Satellite TV Technical Information

One of the most common methods of TV signal distribution is via communications satellite. A communications satellite orbits around the equator about 22,300 mi out in space. It rotates in synchronism with the earth and therefore appears to be stationary. The satellite is used as a radio relay station. The TV signal to be distributed is used to frequency modulate a microwave carrier, and then it is transmitted to the satellite. The path from earth to the satellite is called the uplink. The satellite translates the signal to another frequency and then retransmits it back to earth. This is called the downlink. A receive site on earth picks up the signal. The receive site may be a cable TV company or an individual consumer. Satellites are widely used by the TV networks, the,premium channel companies, and the cable TV industry for distributing their signals nationally.

A newer form of consumer satellite TV is direct broadcast satellite (DBS) TV The DBS systems are designed specifically for consumer reception directly from the satellite. The new DBS systems feature digitally encoded video and audio signals, which make transmission reception more reliable and provide outling picture and sound quality. By using r frequency microwaves, higher powered lite transponders, and very low noise sFETs in the receiver, the customer's satelfish.can be made very small. These systypically use an 18 in dish as opposed to 5 to 12 ft diameter dishes still used in y satellite TV systems.

Sallite Transmission

TV signal to be uplinked to the satellite from a ground station is used to modulate a carrier in one of several available microwave satellite bands. The C band between approxitly 3.7 to 4.2 GHz is the most commonly used. The video signal frequency modulates microwave carrier on one of 24 channel frequencies.

The audio accompanying the video frequency modulates a subcarrier in the 5 to 8 MHz range. The 6.2 and 6.8 MHz subcarriers are the most common. Stereo sound is used to modulate the two subcarriers on 5.58 and 5.76 MHz. The video occupies the spectrum of approximately 0 to 5Hz. The composite spectrum of the video and audio subcarrier signals used to frequency modulate the uplink transmitter is illustrated in fig.1. This is the signal that must be recovered by the satellite receiver.

The signal is received by the satellite, and filters pass the signal through the selected transponder. In the transponder the signal is down-converted to a lower frequency, amplified, and retransmitted.

Satellite Receivers

A satellite receiver is a special subsystem de signed to work with a consumer TV set. It con sists of a parabolic dish antenna, a low noise amplifier and down converter, an IF section with appropriate demodulators for both video and sound, and a method of interconnecting it to the conventional TV set. In addition, most satellite receivers contain circuitry for control ling the positioning of the satellite dish an tenna. You will sometimes hear the satellite receiver referred to as TVRO, or TV receive only, system. The following section describes (TvRo) system the basic organization and operation of typical TVRO satellite receivers.

Satellite TV Antenna

The antenna is more critical in a satellite TV receiver than in any other kind of receiver. The signal from the satellite 22,300 mi away is extremely weak. In addition, there are hundreds of satellites in orbit above the earth, and their spacing is getting closer each year as the number of satellites continues to in crease. A high gain, highly directional para bolic dish antenna is used to select only the signal from the desired satellite and provide very high gain. Most satellite TV antennas range in size from approximately 6 to 15 ft. Over the years, lower noise, higher gain am plifiers have been developed using gallium ar senide (GaAs)field effect transistors. This has permitted dish antenna sizes to be reduced in size. In some of the newer systems, antennas as small as 3 to 4 ft in diameter are available. However, in most cases, the larger the dish, the higher the gain and the better the performance.

A part of determining antenna size is based on the location of the receiver in the United States. The signal strength of the satellite down link signal varies considerably over the United States. In most cases, the signal strength is higher in the center of the country and consid erably lower on the coasts. As a result, if the receiver is located on the east or west coast, higher gains and larger antennas are required for satisfactory reception. Figure 2 shows the "footprint" of the satellite antenna on earth. The contour lines indicate different signal levels, the strongest being in the center and the weakest being on the outside. Since the signal is the strongest in the center, smaller antennas can be used. Larger dishes must be used on the outer areas to receive an adequate signal level.

The antenna is a horn located at the focal point of a parabolic reflector (see Fig. 3). Signals picked up by the dish are focused on the horn, giving very high gain and exceptionally narrow directional characteristics. The antenna is built so that it can receive both horizontally and vertically polarized signals. The horn is usually coupled by a short piece of coaxial cable to the receiver input.

Satellite TV Receiver

A satellite TV receiver is like any other cornmunications receiver in that it is usually of the superheterodyne type. See Fig. 5. A single- conversion receiver usually has a 70 or 140 MHz intermediate frequency (IF). A dual-conversion receiver uses a first IF of 770 MHz, although you may find other values in the 600to 1500 MHz range. A second IF of 140 or 70 MHz follows. These values are chosen to minimize images.

In some receivers the RF amplifier, first mixer, and local oscillator are located directly at the horn antenna on the dish. This is done to avoid the massive attenuation in a coaxial cable from the antenna to the receiver front end. Coaxial cable has a massive loss at 4 to 6 GHz (C band) and even more in the 11 to 18 GHz range (Ku band) used by direct broadcast satellites. With the first mixer at the antenna, the signal can be down-converted to a frequency that will produce less loss. Typically, a broadband converter is used to elimitiate the need to tune the local oscillator. The entire bandwidth of the received signal is converted to a frcquency that is usually in the 900 to 1400 MHz range, where coax attenuation is not so severe. This broadband signal enters the receiver and is further down-converted to the selected IF, either 70 or 140 MHz. From there it is demodulated and the resulting video and audio signals are used to remodulate a signal on VHF channel 3 or 4 to create a standard TV signal that is then connected to the antenna input of a conventional TV receiver.

Direct Broadcast Satellite TV System

The direct broadcast satellite (DBS) system is the newest form of satellite TV available to consumers. It was designed specifically to be an all-digital system in contrast to the analog systems currently in use. Data compression techniques are used to reduce the data rate re quired to produce high quality picture and sound. The DBS system has almost totally re placed the older C band TV receivers. The DBS system features entirely new digital uplink ground stations and satellites. Since the satellites are designed to transmit directly to the home, extra high power transponders are used to ensure a satisfactory signal level.

To receive the digital video from the satellite, a consumer must purchase a satellite TV receiver and antenna. These are similar to the satellite receivers just described; however, they work with digital signals and operate in the Ku rather than the C band. By using higher frequencies as well as higher power satellite transponders, the necessary dish antenna Can be extremely, small. The new satellite DBS system antennas are only 18 inches in diameter. There are several special digital broadcast satellites in orbit, and some of the direct satellite TV sources include DirecTV, USSB, and PrimeStar. All provide full coverage of the major cable networks and the premium channels usually distributed to homes by cable TV, and all can be received directly. In addition to purchasing the receiver and antenna, the consumer must subscribe to one of the services supplying the desired channels.

Satellite TV Transmission

The video to be transmitted must first be placed into digital form. To digitize an analog signal, it must be sampled a minimum of two times per cycle in order for sufficient digital data to be developed for reconstruction of the signal. Assuming that video frequencies of up to 4.2 Mbits/s are used, he minimum sampling rate is twice this, or 8.4 Mbits/s. For each sample, a binary number proportional to the light amplitude is developed. This is done by an A/D converter, usuIly with an 8 bit output. The resulting video signal, therefore, has a data rate of 8 bits times 8.4 Mbits/s, or 67.2 Mbits/s. This is an extremely high data rate. However, for a color TV signal to be transmitted in this way, there must be a separate signal for each of the red, green, and blue components making up the video. This translates to a total data rate of 3 X67.2, or 202 Mbits/s. Even with today's tech dogy, this is an extremely high data rate that is hard to achieve reliably.

In order to lower the data rate and improve reliability of transmission, the new DBS stem uses compressed digital video. Once video signals have been put into digital form, they are processed by digital signal processing (DSP) circuits to minimize the full ount of data to be transmitted. Digital compression greatly reduces the actual transmitting speed to somewhere in the 20 to 30 Mbits/s range. The compressed serial digital signal is then used to modulate the uplinked carrier using BPSK. The DBS satellite uses the Ku band with a frequency range of 11to 14 GHz. Uplink signals are usually in the 14 to 14.5 GHz range and the downlink signals usually cover the range of 10.95 to 12.75 GHz.

The primary advantage of using the Ku band rather than the C band is that the receiving antennas may be made much smaller for a given amount of gain. However, these higher frequencies are more affected by atmospheric conditions than the lower microwave frequencies. The biggest problem is the increased attenuation of the downlink signal caused by rain. Any type of weather involving rain or water vapor, such as fog, can seriously reduce the received signal. This is because the wavelength of Ku band signals is near that of water vapor. Therefore, the water vapor absorbs the signal. Although the power of the satellite transponder and the gain of the receiving antenna are typically sufficient to provide solid reception, there can be fadeout under heavy downpour conditions.

Finally, the digital signal is transmitted from the satellite to the receiver using circular polarization. The DBS satellites have right hand and left hand circularly polarized (RHCP and LHCP) helical antennas. By transmitting both polarities of signal, frequency reuse can be incorporated to double the channel capacity.

DBS Receiver

A block diagram of a typical DBS digital receiver is shown in Fig.6. The receiver subsystem begins with the antenna and its low-noise block converter. The horn antenna picks up the Ku band signal and translates the entire 500 MHz band used by the signal down to the 950 to 1450 MHz range, as explained earlier. Control signals from the receiver to the antenna select between RHCP and LHCP. The RF signal from the antenna is sent by coaxial cable to the receiver.

A typical DBS downlink signal occurs in the 12.2 to 12.7 GHz portion of the Ku band. Each transponder has a bandwidth of about 24 MHz. The digital signal is usually occurring at a rate of approximately 27 Mbits/s.

Figure 7 shows how the digital signal is transmitted. The digital audio and video signals are organized into data packets. Each packet consists of a total of 147 bytes. The first 2 bytes (16 bits) contain the service channel identification (SCID) number. This is a 12 bit number that identifies the video program being carried by the packet. The 4 additional bits are used to indicate whether the packet is encrypted and if so, which decoding key to use. One additional byte contains the packet type and a continuity counter.

The data block consists of 127 bytes, either 8 bit video signals or 16 bit audio signals. It may also contain digital data used for control purposes in the receiver. Finally, the last 17 bytes are the error detection check codes. These 17 bytes are developed by an errorchecking circuit at the transmitter.The appended bytes are checked at the receiver to detect any errors and correct them.

The received signal is passed through another mixer with a variable frequency local oscillator to provide channel selection. The digital signal at the second IF is then demodulated to recover the originally transmitted digital signal, which is passed through a forward error correction (FEC) circuit. This circuit is designed to detect bit errors in the transmission and to correct them on the fly. Any bits lost or obscured by noise during the transmission process are usually caught and corrected to ensure a near perfect digital signal.

The resulting error corrected signal is stored in random access memory (RAM), after which the signal is decoded to separate it into both the video and the audio portions. The resulting signals are then sent to the audio and video decompression circuits. The DBS TV system uses digital compression-decompression standards referred to as an MPEG2. MPEG means "Motion Picture Experts Group," which is a standards organization that establishes technical standards for movies and video. MPEG2 is its latest and best compression method for video.

Although the new DBS digital systems will not replace cable TV, they provide the consumer with the capability of receiving a wide range of TV channels. The use of digital techniques provides an unusually high quality signal.

Finally, the video and audio signals are converted to analog by D/A converters and sent to an RF modulator that develops a conventional TV signal.