DIGITAL DATA, ANALOG SIGNALS

DIGITAL DATA, ANALOG SIGNALS

We turn now to the case of transmitting digital data using analog signals. The most

familiar use of this transformation is for transmitting digital data through the public

telephone network. The telephone network was designed to receive, switch, and

transmit analog signals in the voice-frequency range of about 300 to 3400 Hz. It is

not at present suitable for handling digital signals from the subscriber locations

(although this is beginning to change). Thus, digital devices are attached to the network

via a modem (modulator-demodulator), which converts digital data to analog

signals, and vice versa.

For the telephone network, modems are used that produce signals in the

voice-frequency range. The same basic techniques are used for modems that produce

signals at higher frequencies (e.g., microwave). This section introduces these

techniques and provides a brief discussion of the performance characteristics of the

alternative approaches.

Encoding Techniques

We mentioned that modulation involves operation on one or more of the three

characteristics of a carrier signal: amplitude, frequency, and phase. Accordingly,

there are three basic encoding or modulation techniques for transforming digital

data into analog signals, as illustrated in Figure 4.7:

Amplitude-shift keying (ASK)

Frequency-shift keying (FSK)

Phase-shift keying (PSK)

In all these case;, the resulting signal occupies a bandwidth centered on the

carrier frequency.

In ASK, the two binary values are represented by two different amplitudes of

the carrier frequency. Commonly, one of the amplitudes is zero; that is, one binary

digit is represented by the presence, at constant amplitude, of the carrier, the other

by the absence of the carrier. The resulting signal is

The ASK technique is used to transmit digital data over optical fiber. For

LED transmitters, the equation above is valid. That is, one signal element is represented

by a light pulse while the other signal element is represented by the absence

of light. Laser transmitters normally have a fixed "bias" current that causes the

device to emit a low light level. This low level represents one signal element, while

a higher-amplitude lightwave represents another.

In FSK, the two binary values are represented by two different frequencies

near the carrier frequency. The resulting signal is

Figure 4.8 shows an example of the use of FSK for full-duplex operation over

a voice-grade line. The figure is a specification for the Bell System 108 series

modems. Recall that a voice-grade line will pass frequencies in the approximate

range of 300 to 3400 Hz, and that full-duplex means that signals are transmitted in

both directions at the same time. To achieve full-duplex transmission, this bandwidth

is split at 1700 Hz. In one direction (transmit or receive), the frequencies used

to represent 1 and 0 are centered on 1170 Hz, with a shift of 100 Hz on either side.

The effect of alternating between those two frequencies is to produce a signal whose

spectrum is indicated as the shaded area on the left in Figure 4.8. Similarly, for the

other direction (receive or transmit) the modem uses frequencies shifted 100 Hz to

each side of a center frequency of 2125 Hz. This signal is indicated by the shaded

area on the right in Figure 4.8. Note that there is little overlap and, consequently,

little interference.

FSK is less susceptible to error than ASK. On voice-grade lines, it is typically

used up to 1200 bps. It is also commonly used for high-frequency (3 to 30 MHz)

radio transmission. It can also be used at even higher frequencies on local area networks

that use coaxial cable.

In PSK, the phase of the carrier signal is shifted to represent data. The bottom

of Figure 4.7 is an example of a two-phase system. In this system, a binary 0 is represented

by sending a signal burst of the same phase as the previous signal burst. A

binary 1 is represented by sending a signal burst of opposite phase to the preceding

one; this is known as differential PSK, as the phase shift is with reference to the previous

bit transmitted rather than to some constant reference signal. The resulting

signal is

Thus, each signal element represents two bits rather than one.

This scheme can be extended. It is possible to transmit bits three at a time

using eight different phase angles. Further, each angle can have more than one

amplitude. For example, a standard 9600 bps modem uses 12 phase angles, four of

which have two amplitude values (Figure 4.9).

This latter example points out very well the difference between the data rate

R (in bps) and the modulation rate D (in bauds) of a signal. Let us assume that this

scheme is being employed with NRZ-L digital input. The data rate is R = l/tB

where tB is the width of each NRZ-L bit. However, the encoded signal contains

b = 4 bits in each signal element using L = 16 different combinations of amplitude

and phase. The modulation rate can be seen to be R/4, as each change of signal element

communicates four bits. Thus, the line signaling speed is 2400 bauds, but the

data rate is 9600 bps. This is the reason that higher bit rates can be achieved over

voice-grade lines by employing more complex modulation schemes.

To repeat

Performance

In looking at the performance of various digital-to-analog modulation schemes, the

first parameter of interest is the bandwidth of the modulated signal. This depends

on a variety of factors, including the definition of bandwidth used and the filtering

technique used to create the bandpass signal. We will use some straightforward

results from [COUC95].

which can be accomplished by increasing the bandwidth or decreasing the data

rate-in other words, by reducing bandwidth efficiency

Thus, digital signaling is in the same ballpark, in terms of bandwidth efficiency, as

ASK, FSK, and PSK. Significant advantage for analog signaling is seen with multilevel

techniques.