No matter how much the enlightened community criticizes television for its negative impact on consciousness, the television signal is present in almost all residential (and many non-residential) spaces. In large cities, it's almost always cable television, even if everyone around still calls it an 'antenna.' While the system for receiving over-the-air television is fairly straightforward (although it can differ from the usual rabbit-ear antenna in the window, which I will definitely discuss later), the cable television system may unexpectedly appear quite complex in its operation and architecture. I present a series of articles on this topic. I want to introduce those interested to the principles of cable television networks, as well as their operation and diagnostics.
- Part 1: General architecture of the CATV network
- Part 2: Composition and form of the signal
- Part 3: Analog component of the signal
- Part 4: Digital component of the signal
- Part 5: Coaxial distribution network
- Part 6: RF-signal amplifiers
- Part 7: Optical receivers
- Part 8: Optical backbone network
- Part 9: Headend
- Part 10: Troubleshooting in the CATV network
I do not claim to be writing a comprehensive textbook; rather, I will strive to remain within the realm of popular science and not overload the articles with formulas and technical descriptions. For this reason, I have left 'smart' words in the text without explanations; searching them online will allow you to delve as deeply as you need. After all, everything is well described individually, and I will just explain how it all fits together in a cable television system. In the first part, I will superficially describe the network structure, and later I will go into more detail about the principles of the entire system.
A cable television network has a tree-like structure. The signal is generated at the head end, which collects signals from various sources, combines them into a single signal (according to a specified frequency plan), and sends it into the trunk distribution network in the required format. Today, the trunk network is, of course, optical, and the signal only transitions to coaxial cable within the final building.
Head Station
The sources of the signal for the head end can be satellite antennas (of which there may be a dozen) as well as digital streams directly provided by TV channels or other communication operators. To receive and assemble the signal from different sources, multi-channel multi-service decoders/modulators are used, which consist of rack-mounted chassis with various expansion cards that provide connections for different interfaces, as well as decoding, modulation, and the formation of the required signal.
Here, for example, we see 6 modules for receiving satellite broadcasting signals and two DVB-C output modulators.
This chassis is responsible for demodulating the signal. The CAM modules are visible, the same as those inserted into televisions for receiving encrypted channels.
The result of the operation of this equipment is an output signal containing all the channels that we will distribute to subscribers, sorted by frequencies according to the specified frequency plan. In our network, this range is from 49 to 855 MHz, containing both analog and digital channels in DVB-C, DVB-T, and DVB-T2 formats:
Signal spectrum visualization.
The formed signal is fed into an optical transmitter, which essentially acts as a media converter, transferring our channels into the optical medium at a traditional television wavelength of 1550 nm.
Optical transmitter.
Main distribution network
The optical signal received from the headend station is amplified using an erbium-doped fiber amplifier (EDFA), familiar to any telecommunication engineer.
The signal level of a few dozen dBm taken from the amplifier output can already be split and sent to different regions. The splitting is done using passive splitters, conveniently housed in rack-mounted enclosures.
Optical splitter inside a one-unit optical cross.
The split signal reaches the nodes where it can be amplified if necessary by similar amplifiers or split among other equipment.
This is how a residential area node might look. It includes an optical amplifier, a signal splitter in a rack-mounted cross, and a distribution optical cross, from which fibers spread to the optical receivers.
Subscriber distribution network
Optical receivers, like the transmitter, act as medium converters: they transfer the received optical signal into coaxial cable. Optical receivers vary in type and manufacturer, but their functionality is generally the same: monitoring levels and basic signal adjustments, which I will detail in future articles.
Optical receivers used in our network.
Depending on the architecture of the buildings (number of floors, number of blocks and entrances, etc.), the optical receiver can be placed at the beginning of each riser, or there can be one for several (sometimes, a coaxial cable is laid instead of an optical one between buildings). In such cases, the inevitable losses at the splitters and main lines are compensated by amplifiers. For example, like this one:
Signal Amplifier KTV Teleste CXE180RF
The subscriber distribution network is built on different types of coaxial cables and various splitters, which you can see in the low-voltage panel on your staircase.
Cables entering the apartment are connected to the outputs of the subscriber splitters.
Of course, in most cases, there are several televisions in each apartment, and they are connected through additional splitters, which also introduce losses. Therefore, in certain situations (when there are many televisions in a large apartment), additional signal amplifiers, which are smaller and weaker than the main ones, need to be installed directly in the apartment.
Source: habr.com
