At present, with the acceleration of the popularization process of digital TV, the usage problems encountered by consumers are gradually increasing. One of the most common problems is the frequent appearance of "mosaic" on the TV screen. At the beginning, consumers may mistakenly think that there is a problem with the signal source of the TV station's program, just like when watching a disc player at home and the disc cannot be read. In fact, the mosaic of digital TV is more caused by usage problems.
Different from the analog signal, since the digital signal adopts the QAM modulation mode, besides the amplitude of the carrier carries information, the phase of the carrier also carries information. The main factors affecting the phase characteristics of the network are the multipath effect caused by the reflection caused by the mismatch of the network, or the interference of the same frequency signal nearby.
If there is a mismatch in the cable TV network (for example, no dummy load is connected to the idle output terminal), signal reflection will occur, so that the signal reaching the set-top box is not only transmitted by the direct path, but also transmitted by the reflected path. There is a time difference between the direct path and the reflection path. From the carrier frequency point of view, as long as the time difference between the arrival of the two signals changes by 1/fc (fc is the radio frequency carrier frequency), the phase difference between the two will change by 2π radians, which means it is very small. The time difference of the two signals will cause a large phase difference. When the phase difference of the two signals is 2π, the two are added in phase, and the amplitude of the composite signal is the largest; . The time-delayed signal is mixed with the direct signal arriving at the receiver, which will not only affect the amplitude, but also affect the correct decoding of the digital signal from the phase.
Causes of network reflection include: physical damage of the cable, oxidized water ingress of the connector, no dummy load connected to the idle port of the main line, no dummy load connected to the air interface of the distributor, etc. These all cause network mismatch and cause reflection.
At present, most of the wired digital networks stay at 550MHz bandwidth, and supplementary bandwidth has to be used when transmitting new channels. In fact, regardless of the bandwidth, the cable TV is not necessarily used independently. The channel is interfered by the outside world, which will also cause the problem of signal superposition and phase noise.
The influence of phase noise on analog TV is relatively light. Even if the index of phase noise is poor, it will only appear as chaotic patterns in the dark field of the image. Therefore, the index of phase noise is not investigated in analog TV. For digital TV, this kind of noise will cause some programs to have mosaics or cannot be received.
