Satellite TVRO Part 2
Sep 2, 2009 3:16 PM, By Russell Brown
Figure 4: Determining symbol rate
Select figure to enlarge.
Just as with today’s over-the-air DTV signal that can carry more than one program, so, too, can satellite transmissions. If your station transmits more than one program, then you have two or more encoders whose outputs are combined in a multiplexer and sent on to your digital transmitter. For DTV, we are limited to one 6Mhz-wide channel and the 8-VSB modulation method, which give every station a data rate of 19.4Mb/s. For satellites, a transponder’s bandwidth can be as wide as 70MHz, and the modulation methods have evolved over the years from BPSK and QPSK to SP-QPSK. All these modulation methods provide a higher data rate with which to pass more information within a smaller bandwidth. The “PSK” of all these methods stands for phase shift keying. Binary PSK is where there are only two states of the carrier’s phase, so it can only represent one bit of data, one or zero. Quadrature PSK is where there are four states of the carrier’s phase, which means that it can represent two bits at every state.
Just as in 8-VSB, where there are eight different amplitude levels, each amplitude level represents a single symbol. A symbol corresponds to one of eight possible combinations of three bits, so each symbol sent transmits three bits of data, an eightfold increase of data flow. (See Figure 4.)
8PSK is similar to 8-VSB in that it also has eight different states. One of the newest modulation methods is SP-QPSK (sinusoidal-shaped π/4 QPSK), which is a more efficient way to modulate the satellite carrier and something The Associated Press has just started using.
The symbol rate is the number of symbols sent per second. It contains the data rate and the forward error correction (FEC) data. The symbol and data rates correspond to each other, and a formula can be used to convert between the two. (See Figure 5.)
Figure 5: Data/symbol rate conversion
Select figure to enlarge.
FEC is an important part of digital broadcasting. In satellite transmissions, the amount of error correction is stated as follows: 1/2, 2/3, 3/4, 7/8. The first number is the number of actual data bits, followed by the total number of bits transmitted. The difference is the amount of error correction bits. So for an FEC of 1/2, there is one error correction bit for data every bit. At the other end, an FEC of 7/8 means there is one error correction bit for every seven data bits.
An FEC of 1/2 equals a great deal of error correction, which will tolerate smaller antennas and lower-quality receive equipment, while an FEC of 7/8 means the signal provider is trying to squeeze as much data through as possible and may require a bigger antenna and higher-quality receive equipment.
Conclusion
All of the methods described allow for more data to be sent through fewer frequencies. Satellite transmission requires a great deal of high-tech equipment to be able to reliably send and receive the programs that TV stations use every day.
Acknowledgements
Frank Foge of CALSAT and Ed Johnson of San Francisco International Gateway Teleport assisted with this tutorial.
Next Time
The next “Transition to Digital” tutorial will discuss how to find the satellite you want.
Continue reading part three of the Satellite TVRO series.
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