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U. Glaeser

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FIGURE 39.60<br />

FIGURE 39.61<br />

FIGURE 39.62<br />

aliasing distortion. Figure 39.61 illustrates the effect of changing the sample rate on the images of the inband<br />

signal. The sample rate converter must remove these images in order to obtain high quality conversion.<br />

It is easy to deceive a naïve observer by a sampled waveform. Consider the following time series:<br />

© 2002 by CRC Press LLC<br />

Images above the Nyquist frequency.<br />

Aliases and images from sample rate conversion.<br />

1<br />

0.8<br />

0.6<br />

0.4<br />

0.2<br />

0<br />

-0.2<br />

-0.4<br />

-0.6<br />

-0.8<br />

-1<br />

Simple time series that deceptively appears to represent a square wave.<br />

0.707 0.707<br />

3<br />

� �<br />

Original signal<br />

�������� ������<br />

−0.707<br />

20<br />

�� ��<br />

24<br />

��<br />

28<br />

����� ������ ��� ����� ������<br />

�� ������ ���� ���������� ��<br />

�� ��� ������� ������<br />

−0.707<br />

0.707 0.707<br />

−0.707<br />

−0.707<br />

As shown in Fig. 39.62, the waveform might appear to represent a square wave of peak magnitude<br />

0.707 at exactly one-half the Nyquist frequency. This is incorrect. A true square wave consists of an infinite<br />

45<br />

Image signal caused<br />

by sampling at 48 kHz<br />

�� ��<br />

48<br />

�� ��<br />

����� �� ��������<br />

������<br />

freq (kHz)<br />

��<br />

���� �����<br />

0 1 2 3 4 5 6 7

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