492 Chapter 15
Delta sigma modulators made an early appearance in digital audio effects processors for
a short while. One of the shortcomings of the plain delta modulator is the limitation in the
rate at which it can track signals with high slew rates. As we have shown, each pulse is
worth one unit of charge to the integrator. To make the integrator climb faster the rate of
charge can be increased so that high slew rate signals can be tracked.
Oversampling techniques can be applied to both ADCs and DACs. The oversampling
ratio is usually chosen as a power of 2 in order to make computation more effi cient.
Figure 15.31 shows the idea behind the process for either direction of conversion. The 4
oversampling shown is achieved by adding samples with zero value at each new sample
point. At this stage of the process the spectrum of the signal will not have been altered
and thus there are still the aliases at multiples ofFs. The fi nal stage is to fi lter the new set
Zero
samplesZero
samplesZero
samplesZero
samplesInterpolated
samplesInterpolated
samplesInterpolated
samplesInterpolated
samples(a)
Figure 15.31(a) : The oversampling process adds zero valued dummy samples to the
straight sampled signal. If oversampling is being carried out in the DAC direction,
then the digital sequence of samples is treated as if these extra dummy samples had
been added in. The sequence is then fi ltered using an interpolation fi lter, which
creates useful values for the dummy samples.