Introduction to CATV – from IP Television Magazine.

CATV Headend
by: Lawrence Harte

The head-end is the master distribution center of a CATV system where incoming television signals from video sources (e.g., DBS satellites, local studios, video players) are received, amplified, and re-modulated onto TV channels for transmission down the CATV system.

The incoming signals for headend systems include satellite receivers, off-air receivers and other types of transmission links. The signals are received (selected) and processed using channel decoders. Headends commonly use integrated receiver devices that combine multiple receiver, decoding and decryption functions into one assembly. After the headend receives, separates and converts incoming signals into new formats, the signals are selected and encoded so they can be retransmitted (or stored) in the CATV network. These signals are modulated, amplified and combined so they can be sent on the CATV distribution system.

This article is Part 3 of a 9 Part Series

CATV Series List Month

Introduction to CATV Dec 06

Contribution Network Jan 07

Headend Feb 07

Asset Management Mar 07

Distribution Network Apr 07

CATV Display Devices May 07

CATV Marketplace Jun 07

CATV Systems Jul 07

CATV Services Aug 07

Figure 1.8 shows a diagram of a simple head-end system. This diagram shows that the head-end gathers programming sources, decodes, selects and retransmits video programming to the distribution network. The video sources to the headend typically include satellite signals, off air receivers, microwave connections and other video feed

Figure 1.8., Head-end System

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signals. The video sources are scrambled to prevent unauthorized viewing before being sent to the cable distribution system. The headend receives, decodes and decrypts these channels. This example shows that the programs that will be broadcasted are supplied to encoders and modulators to produce television channels on multiple frequencies. These channels are combined onto a single transmission line by a channel combiner.

A CATV system has expanded to include multiple regions including local headend locations, which may be distributed over a large geographic region. Local headends may be connected to regional headends and regional headends may be connected to a super headend. To reduce the cost of a CATV system, headend systems can be shared by several distribution systems.

Integrated Receiver Decoder (IRD)

An integrated receiver and decoder is a device that can receive, decode, decrypt and convert broadcast signals (such as from a satellite system) into a form that can be transmitted or used by other devices.

In headend systems, IRDs are commonly used to demodulate and decrypt the multi-program transport stream (MPTS) from a satellite antenna. The IRD has a receiver that can select and demodulate a specific channel. The decoder divides an incoming channel into its component parts. A decryptor can convert the encrypted information into a form that can be used by the system. An interface converter may change the format of the media so that it can be used by other devices.

The inputs to an IRD (the front end) can include a satellite

receiver or a data connection (such as an ATM or IP data connection). The types of processing that an IRD performs can vary from creating analog video signals to creating high definition video digital formats. The outputs of an IRD range from simple video outputs to high-speed IP data connections. Companies that produce IRDs commonly offer variations of IRD (such as analog and digital outputs) that meet the specific needs of the CATV system operator.

Figure 1.9 shows the basic function of an integrated receiver device that is used in a cable TV system to receive satellite broadcasted signals and decode the channels. This example shows that an IRD contains a receiver section that can receive and demodulate the MPTS from the satellite. This IRD can decode and decrypt the MPTS to produce several MPEG digital television channels.

Off Air Receivers

An off air receiver is a device or assembly that can select, demodulate and decode a broadcasted radio channel. Broadcast receivers are used in cable television systems to receive local broadcasted channels so they can be re-broadcasted in the local cable television system.

In some countries (such as the United States), CATV operators are required to rebroadcast local television channels on their cable television systems. These "must carry" regulations are government requirements that dictate that a broadcaster or other media service provider must retransmit (carry) or make available to carry a type of program or content.

Off-air receivers contain a tuner (receiver), demodulator and

Figure 1.9., Headend Integrated Receiver and Decoder

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decoder for analog and/or digital television signals. The off-air receiver contain a tuning head that allows it to select (or to be programmed to select) a specific television channel. Off-air receivers may be simple analog television tuners (e.g. NTSC, PAL or SECAM) or they may be capable of demodulating and decoding digital television channels (e.g. DTT).

Encoders

An encoder is a device that processes one or more input signals into a specified form for transmission and/or storage. A video encoder is device used to form a single (composite) color signal from a set of component signals. An encoder is used whenever a composite output is required from a source (or recording) that is in a component format.

Transcoders

A transcoder is a device or assembly that enables differently coded transmission systems to be interconnected with little or no loss in functionality.

Rate Shaper

Rate shapers are devices or assemblies in a communication system that adapt and/or transform the transmission rate of one system to the transmission rate of another system. Rate shapers are used as digital turnaround products in a media distribution system (such as a cable television system).

Rate shapers can be used to adjust the data transmission rates for multiple channels that have variable bandwidth rates so they can operate over transmission channel that have constant or limited maximum transmission bit rates. A variable bit rate feed is a media source that has a data transmission rate that varies over time (such as digital video). A constant bit rate feed is a media source that has a data transmission rate that does not vary over time (such as a digital transmission channel).

Figure 1.10 shows how a rate shaper can combine multiple digital television channels to help adjust the maximum bandwidth usage. This example shows 3 digital television channels that have variable bandwidth due to high video activity periods (action scenes with high motion). The combined data rate is shown at the bottom. The combined data rate has a peak data rate that is larger than the 30 Mbps transmission channel can allow. As a result, one or more of the input MPEG channels must use a higher compression rate (temporary lower picture quality).

Figure 1.10., MPEG Statistical Multiplexing

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Channel Modulators

A channel modulator is used to convert video signals into television broadcast channels. Channel modulators are used in cable-TV networks to convert a video program signal (such as CNN or MTV) and converts it with an RF carrier frequency for a television channel that is distributed through the CATV network. The modulator converts both video and audio signals. The frequency of this channel modulator carrier determines the television channel number (e.g., 2 to 120) that the program will be received on by subscribers.

Channel modulators may produce analog or digital signals. Analog modulators take video and audio signals and convert them into a format that can be used by televisions that may be connected to the CATV system (NTSC, PAL or SECAM). Digital modulators convert the digital media programs into a common shared transport stream. Cable television systems commonly use an MPEG transport stream (MPEG-TS) to transfer multiple programs on each RF transmission channel.

MPEG transport streams (MPEG-TS) use a fixed length packet size and a packet identifier identifies each transport packet within the transport stream. A packet identifier in an MPEG system identifies the packetized elementary streams (PES) of a program channel. A program (such as a television show) is usually composed of multiple PES channels (e.g. video and audio).

Channel Processors

A channel processor is a device or assembly that can receive, modify and produce a new channel signal. A common CATV television channel processing function is to change the frequency of the channel signal so that a channel that is received on one frequency can be shifted (translated) to another frequency that is sent on a transmission system.

Figure 1.11., CATV Channel Combiners

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Channel Signal Combiners

Channel combiners are devices or filter assemblies that allows several modulated carrier signals (physical channels) to be grouped on to the same transmission channel or antenna system.
Each transmission (transport) channel operates on a separate frequency band. CATV systems may allow up to 120 RF channels on a single transmission channel. Because each transport channel can carry multiple logical channels (typically 4 to 6 TV channels per carrier), a digital cable television system can provide hundreds of television channels.

Channel combiners allow multiple RF channels to connect to the same transmission line. To keep signals from the transmitter of one RF channel from being received and interfering with RF transmitters of other transmission channels, the channel combiner provides some port to port isolation (attenuation between the ports).

The types of combiner networks include active and passive combiners. Active combiners include amplifiers to increase the signal level as it passes through the combining network. Passive combiners use filters to isolate the signals from each other.

Figure 1.11 shows how CATV channel combiners are used to combine different RF channel signals into a common

transmission line. This diagram shows a passive CATV channel combiner that is composed of multiple signal couplers. Each coupler (tap) allows signals to pass through to the transmission line while providing some isolation (signal attenuation) from other transmitters.

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Introduction to CATV, 2nd Edition
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