Page 329 - Physics Coursebook 2015 (A level)

Chapter 20: Communications systems
The maximum sampling rate required is only twice the highest frequency present in the signal (this is known as Nyquist’s theorem). The human ear can hear up to 20 kHz and so the maximum sampling rate needed is 40 kHz
for music. For every second of music on a compact disc storing 8-bit numbers sampled 40 000 times a second, 320 000 separate binary digits are stored. The sampling rate required for a telephone system will be lower, since only frequencies up to about 3400 Hz are required for basic speech recognition. Many digital telephone systems sample the input signal 8000 times a second and so only transmit frequencies below 4 kHz.
Channels of communication
The term channel of communication refers to the medium, the path or even the actual frequency range used to convey information from a transmitter to a receiver.
When you listen to a radio, the radio signal may have travelled through the air by a number of different routes. When you talk to someone on a telephone in a different country then the signal may have passed along a wire-pair, a coaxial cable, through the air by a microwave link or been converted into pulses of light and then transmitted down an optic fibre. These are all different channels of communication.
Before comparing the different channels you will need to understand another technical term, crosslinking, and be able to calculate signal attenuation.
Crosstalk
You may have experienced crosstalk or crosslinking when using a radio or a telephone. If you tune your radio set to one radio station, sometimes you can also hear another station. When talking on the telephone you can suddenly find yourself listening to a completely different telephone conversation; the telephone network has connected your telephone to someone you have not dialled. Crosslinking occurs when a signal, transmitted on one circuit or channel, creates an undesired effect in another circuit or channel.
Signal attenuation
Attenuation is the gradual decrease in the power of a signal the further it travels. The causes of energy loss depend on the type of signal:
■■ As an electrical signal passes along a wire, there is a voltage drop across the resistance of the wire itself. This reduces the voltage of the signal that arrives at the end of the wire. The energy loss in the wire causes electrical heating in the resistance of the wire (I 2Rt).
■■ A radio wave spreads out from a transmitter. On its own this spreading causes a decrease in intensity, but there is also a loss in signal strength because of the absorption of energy by the material through which the wave travels.
■■ Light travelling through an optic fibre may be scattered or absorbed by irregularities in the glass structure.
The decrease in signal power from the transmitted value P1 to that received P2 can be very high. The ratio P2 to P1 is measured using a logarithmic scale rather than by the simple ratio of the two powers.
The logarithm to base 10 of the ratio gives us the number of bels (B). When multiplied by 10 we obtain the number of decibels (dB). Your calculator may show logarithms to base 10 as log10.
QUESTIONS
10 Convert the following decimal numbers into binary numbers:
a 14 b 16
11 Convert the following binary numbers into decimal numbers:
a 1111
b 0001011
12 The diagrams in Figure 20.12 show a digital signal at the start of a long cable and at the end of the cable. Both diagrams are drawn to the same horizontal scale (time) and vertical scale (voltage).
before transmission
at the end of a long cable
Figure 20.12 For Question 12.
a Explain what feature of the top diagram shows that the signal is digital.
b State and explain two advantages of digital transmission over analogue transmission of data.
c State and explain two reasons why the signal at the end of the long cable differs from the signal at the start.
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