Cable transmission distortion and frequency weighting compensation

When video signals are transmitted directly through coaxial cables, amplitude attenuation and frequency distortion will occur. That is, different frequency components attenuate differently, and the higher the frequency, the greater the attenuation. Amplitude attenuation causes a decrease in contrast. High-frequency attenuation causes a decrease in clarity. Actual measurements show that the distortion caused by the commonly used 75-7 cable exceeds the requirements of my country's television standards when it is 250-300 meters long. The EV-2010A weighted video amplifier can effectively compensate for the actual attenuation characteristics of 75-ohm coaxial cables over full distances and frequency bands. The maximum effective compensation distance can reach more than 2,500 meters (75-5 cables can be converted at 60%, reaching more than 1,500 meters). Here are some views on the relevant issues for discussion.

1) PAL video standard: The frequency response limit of video transmission 0-6M is within 0.5-5M bandwidth, the upper and lower fluctuations are limited to ± 0.75db, and 6M is limited to +0.75 ∽ -3db; this is the main feature, and there are other distortion index requirements. Generally, for medium and high-definition camera video signals of 420-480TVL, the frequency components of 4-6M cannot be ignored. Of course, the frequency components of the video signal are different depending on the shooting lens (viewing). The high-frequency components of the lens with small detail changes (such as a light wall) are weak; the high-frequency components of the lens with many detail changes (such as flowers and hair) are very rich. Therefore, judging the image quality based on a certain TV screen is limited. The responsibility of the contractor should be to reduce the distortion of the video signal as much as possible and give full play to the performance level of the camera to achieve the best possible satisfaction.

2) Video standard signal source
In order to detect the quality of video control equipment, conversion (switching, distribution, amplification,) equipment, and transmission (different media, different conversion and modulation and demodulation) equipment, a standard video signal source is required to generate several special video signals with zero distortion. One is a continuous sweep video waveform with a sweep range of 0.2-6M and an amplitude of 700Mv. The frequency response of the equipment can be detected very intuitively. There is also a similar multi-wave group video signal. Each group is a fixed frequency, and the six groups from low to high are 0.5/1.0/2.0/4.0/4.8/5.8M respectively. Using this waveform to study cable attenuation and compensation is very intuitive and easy to understand.

3) Frequency distortion of video cables
Different types of cables have different characteristics, and the performance of cables from different manufacturers and batches also varies. Here we take a typical 75-7 cable as an example for analysis and introduction. The transmission of video signals through cables will cause low-frequency amplitude attenuation and high-frequency distortion. The higher the frequency, the greater the attenuation. At the same time, it is noted that the horizontal synchronization header is distorted and the color synchronization header amplitude is seriously reduced. A set of typical 75-7 cable 1000M at different frequency attenuation test data is shown in Table 1:
Length (1000m) 0.5M---1.0M---2.0M---4.0M---4.8M---5.8M
Decibels....-4.18db-6.15db-8.52db-11.0db-11.6db-12.6db
Multiples......61.8%...49%....37.5%...28.2%...26.3%..23.4%
For the 5.8M frequency component, the attenuation of 2500m cable can reach -31.5db (2.67%). This is the actual concept and meaning of frequency distortion. Based on the above analysis and practical experience, we summarize the following opinions for reference:

1. Video coaxial cable, from its different attenuation characteristics for different frequency components, can be understood as a frequency "de-emphasis" device;
2. Cable has frequency "de-emphasis" characteristics, so "cable video compensator" should be a device with frequency "emphasis" (or "weighting") characteristics. We call it "weighted video amplifier", referred to as "weighted video amplifier"; 3.
Weighted compensation should be considered for 75-7 cables over 200m.
4. The requirements for video weighted compensation are:
① The amplitude should be able to be increased to greater than 1Vp-p;
② The frequency compensation must be weighted compensation in the full frequency range of 0-6M (low end light, high end heavy);
③ The compensation characteristics must be opposite to the cable attenuation characteristics;
④ The compensation distance should be as large as possible;
⑤ The compensator must have full-range continuous and adjustable effective control functions for the above compensations;
⑥ The problem of convenient and accurate debugging methods at the engineering site must be solved (many products have not solved this fundamental practical use problem).

IV) The problem of "front compensation" and "rear compensation"
Whether the compensation position is placed at the front end or the end of the cable, it seems the same on the surface. So some product introductions say: one amplifier can compensate for a distance of 1500 meters, and one amplifier can be placed before and after the cable to compensate for 3000 meters. We still find it hard to believe this. Because the two are completely different in terms of technical implementation. Rear compensation is the amplification and compensation of small signals below 1 volt, and the maximum dynamic range is 1 volt; while front compensation is the opposite, it is to first boost the amplification and frequency weighting of the 1 volt standard signal to compensate for the cable attenuation. For example, for a 1500-meter 75-7 cable, the attenuation of the 5.8M frequency is -18.9db, which is 8.8 times the attenuation. When pre-compensating, this frequency component must be amplified 4.26 times. This requires reaching 0.7V*8.8=6.16V. The horizontal synchronization head attenuation is -6.27db (2.06 times), which means it is required to be increased to 0.3*2.06=0.618V. The sum of the two items is the "peak-to-slit" value of the video signal, which is 6.16+0.618=6.778V. This is the dynamic range on a 75 ohm load, and the actual dynamic range of the amplifier output should be 6.778*2=13.556V, which is difficult to achieve with conventional circuits. In other words, post-compensation is effective for long-distance cables, but pre-compensation is ineffective and even harmful due to distortion.