed on alternated video lines. SECAM does not need a color burst signal.