Cable television headend

A cable headend (including cable head-end or shortly headend, English Cable television headend ) is part of a broadband cable network and is used to supply all available in the relevant grid signals. Broadband cable networks for the distribution of radio and television programs ( analog and digital) are constructed in a tree structure. At the root of this tree is the cable headend.

Technical structure

A cable headend can be constructed in various sizes, from the small plant for apartment buildings to serve many cities, counties and districts, ie up to several hundred thousand housing units. It is located in the hierarchy of grid levels in the cable network of order number 2

In the cable head end, the received signals from the satellite (analog and / or digital) or the terrestrial antenna converted ( analog / digital) in a frequency range of 47 or 87.5 to 862 MHz, so that the subscribers in the cable network, the programs according to its devices such as a TV can play with analog reception, set- top box for digital reception or radio receiver for FM reception. In addition, other signals for bidirectional services such as Internet or telephony (triple play ) in anticipation (Downstream ) is added and from the return channel ( upstream) can be received in the headend.

A station consists in principle

From an antenna system that receives terrestrial and transmitted by satellite programs,
From a receiver section in which all the necessary receiving equipment are summarized, and
Of transmitter modules that modulate the information supplied by beneficiaries individual signals on carrier waves and feed it into the trunk line of the network for retransmission.

A cable head end can be compared to a large community antenna system. With the terrestrial antenna system be with the help of several directional antennas with high gain antenna, the incident in a region terrestrial channels ( FM radio and television programs) received. For this reason, cable headends to be built in topographically favorable sites, such as on mountains or high buildings. The satellite programs are captured by several large satellite dishes and fed to the corresponding satellite receivers. The parabolic mirrors are oversized to always ensure a high quality reception even in the worst weather conditions.

Aufmodulation

For each program fed a separate receiver and a separate sending unit out of the station is required. The FM radio channels are fed in conventional FM radio range 87.5 to 108 MHz in the usual frequency modulation ( FM) in the network so that they can be received with the normal VHF FM receivers. The programs are not further transmitted on the frequencies on which they were received, but they are assigned frequencies, which apply only to a cable network. To avoid interference in the network must be given to the frequency division special attention.

The TV channels ( terrestrial or received from satellites) are in the usual in Germany television standard ( CCIR Gerber standard, vestigial sideband amplitude modulation, PAL) in the frequency range 47-300 MHz in the power fed ( re-modulation ). In the network of Swiss Cablecom the band I ( 47-68 MHz) has been cleared to make room for the wider rear channel in the second half of 2008. This results in the following frequency distribution: 5-65 MHz return path for IP connectivity, 87.5-108 MHz FM Radio, 108-862 MHz TV channels. Moreover, this frequency range are at the top of transmitted digital TV programs (eg pay programs). For this, however, the subscriber requires a special digital receiver (Set-Top - Box), which converts the digital programs in the traditional television standard for televisions.

Pilot frequencies

In order to ensure at the transfer points of the client signal level constant, non-modulated carrier wave ( pilot signals ) are provided by the head-end to at least two frequencies in the vicinity of the lower and upper end of the frequency band transmitted. Whose levels are analyzed in the different repeater stations and control the gains in accordance with such a manner that the most distant transition point is a stable minimum level available. Thus, the temperature and frequency variations caused by attenuation of the transmission cable can be compensated.

Transmission in the reverse direction

Modern cable networks are also able to transmit signals by the end users to the headend. Here, frequencies of 3-30 MHz, and more recently used 5-65 MHz, the repeater stations to be bypassed at the moment. This is done with appropriately sized crossovers. Such return channel capability cable networks can be used in cable television for telephony ( VoIP) and Internet connectivity. In Switzerland, most cable networks are return path compatible.