Typical processing steps of digital TV

The following introduces several typical processing processes of digital television.

Figure 07-04-9 is a digital processing block diagram of the studio. The signal source is the color camera sending out R, G, B signals, which are converted into digital video signals through A/D and sent to the program production unit. At the same time, the signals sent by the digital video recorder, digital special effects and digital format converter are also sent to the program production unit. After the program is produced, the signal is sent to the D/A converter, converted into an analog signal, and then sent to the TV transmitter.

Figure 07-04-9 Studio digital processing block diagram

Figure 07-04-10 is a block diagram of a digital TV transmission system. The color TV image is generated by a camera at the transmitting end. After A/D conversion, it becomes a digital video signal and is sent to the source coding. Source coding is responsible for image data compression. It removes the redundant part of the signal and reduces the transmission bit rate. The image signal after source coding is sent to the multiplexer for multiplexing with the digital audio signal, and then sent to the channel encoder. Channel coding is error correction coding, which improves the signal's anti-interference ability during transmission. This is because the data code stream will inevitably introduce noise and cause bit errors after long-distance transmission. Therefore, error correction codes are added to improve its anti-interference ability. The signal after error correction coding is sent to the output interface circuit. The output interface circuit plays a role in code type conversion, that is, converting the unipolar code into a bipolar code that is conducive to transmission. For long-distance transmission, digital microwave lines or digital optical fiber lines can be used. In the relay transmission mode, the distance between stations can reach 50 kilometers. The process at the receiving end is opposite to that at the transmitting end. After the receiving end receives the signal, the input interface circuit converts the bipolar signal into a unipolar signal, and then sends it to the channel decoding. The channel decoding can correct the bit errors caused by transmission. The signal is then sent to the demultiplexing circuit and then to the video and audio processing circuits respectively to be restored to analog video and audio signals.

Figure 07-04-10 Block diagram of digital TV transmission system

The block diagram of a digital TV receiver is shown in Figure 07-04-11. The signal received by the TV receiving antenna is amplified through the high-frequency channel and the intermediate frequency, and then sent to the video detector to obtain an analog signal. After A/D conversion, it becomes a digital signal and is sent to the digital processor for digital noise reduction, digital contour correction, digital ghosting removal, line frequency doubling, flicker removal, picture-in-picture processing, etc. Finally, it is sent to the D/A converter to become an analog signal for display on the picture tube. Both the TV transmission high-frequency part and the TV reception high-frequency channel part are analog systems. This is because it is impossible to achieve digitization at a frequency close to 1000MHz based on the current technical level.

Figure 07-04-11 Block diagram of digital TV receiver

The encoding of digital TV signals can be divided into full signal encoding and component encoding, as shown in Figure 07-04-12. Figure (a) is full signal encoding, and Figure (b) is component signal encoding. As can be seen from Figure (a), the analog full TV signal is converted into a digital signal through A/D conversion, and then sent to D/A conversion after digital processing to restore it to an analog full signal. Full signal encoding seems very simple from the block diagram, but this method is prone to brightness and color interference. Especially for the SECAM system (the color TV system of Eastern European countries), since the modulation method of the color difference signal on the subcarrier frequency is frequency modulation, it is difficult to use full signal encoding, and only component encoding can be used. Figure (b) is a component signal encoding block diagram. After the analog full signal is converted by A/D, it is separated into digital brightness and color, and is divided into digital brightness signal and digital color difference signal, which are sent to the digital signal processor. The processed signal is then encoded by digital full signal, and then converted into an analog full signal through D/A. Although the image adds digital brightness and color separation and digital full signal encoding, the digital processing is performed on the component signal, eliminating the brightness and color interference. This processing method can improve the image quality, so it is widely used, and the international standard also recommends the use of component signal encoding.

Figure 07-04-12 Full signal encoding and component encoding