Applications of twisted pair wires and coaxial video transmission

During the differential transmission of twisted pair wires, for external interference signals, the two twisted pair wires are equivalent to the Same conductor, and the interference signals have the same amplitude and polarity on the two wires. At the input end of the differential amplifier, if the CMR value of the two-wire common mode input is large enough to receive the difference amplifier, it can be well restrained.

Generally, the characteristic impedance of the Communication twisted pair is not 75Ω. in order to match the input video signal source with the output video device, the receiver and transmitter also need to change the impedance. The receiving and sending ends are connected with the matching impedance RZ. Transmitter A converts the 75 Ω input impedance of the signal source to the 50 Ω output of each line through the RZ between the external Z + and Z;

When receiving, the network impedance of each 50Ω of the two lines is 100 Ω in the RZ of the receiver. In order to solve the problem that the distribution of the twisted pair is not constant, some circuits set RX and CX between Z + and Z-to ensure correct matching.

The coaxial cable transmission video achieves the desired effect by means of Impedance Matching and unbalanced outer-end equipotential (shielding), while the twisted pair wire transmission video uses parasitic impedance differential offset to ensure frequency response, differential reception is used to suppress external interference to ensure indicators.

Because the characteristic impedance of the twisted pair is not constant, the changes vary depending on the type, length, and wiring environment of the twisted pair. The above impedance change adjustment is only the approximate impedance of a typical twisted pair, in the actual project, the wiring environment varies widely (the Environment Medium cannot be unified). The broken line is inevitably discounted and the exact length of the used line is difficult to grasp, making the characteristic impedance unable to be accurately adjusted. This kind of common mode rejection ratio decrease and reflection distortion is caused by the mismatch of ideal differential balanced Transmission Conditions and impedance, which is an important aspect of twisted pair wires during video transmission.

Application Comparison and precautions

1) high-frequency response to videos twisted pair wires are better than Coaxial Cables

Due to the large parasitic impedance of the coaxial cable (although its value is constant), if high-frequency compensation or balance is not added, it is only suitable for transmission definition 300 ~ A video signal of less than 350 lines. The parasitic Impedance Between twisted pair wires is small (ideally tends to be zero), and the high-frequency component loss of transmitted videos is small. This is especially suitable for transmitting videos with high-definition details over 400 TV lines and transmitting videos with twisted pair wires, some dedicated chips can even be used to transmit video signals modulated into RF.

2) transmission distance

The coaxial cable for non-relay transmission is not as far as the twisted pair cable. Technical Specifications of typical dedicated chips for transmitting videos by twisted pair wires: The valid transmission distance between one pair of receivers is 1 000 feet (304.8 m ). Adding a relay transmission twisted pair cable is not as good as a coaxial cable. Adding Multiple Relay amplification between twisted pair transceiver may cause some problems and the effect may not be satisfactory. The main problem may be that the balancing conditions are not satisfying, and the causes of compensation and impedance matching.

The coaxial cable does not have these problems. Regardless of the actual wiring environment and wire diameter, the signal line and the ground line (Shield) can always be in a constant state. With the extension of the transmission distance, you can determine the balance compensation required by the cable according to the mature formula. When the distance is far, you can add several additional relay amplification.

3) Interference and anti-interference

As long as the shielding layer is grounded and in contact with each other, the coaxial cable will not be disturbed or interfere with others. The degree of interference between twisted pair wires depends on the CMR value of the difference amplifier at the receiver end. The CMR value of commonly used dedicated chips is generally 70 ~ Between 90 dB (10 MHz.

I use a four-category Non-shielded line (containing 4 pairs of twisted pair wires) of AT&T to transmit 2 videos, 2 audios, 1 fmbroadcast, computer data signals, and 1 voice call at the same time, the work shows that the signal transmitted in the same line is either digital or analog, modulated or unmodulated, video or audio, as long as the balanced transmission mode is used, generally, mutual interference and Crosstalk will not occur. If there is a signal using unbalanced transmission mode, such as computer data streams using RS-232C or RS-423 format or audio broadcast using single-ended transmission mode, there may be interference and the phenomenon of mutual Signal Crosstalk.

In the application of twisted pair wires for video transmission, you must pay attention to the following two problems in addition to the above:

(1) Not all twisted pair wires can be used to transmit videos. Low-frequency twisted pair wires such as telephone lines and power supply wires are not suitable for video transmission due to limited band availability. In parallel data transmission, the balanced interference suppression conditions of multiple parallel lines can be well met, but it cannot offset the parasitic assignees like twisted pair wires, and the attenuation of high-frequency components is severe. For formal projects, we should select standard communication lines with transmission rates of 10 Mbps or higher for EIT/TIA. For all transmitted video, voice, data, and other information, we should use balanced plug sockets such as RJ45 or ipv11.

(2) Balanced transmission of twisted pair wires is most suitable for transmitting compound full-TV signals, transmitting component-based videos (YUV, YCaCr, YIQ, RGB, etc.) and Computer VGA or RGB signals, balance and phase conditions are difficult to grasp.

To sum up, it can be considered that in general cases, if the twisted pair wires are used to transmit videos, the signal transmission quality can be guaranteed as long as the balance conditions are met. When digital data signals and videos need to be transmitted at the same time, twisted pair cables are more advantageous than coaxial cables and comply with the principle of integrated wiring, it has great application advantages in fields such as e-education, monitoring, and computer modulus hybrid network.