Basic principle of high frequency head

High-frequency head: commonly known as tuner, it is the first part of the public channel of high-frequency signal of television. The high-frequency heads used in television sets are generally divided into digital signal high-frequency head (referred to as digital high-frequency head) and analog signal high-frequency head (referred to as analog high-frequency head). The function of digital high-frequency head is to receive high-frequency signal of digital television, select channels, amplify high-frequency signal and process frequency conversion. Some of them also have the functions of intermediate frequency signal amplification and high-frequency digital signal demodulation. After the high-frequency digital signal is demodulated, the output digital signal is TS (Transport Stream). TS stream: also called transport stream, it is a digital signal transmission stream in units of "frames". Each frame of digital signal contains signals such as synchronization header data end. For MPEG2 digital signal, each frame of signal is composed of binary signal packets with a length of 188 bytes, and its content contains one or more programs. The concept of "frame" here is very similar to the frame in television image, but the content is different. One frame of MPEG2 digital signal corresponds to one frame of image, which is only equivalent to a few pixels in the content of an image. ; According to the modulation method of receiving high-frequency digital signals, digital high-frequency heads are divided into QPSK (Quadrature Phase Shift Keying) modulation high-frequency heads and QAM (Quadrature Amplitude Modulation) modulation high-frequency heads. QPSK modulation high-frequency heads are mainly used for satellite TV signal reception; QAM modulation high-frequency heads are mainly used for cable TV signal reception. ; The function of analog high-frequency heads is to receive analog TV high-frequency signals, select channels, amplify high-frequency signals and perform frequency conversion processing. Analog high-frequency heads generally do not have intermediate frequency signal amplification and high-frequency signal demodulation functions, so analog TVs also need to add an intermediate frequency amplifier and high-frequency signal demodulator. Generally, the receiving, amplifying, and demodulating circuits of analog high-frequency signals need to be strictly adjusted to meet the requirements of the whole machine. Therefore, it is difficult to complete all the functions of high-frequency signal receiving, amplifying, and demodulating by the high-frequency head. Therefore, the main task of the analog high-frequency head is to select channels. Another task is to reduce the frequency, which is to reduce the received high-frequency signal to a fixed frequency. This fixed frequency signal is the intermediate frequency signal, and its frequency is generally 38MHz. For video, the intermediate frequency signal is still a high-frequency signal. It needs to be further amplified before demodulation and various processing (such as: synchronous separation, bright color signal separation, etc.). The main task of the intermediate frequency amplifier circuit is to amplify the intermediate frequency signal and demodulate the audio and video signals. In addition, the intermediate frequency amplification is also very special for video signal demodulation. Generally, synchronous detection is used, and the envelope distortion is very small. After demodulation, the intermediate frequency signal outputs the video signal and audio signal, that is, the AV signal. The AV signal needs to be further processed (decoded) by the color signal to become the three primary color signals of R, G, and B (red, green, and blue). At present, there are not many TVs that can receive digital signals (such as satellite TV), and most of them are analog TVs.