796 Chapter 27
we know that we want to be able to control the gain of each signal before it is mixed. So
multiplication must be needed too. So far, the performance of the digital signal processing
“ block ” is analogous with its analogue counterpart. The simplest form of digital audio
mixer is illustrated in Figure 27.24. In this case, two digital audio signals are each
multiplied by coeffi cients (k 1 and k 2 ) derived from the position of a pair of fader controls;
one fader assigned to either signal. Signals issuing from these multiplication stages are
subsequently added together in a summing stage. All audio mixers possess this essential
architecture, although it may be supplemented many times over.
But, in fact, the two functions of addition and multiplication, plus the ability to delay signals
easily within digital systems, allow us to perform all the functions required within a digital
sound mixer, even the equalization functions. That’s because equalization is a form of
signal fi ltering on successive audio samples, which is simply another form of mathematical
manipulation, even though it is not usually regarded as such in analogue circuitry.
27.8.3 Digital Filtering
The simplest form of analogue low-pass fi lter is shown in Figure 27.25. Its effect on a fast
rise-time signal wave front (an “ edge ” ) is also illustrated. Note that the resulting signal has
its “ edges ” slowed down in relation to the incoming signal. Its frequency response is also
illustrated, with its turnover frequency. Unfortunately, in digital circuits there are no such
things as capacitors or inductors, which may be used to change the frequency response of
a circuit. However, if you remember, we’ve come across situations before in sections as
diverse as microphones to fl anging, phasing, and chorus wherein a frequency response was
altered by the interaction of signals delayed with respect to one another. This principle is
ΣXXabk 1k 2(k 1 .ak 2 .b)Figure 27.24 : Simple digital audio mixer.