Television Signal Spectrum

Television Signal Spectrum


the set of harmonic components in a television signal. The width and structure of the spectrum depend on the scanning parameters and content of the image being transmitted.

The lower limit of a television signal spectrum with line-by-line scanning is assumed to be equal to the frame frequency or, with interlaced scanning, to the field frequency. A DC component, characterizing the average brightness of the image, is usually not present as such in a television signal. The upper limit of the spectrum fmax is determined from the transmission conditions of the major harmonic components for alternating black and white image elements along a line; fmax = 1/2KnpZ2, where K is a constant (usually 0.6–0.9), n is the frame frequency, ρ is the aspect ratio of the frame (ratio of width to height), and Z is the number of lines. With the television standards adopted in the USSR, n = 25 per sec, Z = 625, p = 4/3; when K = 0.9, fmax = 6 megahertz (MHz).

The television signal spectrum for a stationary black-and-white image, as well as the signal spectrum for the brightness of a stationary color image, is discrete and consists of separate groups of spectral lines, which are formed by harmonics of the line frequency fine and by side lines. In each group, the most intense harmonic is fnne. When objects move and the content of the transmitted images changes, side bands having a continuous spectrum appear around the discrete spectral lines; the width of a band usually does not exceed several hertz.

In compatible color-television systems, the signal spectrum of the color subcarrier is located in the high-frequency part of the brightness signal spectrum. The frequency and the method of modulation of the signal of the color subcarrier are chosen in such a way that the corresponding side spectral lines appear in the unoccupied parts of the brightness signal spectrum. In the system known as SECAM (séquentiel couleur à mémoire), for example, the frequencies of the color subcarriers are 272 and 282 times fline, and frequency modulation is employed. The width of the signal spectrum for the color subcarrier in the brightness signal spectrum does not exceed 3 MHz.