The signal transmitted over the cable television network is a broadband, frequency-separated spectrum. Signal parameters, including frequencies and channel numbers in Russia, are regulated by GOST 7845-92 and GOST R 52023-2003, but the operator is free to choose the content of each channel at his discretion.
Contents of the article series
- Part 2: Composition and waveform
- Part 3: Analog component of the signal
- Part 4: Digital component of the signal
- Part 5: Coaxial Distribution Network
- Part 6: RF Amplifiers
- Part 7: Optical Receivers
- Part 8: Optical backbone network
- Part 9: Headend
- Part 10: Troubleshooting on the cable TV network
Let me remind you that I am not writing a textbook, but an educational program to broaden my horizons and enter the world of cable TV. Therefore, I try to write in simple language, leaving key words for those who are interested and not delving into the description of technologies that have been perfectly described hundreds of times without me.
What do we measure
To obtain information about the signal in the coaxial cable, our technical staff mainly uses the device Deviser DS2400T
In fact, this is a television receiver, but instead of an image and sound, we see quantitative and qualitative characteristics of both the entire spectrum and individual channels. The following illustrations are screenshots from this instrument.
Such a Deviser even has somewhat redundant functionality, but there are devices that are more abrupt: with a screen showing a TV image directly, receiving an optical signal and, what Deviser lacks, receiving a DVB-S satellite signal (but that's a completely different story).
Signal spectrum
The spectrum display mode allows you to quickly assess the state of the signal "by eye"
In this mode, the device scans channels in accordance with the specified frequency plan. For convenience, frequencies not used in our network have been removed from the full spectrum, so the resulting image is a palisade of channels.
Digital channels are in blue, analog channels are in yellow. The green part of the analog channel is its audio component.
The difference in the levels of different channels is clearly visible: a separate unevenness depends on the settings of the transponders at the headend, and the general difference between the upper and lower frequencies has a certain meaning, which I will discuss below.
In this mode, strong deviations from the norm will be clearly visible, and if there are serious problems in the network, this will immediately become visible. For example, the above image shows the passage of two digital channels in the high-frequency zone: they are present only in the form of short strips, barely reaching the level of 10dBμV (the reference level of 80dBμV is indicated at the top - this is the upper boundary of the graph), which is actually the noise that the cable receives on itself as an antenna or contributed by active equipment. These two channels are test channels and were turned off at the time of this writing.
Bewilderment can cause uneven arrangement of digital and analog channels. This, of course, is not correct and happened due to the evolutionary development of the network: additional channels were simply added to the frequency plan in the free part of the spectrum. When creating a frequency plan from scratch, it would be correct to place all analogue in the lower part of the spectrum. In addition, station equipment designed to generate a signal for European countries has restrictions on the use of frequencies for broadcasting a digital signal, and although there are no such restrictions in our country, using such equipment it is necessary to place digital channels in the spectrum contrary to logic.
Waveform
As is known from fundamental physics, the higher the frequency of the wave, the stronger its attenuation as it propagates. When transmitting such a broadband signal as available in the cable TV network, the attenuation in the distribution network can reach tens of decibels per shoulder, and in the lower part of the spectrum it will be several times less. Therefore, having sent an even signal to the riser from the basement, on the 25th floor we will see something like the following:
The level of the upper frequencies is noticeably less than the lower ones. In a real situation, the TV, without understanding, can consider weaker channels as just noise and filter them out. And if an amplifier is installed in the apartment, then when you try to set it up for high-quality reception of channels from the upper part of the range, overamplification will occur in the lower one. Standards regulate the difference is not more than 15 dBuV for the entire range.
To avoid this, when setting up active equipment, a higher level is initially laid in the high frequency zone. This is called "straight slope", or simply "tilt". And what is shown in the image is “reverse tilt”, and such a picture is already an accident. Or, at least, an indication that the cable to the measurement point is a disaster.
There is also the opposite situation, when low frequencies are practically absent, and the upper ones barely break through above the noise level:
This also tells us about damage to the cable, namely its central core: the higher the frequency, the closer to the edge of the waveguide it propagates (skin effect in a coaxial cable). Therefore, we see only those channels that are distributed at high frequencies, but, as a rule, the TV will no longer be able to receive them with such a level.
Source: habr.com