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 f_max is determined from the transmission conditions of the major harmonic components for alternating black and white image elements along a line; f_max = 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, f_max = 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 f_ine and by side lines. In each group, the most intense harmonic is f_nne. 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 f_line, 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.

N. G. DERIUGIN