The digitization of analogue waveforms by pulse code modulation is accomplished in two stages. First the waveform is sampled to produce pulse amplitude modulation (PAM). Short-duration samples are taken at regular intervals which are long compared with the sampling time but short in relation to the highest signal frequency. The result is a train of pulses whose amplitude envelope is the same as the analogue waveform. The envelope (Figure 14.1) will contain:
• clock frequency fc, the sampling rate
• all the signal frequencies contained in the waveform from the lowest, f1, to the highest, f2
• upper and lower side bands, (fc − f2) to (fc − f1) and (fc + f1) to (fc + f2)
• harmonics of fc and the upper and lower side band frequencies
• a DC component equal to the mean value of the PAM waveform.
The envelope contains the original signal frequencies and can be demodulated by a low-pass filter which will pass f2 but not the clock frequency. The clock frequency must therefore be higher than 2f2. For line communications in the UK a clock frequency of 8 kHz is used with a maximum modulating frequency of 3400 Hz.