Satellite Communications System

A broadcast signal is typically delivered to a teleport as an analog video and associated audio signal—and is either FM modulated or digitized—and is then upconverted into the 6 GHz, 14 GHz or 17 GHz frequency band. As the signal strength is very low it must be routed through a high power RF (Radio Frequency) amplifier. The amplified signal is then routed through waveguide to a high gain antenna for transmission to the satellite.

Satellite Transmission

The television signal is sent over the satellite on one of several frequency bands: Ku-band (14/12 GHz), Ku-band DBS (17/12 GHz), or C-band (6/4 GHz). Each of these frequency bands has different advantages for broadcasting applications. C-band has traditionally been the frequency of choice for broadcasters, due to its wide area coverage for broadcast applications and minimal disturbance due to rain attenuation. On the Anik E's and Anik F1 satellites, broadcasters can reach into the far north of Canada and into the United States, as far south as Florida. Telesat's C-band RF channels have 36 MHz transponders.

Ku-band has gained popularity both in Canada and in the U.S. as it is less prone to terrestrial interference. As well, Ku-band transmissions using digital video compression can be received with smaller receive antennas —as small as 0.6 meters. This is very attractive to broadcasters wishing to transmit directly to a subscriber's home. Anik E Ku-band RF transponders have 54 MHz bandwidth, available in 27 MHz increments, while Anik F1 Ku-band transponders each have 27 MHz bandwidth.

Telesat's two Nimiq Direct Broadcast Satellites operate at 17/12 GHz and have 32 channels each of 24 MHz.

Broadcast Receive Signal

The low level RF signal is received by the earth station antenna and is amplified by a small amplifier which is typically located at the focal point of the antenna. The signal is downconverted into the L band frequency and fed via coax cable to the receiver where it is demodulated and decoded; then routed to the cable company head-end or directly to the subscribers television set.

Digital Video Compression

Digital Video Compression (DVC) systems have had a dramatic effect on the way that television signals are transmitted from their origination to the subscriber's home. In the past, most television signals were transmitted by satellite using an analog format. Analog transmission requires the full RF transponders on C-band transmissions (36 MHz) and half of a Ku-band transponder (27 MHz).

DVC technology allows customers to put multiple signals into a single satellite transponder, thus making the transmission of television signals increasingly more economical. Digital Video Compression manufacturers are able to transmit up to 10 television signals in a 36 MHz C-band transponder, or up to 8 signals in a 27 MHz Ku-band channel where previously only one channel could be transmitted.

DVC works by sampling the video at a very high rate. After this, complex algorithims and techniques are used to remove redundant video information, which greatly reduces the transmitted bit rate. At the receive location, the image is then reconstructed to its original format by sophisticated decoders.

Without compression, a broadcast quality television signal requires up to 240 Mbits of information to be transmitted per second. With DVC, the video requires between 6 and 8 Mbits/sec and provides similar visual quality as the higher rate. This results in significant reductions in the bandwidth required for the television signal.