waveform Original waveforms (b) Time division multiplexed pulses for three signals Figure 3.16 Pulse amplitude modulation Another method of pulse modulation involves varying the width of the pulses according to the amplitude of the waveform. This is called pulse width modulation (PWM). An example is shown in Figure 3.17 and from this it can be seen that the width of the pulses increases in line with the instantaneous value of the waveform. Both PAM and PWM are essentially analogue methods of pulse modulating a signal. There are many advantages to digitizing the signal and using a digital train of pulses to modulate the carrier. This form of pulse code modulation (PCM) is widely used for telecommunications links. The advantage is that once the signal is digitized, the system only has to recognize whether a pulse is present or not, i.e. a logical one or zero. If the noise level is not large then there is a very small chance of there being any errors. It is found that if the RMS noise level is 20 dB below the peak pulse level then there is a negligible chance of there being an error and at 13 dB the chance of an error becomes one in a hundred. As a result PCM systems are more resilient to noise than their analogue equivalents. This has the advantage that repeater stations to boost the signals are required less frequently giving a significant saving in costs. 70 Newnes Guide to Radio and Communications Technology Figure 3.17 Pulse width modulation To generate a PCM signal, the modulating waveform is sampled as before at distinct points. These samples are then converted into a digital format using a circuit called an analogue to digital converter. This circuit generates a binary code that in turn is converted into a group of pulses. A group of n pulses can represent 2nd different levels, i.e. if three pulses are generated then there can be 23 or 8 levels. In reality eight pulses are usually used for communications links giving a total of 256 levels. To reconstitute the waveform the incoming pulses are converted into a