Cable headend
A cable headend (also cable headend or headend for short ; English Cable television headend ) is part of a broadband cable network and is used to feed in all signals available in the corresponding network. Broadband cable networks for the distribution of radio and television programs (analog and digital) are structured in a tree structure. The cable head end is on the trunk of this tree.
technical structure
A cable headend can be constructed in various sizes, from the small system for apartment buildings to the supply of several cities, counties and districts, i.e. up to several hundred thousand residential units. It is located in the hierarchy of the network levels in the cable network at order number 2, also known as NE2 .
Within the cable headend, the signals received from the satellite (digital) or the terrestrial antenna (digital) are converted into a frequency range of 47 or 87.5 to 862 MHz, so that the subscriber in the cable network can view the programs accordingly on his end devices such as television sets with digital reception, Set-top box for digital reception or radio receiver for FM reception . In addition, further signals for bidirectional services such as Internet or telephony ( triple play ) can be added in advance ( downstream ) and received from the return channel ( upstream ) in the headend .
A station basically consists of:
- from an antenna system that receives terrestrial and satellite programs,
- from a receiver section in which all necessary receiving devices are combined, and
- consisting of transmitter modules that modulate the individual signals supplied by the receivers onto carrier waves and feed them into the network's trunk line for further distribution.
A cable headend can be compared to a large community antenna system. With the terrestrial antenna system, the incoming terrestrial programs (VHF radio and television programs) in a region are received with the aid of several directional antennas with a high antenna gain . For this reason, cable headends are usually installed at topographically favorable locations, e.g. B. on mountains or tall buildings. The satellite programs are picked up by one or more large parabolic antennas and mostly converted into the frequency range up to 862 MHz with a channel converter or fed directly to the corresponding satellite receivers . The parabolic mirrors are very large in order to always guarantee high quality reception even in the most unfavorable weather conditions.
More and more often, the cable headends are being replaced by playout centers in connection with fiber optic cables, as the susceptibility to failure z. B. Reduced heavy rain and increased flexibility.
Modulation
A separate receiver and a separate transmitter unit at the station exit is required for each program fed in . The VHF radio programs are usually fed into the network in the conventional VHF radio range between 87.5 and 108 MHz in the usual frequency modulation (FM) so that they can be received with normal VHF FM receivers. The programs are not transmitted on the frequencies on which they were received, but are assigned frequencies that only apply to a cable network.
The television programs, which are received by network operators via networks such as ATM, terrestrial or from satellites, are fed in analogue in common television standards such as PAL . In addition, digital television transmission methods such as DVB-C are used, which are specially designed for the special features of cable networks. Terrestrially received digital television programs in DVB-T or satellite programs in DVB- S are converted in the cable headend to DVB-C by means of a so-called transcoding.
In order to avoid disturbances in the network, special attention must be paid to the frequency distribution. In cable networks, the main source of interference is the intermodulation between different channels caused by the broadband amplifiers .
Pilot frequencies
In order to ensure constant signal levels at the customer transfer points , the head-end station also sends unmodulated carrier waves ( pilot tones ) on at least two frequencies near the lower and upper end of the frequency band . Their levels are evaluated in the various repeater stations and adjust the amplifications so that a stable minimum level is available at the most distant transfer point. In this way, the temperature and frequency-related fluctuations in attenuation of the transmission cables can be compensated.
Return channel suitability
Some cable networks are also able to transmit signals from end users to the head end. Frequencies between 3 MHz and 68 MHz are used here; specific values vary depending on the cable network operator. The amplifier stations must be suitable for return channels and have crossovers . Cable networks with return channel capability can also be used for telephony ( VoC ) and cable internet . Most cable networks in Germany, Switzerland and Austria are suitable for return routes.
literature
- Hermann Kenter: Sound and television transmission technology and technology of lead-bound BK systems . Ed .: Central Telecommunications Office, Darmstadt. tape 10 . RV Deckers Verlag, G. Schenck, 1988, ISBN 3-7685-2787-5 .