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Figure 2.18: Some examples of sampled systems in which filtering is inadequate or absent.
It should be stressed that in real systems there will often be more than one aperture effect. The result is that the
frequency responses of the various aperture effects multiply, which is the same as saying that their impulse
responses convolve. Whatever fine words are used, the result is an increasing loss of high frequencies where a
series of acceptable devices when cascaded produce an unacceptable result.
In many systems, for reasons of economy or ignorance, reconstruction is simply not used and the system output is
an unfiltered ZOH waveform. Figure 2.18 shows some examples of this kind of thing which are associated with the
'digital look'. It is important to appreciate that in well-engineered systems containing proper filters there is no such
thing as the digital look.
2.8 Choice of audio sampling rate
The Nyquist criterion is only the beginning of the process which must be followed to arrive at a suitable sampling
rate. The slope of available filters will compel designers to raise the sampling rate above the theoretical Nyquist
rate. For consumer products, the lower the sampling rate, the better, since the cost of the medium or channel is
directly proportional to the sampling rate: thus sampling rates near to twice 20 kHz are to be expected.
Where very low bit-rate compression is to be used, better results may be obtained by reducing the sampling rate so
that the compression factor is not as great.
For professional products, there is a need to operate at variable speed for pitch correction. When the speed of a
digital recorder is reduced, the offtape sampling rate falls, and Figure 2.19 shows that with a minimal sampling rate
the first image frequency can become low enough to pass the reconstruction filter. If the sampling frequency is
raised without changing the response of the filters, the speed can be reduced without this problem. It follows that
variable-speed recorders, generally those with stationary heads, must use a higher sampling rate.
Figure 2.19: At normal speed, the reconstruction filter correctly prevents images entering the baseband, as in (a).
 
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