Representation of Audio Signals 479(a)AmplitudeTimeFigure 15.20 : (a) In the time domain the process of sampling is like one of using a sequence
of pulses, whose amplitude is either 1 or 0, and multiplying it by the value of the sinusoidal
waveform. A sample and hold circuit holds the sampled signal level steady while the
amplitude is measured. (b) At a higher frequency, sampling is taking place approximately
three times per sinusoid input cycle. Once more it is possible to see that even by simply
joining the sample spikes the frequency information is still retained. (c) This plot shows
the sinusoid being under sample, and on reconstituting the original signal from the spikes
the best-fi t sinusoid is the one shown as the dashed line. This new signal will appear as a
perfectly proper signal to any subsequent process and there is no method for abstracting
such aliases from properly sampled signals. It is necessary to ensure that frequencies greater
than half of the sampling frequency Fs are fi ltered out before the input signal is presented to
a sampling circuit. This fi lter is known as an antialiasing fi lter.
(b)AmplitudeTime(c)AmplitudeTime