DIGITAL TELEVISION: WHAT IS IT?
The difference in the propagation conditions of the signals according to their path (terrestrial, cable or satellite) is not only physical. The way to use the frequency range is also different.
It is time to remind the reader that any information can be transmitted over a radio channel (that is, not over wires) using a high frequency signal. For this, the information to be transmitted is presented in the form of a low frequency signal and with its help a change in any characteristic of the high frequency signal occurs. Thus, the useful (informative) signal to be transmitted results, as it were, enclosed in a high-frequency signal that is easily transmitted. This process is called modulation, the high frequency signal (information carrier) is called the carrier, and the signal received after modulation is called the modulated carrier. The reverse process of recovering the modulated carrier information signal at the receiver is called demodulation.
With a land path, when signals are transmitted through the air over a distance of several kilometers, their quality is adversely affected by pollution in the lower layers of the atmosphere, reflections from various landforms and structures, etc. Although in the dedicated VHF (VHF) and UHF (UHF) frequency ranges, the signals propagate almost linearly, they are well reflected from various obstacles, therefore, even when the transmitter is in line of sight, the antenna receives not only direct waves, but and reflected by various obstacles, as well as by waves emitted by other transmitters on the same frequency. All this leads to distortions, if not the disappearance of the signal. To avoid this, with the terrestrial method, the transmission is carried out simultaneously at different carrier frequencies. Nearly several thousand carrier frequencies are combined into one multicarrier waveform, called orthogonal multiplexing (frequency division multiplexing) of COFDM (coded orthogonal frequency division multiplexing) channels. The full version of this system allows the use of 6785 individually modulated carriers.
For cable networks with good dielectric and electromagnetic properties and almost no interference, the quadrature amplitude modulation method QAM (Quadrature Amplitude Modulation) is used. Double your bandwidth efficiency by transmitting two signals simultaneously on the same carrier. This type of modulation is used in the ETS 400429 cable transmission standard.
When transmitting from satellites, the signal path is characterized by low interference and good reception conditions, which, however, can deteriorate in bad weather, for example rain, fog, snowfall, as well as under the influence of strong winds. that disturb the correct orientation of the antenna. The peculiarity of this method is the low power of the received signal. Therefore, the antenna must be installed at a sufficient height and have a sufficient parabola diameter, and the converter (amplifier and frequency converter) must have the necessary gain and low noise factor. For satellite transmission, the QPSK (Quadrature Phase Shift Keying) method was invented, which transmits two bits per symbol and thus allows you to double the bandwidth.
Signal coding
In digital television broadcasting, digital signals that have passed through encoders are first multiplexed (several input streams are combined into a single output stream) into a single elemental stream of a program, and then the elemental streams of all programs are multiplexed into a single elementary transport stream (also called transport multiplex).
Then, the digital signal during channel coding is complemented by a security code that protects against transmission errors, modulated and transmitted through the appropriate channel (terrestrial – terrestrial, satellite or cable) to the viewer.
Before encoding, the component signals for luma (Y), chrominance (CR and CB), and left and right audio channels (R and L, respectively) are sampled on an ADC. The sampling rate according to the CCIR 601 unified digital television standard is 13.5 MHz, the bit rate for the eight-bit quantization is 216 Mbps, and for the ten-bit quantization 270 Mbps.
When encoding sound signals, the psychoacoustic properties of a person are mainly used, for example a masking effect, in which loud sounds make silent sounds of other frequency subbands inaudible simultaneously with them.
