Complete Communications Engineering

The major analog TV standards are NTSC, PAL and SECAM. The video signals consist of one luma signal and two chroma signals. Luma contains information about black and white video. Chroma contains additional information for black and white video to be converted to color video. Depending on the standard, the bandwidth of luma and chroma signals is limited to particular values. The spectra of the chroma signal has a much narrower bandwidth than luma spectrum.

Fig.1 Block diagram of analog TV reciever

NTSC, PAL and SECAM are interlaced video standards. A TV frame consists of two fields, top and bottom, following sequentially one after another. For NTSC the frame rate is 29.97(30) frames per second (fps) that equal 59.94 (60) fields per sec. For PAL and SECAM. 25 fps or 50 fields per sec.

For NTSC, one frame consists of 525 lines (263 for odd lines and 262 for even), each line follows with a rate 15.734 kHz. For PAL and SECAM, one frame consists of 625 lines (313 for odd lines and 312 for even), each line follows with a rate 15.625 kHz.

Each line consists of horizontal blanking interval and active line that contains direct luma video information with embedded chroma on chroma subcarrier. A horizontal sync pulse, followed by a chroma signal carrier burst (color burst) are located inside the horizontal blanking interval. Horizontal synch pulses are used for line synchronization. Color burst is utilized for chroma decoder carrier synchronization.

A vertical synchronization signal is located between video fields during the vertical blanking interval. It consists of a series of much longer pulses than the horizontal pulses, indicating the start of a new field.

The key difference for NTSC, PAL and SECAM standards is the method for chroma signal encoding. Two chroma signals are modulated on subcarrier(s) and added to the luma signal. On the receiver side, they are separated by a special com filter.

NTSC uses quadrature amplitude modulation (QAM) of two chroma signals to chroma subcarrier. For PAL, a similar modulation is used except the phase of part of the colour information is reversed with each line to reduce phase errors in the transmission, albeit at the expense of vertical frame colour resolution. SECAM is based on frequency modulations. For each of the chroma signals slightly different subcarriers are used and two modulated chroma signals are embedded into luma in sequential order, i.e. each line has one or the other chroma signal.

A video signal with direct luma and embedded chroma is called a composite signal. It is modulated with AM/VSB modulation (amplitude modulation with vestigial sideband) to the channel carrier frequency. Usually the signal is first modulated to some intermediate frequency and then shifted to the channel carrier. Analog TV signal in the channel actually consists of two signals: AM/VSB modulated video signal on one frequency carrier, and FM modulated audio signal located on the sound carrier that has a fixed offset from video carrier (typically 4.5 to 6 MHz depending on the standard).

The TV receiver does the next sequence of actions:

  1. Amplify, controlling the gain by automatic gain control (AGC), and prefilter and move the AM/VSB signal with audio signal to the intermediate frequency.
  2. Move AM/VSB signal to baseband and demodulate to composite signal, separate luma and chroma from composite signal and decode chroma modulated signal into two separate baseband chroma signals, and demodulate the FM audio signal.

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