Page 327 - Physics Coursebook 2015 (A level)

Chapter 20: Communications systems
Advantages of digital signals
Most devices such as microphones or thermistors produce analogue voltage signals. However, digital signals have advantages and it is often worthwhile to change an analogue signal into a digital signal.
The major advantage is that digital signals can deal with ‘noise’ produced over long distances. All signals, both analogue and digital, become weaker as they travel and they pick up electrical noise. The decrease in strength is known as attenuation, and can be corrected by amplifying the signal at regular intervals during long-distance transmission.
Noise is electrical interference, caused in a number of different ways: by the spark from a car ignition, by induced voltages from the magnetic fields caused by currents around the home, by the radio signals emitted by a mobile phone nearby, and even by the random thermal motion of electrons in a wire or by vibrating atoms. You may have noticed a background hiss on telephone conversations. This also is an example of noise.
Noise is the random, unwanted signal that adds to and distorts a transmitted signal. Amplification of a signal amplifies the noise at the same time as the signal.
Other advantages of using digital signals are:
■■ Digital signals are compatible with modern technology and can be stored and processed more easily, for example in a computer or on a compact disc (CD).
■■ Digital electronic systems are, in general, more reliable and easier to design and build.
■■ Digital signals build in safeguards so that if there is an error in reception it is noticed and parts of the signal can be sent again.
Analogue-to-digital conversion
The key to the digital revolution has been the ability to change speech and music from analogue into digital form in analogue-to-digital conversion (ADC) and then convert them back again into analogue form in digital-to-analogue conversion (DAC).
In order to understand this process, you need to be able to count using binary numbers as well as ordinary decimal numbers. The decimal system has base 10 and the number of digits increases by one when going from the number
9 to the number 10. The number 9 has only one digit whereas the number 10 has two digits. The binary number system has base 2 and the number of digits increases when going up from the number 1; so the next number above 1 is 10. The binary number 10 is not the same as the decimal number 10. Table 20.2 compares counting in the decimal system and in the binary system.
3 0
Time
Decimal number
Binary number
Decimal number
Binary number
Figure 20.9 Weakened and noisy signals.
Figure 20.9 shows the signals from Figure 20.8 after
they have travelled a long distance. You will see that they are lower in amplitude and have unwanted variations, or noise.
There is little improvement possible for an analogue signal; amplification will not remove the noise. However, regeneration will remove the noise from a digital signal. The signal is ‘cleaned’ of the noise and returned to its initial shape.
At the end of a long-distance transmission, an electronic circuit, the regeneration amplifier, receives the digital signal. This electronic circuit expects to receive a pulse of a few volts or no pulse at all; any small variations added to the pulse or the 0 V make no difference. The regeneration amplifier can only give a 0 or a 1 as an output. As long as the noise does not completely change the shape of the signal, then the regeneration amplifier returns the digital signal shown in Figure 20.9 to the perfect pulses shown in Figure 20.8.
0
1
2
3
4
5
Table 20.2
0 6 110
1 7 111
10 8 1000
11 9 1001
100 10 1010
101 11 1011
Binary and decimal numbers.
For example:
■■ in the decimal system, the number 243 is a combination of 2×100,4×10and3×1
■■ in the binary system, the number 111 is a combination of 1×4,1×2and1×1.
Counting in the binary system is very similar to counting in the decimal system except that there are only the digits 0 and 1.
Each digit in the binary number is known as a bit. The bit on the left-hand side of a binary number is the most significant bit (MSB) and has the highest value.
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Voltage / V