No matter how the enlightened community scolds television for its negative impact on consciousness, nevertheless, the television signal is present in almost all residential (and in many non-residential) premises. In big cities, this is almost always cable television, even if everyone around it habitually calls it “antenna”. And if the system for receiving terrestrial television is quite obvious (although it may also differ from the usual horned antenna on the windowsill, I will definitely talk about this later), then the cable television system may seem unexpectedly difficult in its work and architecture. I present a series of articles on this. I want to acquaint those interested with the principles of operation of cable TV networks, as well as their operation and diagnostics.

• Part 1: General architecture of a CATV network
• 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

I do not pretend to write a comprehensive textbook, but I will try to stay within the framework of science pop and not overload articles with formulas and descriptions of technologies. That is why I left the “smart” words in the text without explanation, by googling them you can go as deep as you need. After all, everything is well described individually, and I will just tell you how it all adds up to a cable television system. In the first part, I will briefly describe the structure of the network, and in the future I will analyze in more detail the principles of operation of the entire system.

The cable television network has a tree structure. The signal is formed by the head station, which collects signals from different sources, forms them into a single one (according to a given frequency plan) and sends them to the backbone distribution network in the desired form. Today, the backbone network is, of course, optical and the signal goes to coaxial cable only within the end building.

Headend

Signal sources for the headend can be either satellite dishes (of which there may be a dozen), or digital streams given directly by TV channels or other telecom operators. To receive and assemble a signal from different sources, multi-channel multi-service decoders / modulators are used, which are a rack-mount chassis with various expansion cards that provide connection to various interfaces, as well as decoding, modulation and formation of the desired signal.

Here, for example, we see 6 satellite signal reception modules and two DVB-C output modulators.

And this chassis is engaged in signal descrambling. You can see CAM modules, the same as those inserted into TVs to receive closed channels.

The result of the operation of this equipment is an output signal containing all the channels that we will give to subscribers, decomposed into frequencies in accordance with a given frequency plan. In our network, this range is from 49 to 855 MHz, containing both analog channels and digital channels in DVB-C, DVB-T and DVB-T2 formats:

Display of the signal spectrum.

The generated signal is fed into an optical transmitter, which is essentially a media converter and transfers our channels to the optical medium at the traditional television wavelength of 1550nm.

Optical transmitter.

Backbone distribution network

The optical signal received from the headend is amplified using an optical erbium amplifier (EDFA) familiar to any signalman.

A couple of tens of dBm of the signal level taken from the amplifier output can already be divided and sent to different areas. The division is made by passive dividers, for convenience, laid in the body of rack-mounted crosses.

Optical divider inside a one-unit optical cross.

The divided signal gets to the objects, where it can be amplified, if necessary, using the same amplifiers, or divided between other equipment.

This is what a residential area node might look like. It includes an optical amplifier, a signal splitter in a rack-mounted cross-section housing, and a distribution optical cross-section, from which the fibers diverge to optical receivers.

Subscriber distribution network

Optical receivers, like the transmitter, are medium converters: they transfer the received optical signal to a coaxial cable. OPs are different and from different manufacturers, but their functionality is usually the same: level monitoring and basic signal adjustments, which I will discuss in detail in the following articles.

Optical receivers used in our network.

Depending on the architecture of the houses (number of floors, number of buildings and front doors, etc.), an optical receiver can be installed at the beginning of each riser, or it can be one for several (sometimes not an optical cable, but a coaxial cable is laid between buildings), in this In this case, the inevitable attenuation on dividers and mains is compensated by amplifiers. Like this one, for example:

Teleste CXE180RF KTV Signal Amplifier

The subscriber distribution network is built on various types of coaxial cable and various dividers, which you can see in a low-current shield in your stairwell

Cables entering the apartment are connected to the outputs of subscriber splitters.

Of course, in most cases, there are several TVs in each apartment and they are connected through additional splitters, which also introduce attenuation. Therefore, in some cases (when there are a lot of TVs in a large apartment), it is already necessary to install additional signal amplifiers in the apartment, which for these purposes are smaller and weaker than the main ones.

Source: habr.com