Digital Signal Processing
On the previous
page you saw the first and also the last steps of the process. But what can
happen while the signal is in digital form? What effects and tricks can we
do with it?
Actually everything that happens in a digital
effects processor is the result of a few relatively simple operations. Essentially
the operations are addition, multiplication and delay. Everything that the
typical digital effects processor does is based upon these three operations
or combinations of them.
Addition of signals could be the combination
of a number of different audio channels into one or two channels which contain
all the information. For instance, if we record a Hammond organ on one track,
and then along with that, a singer on a different track, if we convert both
tracks to digital information, we can add them together and get a composite
track of the singer and the Hammond together.
Multiplication of the digital information results
in a change in the level, or gain to use the correct term. If the Hammond
was too loud in the above example so that when we combined the tracks by addition
it would drown out the singer, we could multiply the digital information representing
the Hammond track by a factor that is less than one, which would result in
a lowering of the volume or level of that track.
We could also accomplish the same thing by turning
down a volume control with a simple analog signal, but for the purpose of
this discussion which concerns digital signal processing, we'll work with
signals that have been digitized and are for the moment represented by digital
information.
We could also, if the singer was too quiet relative
to the Hammond, multiply the singer's digital track info by a number greater
than one which would then increase the volume of the singer's track.
Suppose that we have a track of a musical instrument
which does not normally have tremolo, such as a piano. If we digitize the
piano track, we can multiply it by a number which changes over time so that
part of the time the number is less than one, and part of the time it is greater
than one. Let's further state that we will do this 6 times per second. The
result will be a periodic and regular variation in the loudness or volume
of the track which is what tremolo is.
Thus we can add tremolo to a piano (or to any
other instrument that we can record and convert to a digital track.) If we
use a microphone and send the piano's signal directly into a digital signal
processor that is set up this way, and then take the output of the processor
and amplify it and send it to a speaker, we'll hear the piano right as we
are playing it and it will have tremolo added.
Now let's make this a little fancier. Let's
take the signal from the piano and then create two simultaneous digital signals,
and we'll do the same thing to both, that is, we'll have both multiplied by
a factor which varies slightly below and slightly over one, but, we'll make
the two channels opposite. That is, when one channel signal is getting multiplied
by a number greater than one, the other is being multiplied by a number less
than one. The result of this is a stereo tremolo where when the signal is
getting louder in one channel, it is getting quieter in the other. This makes
the audible sound from the speakers move back and forth in space which then
adds interesting phase changes and a doppler pitch change effect as well,
resulting in a much more musically interesting effect than just an ordinary
single channel tremolo.
Many years ago, Hammond used a simple volume
varying tremolo in their early instruments because they had not learned how
to make pitch vibrato, and they did it with a mechanical switching device
that sequentially added and then cut out resistors in the signal path. The
beauty of doing this by digital signal processing is that there are no mechanical
parts to wear out, and it is very easy to manipulate digital numbers or "words"
around and then after the manipulation is done, to convert the result back
to an analog signal.
Other digital effects are the result of
delay which is where things become a little more complicated.
