Audio Engineering

Recording Consoles 797

In Out

R

C

Time Time

Freq Freq

Amplitude Amplitude

Figure 27.25 : RC low-pass fi lter.

the basis behind all digital fi ltering and may be extended to include several stages of delay
as shown in Figure 27.26. By utilizing a combination of adder and variable multiplication
factors (between the addition function and the signal taps) it is possible to achieve a very
fl exible method of signal fi ltering in which the shape of the fi lter curve may be varied
over a very wide range of shapes and characteristics. While such a technique is possible in
analogue circuitry, note that the “ circuit ” (shown in Figure 27.26 ) is actually not a real circuit
at all, but a notional block diagram. It is in the realm of digital signal processing that such
a fi ltering technique really comes into its own: the DSP programmer has only to translate
these processes into microprocessor type code to be run on a microcontroller IC, which is
specifi cally designed for audio applications—a so-called DSP IC. Herein lies the greatest
benefi t of digital signal processing—that by simply reprogramming the coeffi cients in the
multiplier stages, a completely different fi lter may be obtained. Not only that, but if this is
done in real time too, the fi lter can be made adaptive, adjusting to demands of the particular
moment in a manner that might be useful for signal compression or noise reduction.