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Understanding Signals and the headend

Q: What are three methods in which a carrier is 'shaped' to contain the original signal?

A carrier is 'shaped' to contain the original signal by varying the carrier's amplitude (signal strength), frequency (number of cycles per second), or phase (a specific stage of a signal during its continued oscillation).

31 important questions on Understanding Signals and the headend

In what frequency bands are over-the-air signals originating from local broadcast stations?

Over-the-air signals originate from local broadcast stations in the VHF (channels 2-13 or 54-88 MHz/174-216 MHz) and UHF (channels 14-69 or 470-806 MHz) frequency bands.

OTA broadcast signals are acquired at the headend using?

Yagi and log periodic antennas

What is a yagi antenna?

The yagi antenna (named after its inventor, engineer Hidetsugu Yagi) receives one primary over-the-air signal. Yagis contain two or more elements that increase the received signal level.

What are log periodic antennas?

Log periodic antennas are used to capture multiple broadcast signals received from the same general direction.

How does the cable operator receive broadcast signals from far away communities?

Some broadcast signals come from communities too far away to be received by typical over-the-air antennas. A microwave antenna on a cable operator's tower can receive them. These dish-shaped receive antennas must be placed in a line-of-sight path with the microwave transmit antennas.

How do transponders on most commercial communications satellites receive and transmit satellite signals?

Most commercial communications satellites contain 24 transponders (a word combining "transmitter" and "responder"), each of which receives an uplink feed from a programmer on Earth. The transponder then downlinks the signals back to Earth.

How does the cable operator acquire signals from satellite transponders?

To acquire satellite transponder signals, the cable operator has stationed at the headend several large-diameter TV receive-only (TVRO) dish-shaped or parabolic antennas, each one aimed at a different satellite. The equipment also can change the incoming signal to a lower frequency and amplify it, if necessary, before feeding it to a satellite receiver in the headend.

How did "community antenna television" systems operate?

Early cable networks, which were built beginning in the late 1940s, were called "community antenna television" (CATV) systems. Those first systems brought two or more local broadcast channels to areas where over-the-air signals were difficult to receive with existing indoor or outdoor TV antennas.

How can analog signals be described?

Just as the voices between two people continuously change in level and pitch from high to low, an analog network sends out continuous signals to convey this information. This looks like a series of waves with curves, peaks, and valleys.

How can digital signals be described?

A digital network takes rapid samples of these continuously varying signals and converts them into data, a stream of 1s and 0s. This looks like a series of square waves with highs and lows.

What are three methods in which a carrier is "shaped" to contain the original signal?

A carrier is "shaped" to contain the original signal by varying the carrier's amplitude (signal strength), frequency (number of cycles per second), or phase (a specific stage of a signal during its continued oscillation).

How are the video, audio, and color carriers each modulated?

The video information is used to vary the amplitude of a carrier to create an amplitude modulated (AM) signal that supplies picture information to the TV set. The audio information uses frequency modulation (FM). The color information uses a combination of amplitude and phase modulation.

Name two unique properties of microwave bands.

Microwave bands possess several unique properties. First, because of their special propagation characteristics, microwave transmit and receive antennas must be aimed directly at each other in an unobstructed, line-of-sight path. Second, microwave frequencies are prone to suffer increased attenuation or signal loss during inclement weather, especially heavy rain (a condition known as "rain fade").

What happens once microwave signals are received?

Once microwave signals are received, they are either sent over waveguide (which handles higher frequencies at low loss) to a microwave receiver in the headend or are downconverted to a lower frequency for delivery to a different type of receiver over coaxial cable. The receiver demodulates the signal into baseband video and audio components, which are routed to a modulator for placement on an RF carrier.

Describe the difference between FM and AML microwave transmission techniques.

The oldest method of microwave transmission is frequency modulation (FM), which transmits a single channel; FM signals can be repeated and amplified over long distances. The amplitude modulated link (AML) method is used by cable operators to send entire blocks of broadband channels between headends and hubs. This system is often employed to transmit signals to remote areas or across mountains and large bodies of water. AML signals are limited to shorter distances because they cannot be repeated and amplified like FM.

What is a geosynchronous orbit?

Satellites used in transmitting cable programming orbit the Earth at an altitude of about 22,300 miles. This is called a geosynchronous orbit because such satellites maintain their position relative to a particular point on the surface of the Earth at all times. They actually orbit in exact synchronization with the rotation of the Earth and thus appear to stationary in the sky.

How do satellites take advantage of each transponder's 36 MHz capacity more efficiently?

Each transponder's channel is an FM signal that occupies 36 MHz of bandwidth. To take advantage of this capacity more efficiently, two different transponder signals reuse the same spectrum but do not interfere with each other. In this method, 12 transponders send and receive RF signals with electrical fields that are horizontally polarized and 12 that are vertically polarized.

What are the uplink and downlink satellite frequencies in the C- and Ku-bands?

Uplink frequencies, which programmers use to send signals to the satellite from a ground-based transmitter, are at 5.9-6.4 GHz. Downlink frequencies, which are used when the satellite delivers its signals to TVRO antennas, are situated between 3.7 and 4.2 GHz. These frequencies are referred to as the C-band. To reduce interference in the C-band, newer satellites operate in the Ku-band, uplinking at 14-14.5 GHz and downlinking at 11.7-12.2 GHz.

What is the purpose of the TVRO antenna's parabolic shape?

The parabolic shape of the TVRO antenna bounces or reflects the downlinked signal from all locations on the dish surface onto a central focal point. This focal point is where the antenna's feedhorn collects the reflections and provides maximum signal reception. The antenna's design is instrumental in providing a small amount of gain that boosts the signal level after its long trip from space.

What happens to the satellite signal once it is received by the feedhorn?

Once the satellite signal is received by the feedhorn, it is directed to a low-noise amplifier (LNA) to increase the signal level. There are two LNAs on the TVRO, one each to amplify horizontally and vertically polarized signals. Instead of LNAs, a low-noise block converter (LNB) or low-noise converter (LNC) can sometimes be used. The LNB includes an LNA for signal amplification as well as a downconverter that lowers the satellite frequency as a single block of spectrum.

What happens as the digital signals are downlinked at the headend?

As the digital signals are downlinked at the local headend, they are captured by an integrated receiver transcoder (IRT), which changes the signals' modulation scheme to one that is compatible with the broadband cable network's transmission requirements. The IRT outputs the modified signals to a frequency upconverter for placement on the appropriate 6 MHz cable channels.

What does each satellite receiver in the headend do?

Each satellite receiver in the headend obtains a specific transponder signal from the TVRO, amplifies it, changes it to a different frequency, and demodulates it for modulating onto a cable channel.

What do filters in the headend do?

Filters in the headend serve to remove noise and distortion, as well as block out unwanted signals from other channels.

What is the function of test equipment in the headend?

The headend usually has test equipment nearby to maintain proper signal levels and perform routine maintenance. Such equipment might include spectrum analyzers, vectorscopes, signal generators, and video measurement sets.

What does EAS equipment do?

EAS equipment receives encoded warnings about pending local or national emergencies through a phone network or radio broadcast. The EAS, which is mandated for all cable operators, immediately transmits a decoded alert that is displayed on certain channels.

What are frequency-agile modulators?

Many modulators are "frequency agile," which means they are adjustable and can place signals onto different carrier frequencies within a certain bandwidth.

What happens after the processing and modulation steps are performed and the RF signals are ready for transmission?

After the processing and modulation steps are performed and the RF signals are ready for transmission, all channels are multiplexed to fit in the broadband spectrum. Headend combiners have multiple input ports that join signals together to produce a single output. The combined signals themselves are coupled together until they produce a single broadband output.

During the 1980's, what led to the expansion in the cable system channel capacity

Fiber-Optic technology

Digital and analog signals are different in that

digital network can combine, or multiplex, several different signals into one transmission

In the cable antenna relay service (CARS)

The over the air signal is converted to a frequency in the 12.7 -13.2 GHz

A satellite transponder

Receive, amplifies, transmit and re-amplifies analog or compressed digital signals

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Understanding Signals and the headend

31 important questions on Understanding Signals and the headend

In what frequency bands are over-the-air signals originating from local broadcast stations?

OTA broadcast signals are acquired at the headend using?

What is a yagi antenna?

What are log periodic antennas?

How does the cable operator receive broadcast signals from far away communities?

How do transponders on most commercial communications satellites receive and transmit satellite signals?

How does the cable operator acquire signals from satellite transponders?

How did "community antenna television" systems operate?

How can analog signals be described?

How can digital signals be described?

What are three methods in which a carrier is "shaped" to contain the original signal?

How are the video, audio, and color carriers each modulated?

Name two unique properties of microwave bands.

What happens once microwave signals are received?

Describe the difference between FM and AML microwave transmission techniques.

What is a geosynchronous orbit?

How do satellites take advantage of each transponder's 36 MHz capacity more efficiently?

What are the uplink and downlink satellite frequencies in the C- and Ku-bands?

What is the purpose of the TVRO antenna's parabolic shape?

What happens to the satellite signal once it is received by the feedhorn?

What happens as the digital signals are downlinked at the headend?

What does each satellite receiver in the headend do?

What do filters in the headend do?

What is the function of test equipment in the headend?

What does EAS equipment do?

What are frequency-agile modulators?

What happens after the processing and modulation steps are performed and the RF signals are ready for transmission?

During the 1980's, what led to the expansion in the cable system channel capacity

Digital and analog signals are different in that

In the cable antenna relay service (CARS)

A satellite transponder

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