Hardware Reference
In-Depth Information
An audio signal, the sort that comes out of an MP3 player or from record decks, is a very
complex waveform, consisting of lots of very rapid changes. he speed of the rapid changes
carry the frequency content information of sound. he size of the waveform - that is, over
what range of voltage values they cover - is the amplitude or loudness information. However,
the amplitude is varying rapidly to convey the frequencies. What you need to do is to isolate
the loudness factor - that is, to measure just the size of the peak of the waveform, but it is
not quite as simple as that. With a loud sound you get a large positive value and a symmetri-
cally large negative one, so in order to get a measure of loudness you have to ignore the nega-
tive value and hold the positive value at its peak. Such a circuit is possible and is called, rather
unsurprisingly, a
peak detector.
Now the beat of music is normally carried in the low frequencies. here are electronic circuits
that will separate or ilter a mishmash of frequencies so that only a speciic range of frequen-
cies get through. he two simplest type are known as
high pass
and
low pass.
In a low-pass
ilter only the low frequencies can pass through it. Exactly how low is low depends on what is
known as the ilter's
break frequency.
his is deined as the frequency where the output is cut
down by half compared with the input. By correct choice of components you can make this
break frequency any value that you want. Most of the low frequencies in music are between
200Hz and 30Hz - any lower and you tend to feel it more than hear it. So to get at the beat
of the music you must ilter it with a low-pass ilter at a break frequency of 200Hz. he key to
ilters is the capacitor component, which acts a bit like a frequency dependent resistor. he
higher the frequency, the lower its resistance is to AC signals or its capacitive reactance.
he inal piece in the jigsaw is called a
comparator,
which compares two voltages and gives a
logic one output if one output is higher than the other or a logic zero if it is lower. By varying
one voltage with a control knob or potentiometer and feeding a varying signal into the other,
you can trigger a digital input when the varying voltage exceeds that set by the knob. If you
feed the output of a peak detector into this you can, by turning the knob, set the level that
will trigger the sequencer to advance to the next step.
Designing the Circuit
So to implement all that you need a couple of components called
operational ampliiers,
or
op
amps
for short. hese are very simple on the outside but quite complex on the inside. Basically
there are two inputs marked + and - with a single output. he way it works is that the output
is equal to the diference in voltage between the two inputs multiplied by a big number called
the
open loop gain,
which is typically 100,000. So you might think that if you put one volt into
the ampliier, you will get 100,000 volts out. Well, you would if you powered it with a 100,000
