Page 128 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 6: Data Communication

The digital data to be transmitted is the binary number 1011. Two amplitudes are used Notes
to directly represent the data, either 0 or 1. In this case, the modulation is called binary
amplitude shift keying or BASK. The signal is divided into four pulses of equal duration
which represent the bits in the digital data. The number of bits used for each character is
a function of the system, but is typically eight, seven of which represent the 128 possible
characters, the last bit is used to check for errors, and is explained at the end of this chapter.

6.4.1.2 FSK

In frequency shift keying, the carrier frequency is changed between discrete values. If only
two frequencies are used then this will be called BFSK, for binary frequency shift keying.
6.4.1.3 PSK

The phase of the carrier wave at the beginning of the pulse is changed between discrete
values. This particular case is the same code shown above but in BPSK.
6.4.1.4 M-ary Frequency/Phase Keying

In binary shift keying, there were only two choices for the parameter of the carrier wave
which was varied in accordance with the digital data. In BASK, there are two possibilities for
amplitude, which corresponded to zero and one. Likewise for BFSK and BPSK. This matches
nicely with the binary number system, which also uses two possibilities for each bit, 0 and 1.
It is possible to increase the data transfer rate by putting more choices into each bit. As and
example, 4-ary (or Quaternary PSK) uses four different phases: 0, 90, 180, and 270 degrees.
This gives four possible values at each pulse, corresponding to two independent streams
(channels) of data. Likewise, 16-ary FSK can send four channels of data at the same time.
6.4.1.5 Amplitude-Phase Keying

This process uses combinations of amplitude and phase keying. For example, if we
use two levels of amplitude and two levels of phase together, there will be a total of
four possibilities. This is used to transmit two independent channels of digital data
simultaneously. This particular case is called Quadrature AM or Quaternary PSK. They
are identical, although it may not be obvious at this level. Because of the equivalence, the
basic process is called amplitude-phase keying. This process may be extended to higher
numbers of possibilities. The completely general term is M-ary APK, which is not specific
about which parameter has which number of possibilities. 16-APK may have 2 amplitudes
and 8 phases or 4 each, it matters little. The upshot is that the number of separate channels
that can be sent simultaneously is increased. If M designates the number of possible
combinations, from the M-ary APK system, then the number of channels of digital data
that may be transmitted simultaneously is given by
N = Log 2M

6.4.1.6 Capacity

All of these methods which utilize a sequence of equally spaced pulses to modulate a
carrier wave have similar bandwidths. The bandwidth determined by the duration of each
pulse, designated as td. It is a general result, that the minimum bandwidth required to
create this pulse, W, is given by
W = 1/(2td)

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