Hardware Reference
In-Depth Information
of the music you must filter it with a low-pass filter at a break frequency of 200Hz. The key to
filters is the capacitor component, which acts a bit like a frequency dependent resistor. The
higher the frequency, the lower its resistance is to AC signals or its capacitive reactance.
The final 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 amplifiers, or op
amps for short. These are very simple on the outside but quite complex on the inside. Basically
there are two inputs marked + and - with a single output. The way it works is that the output
is equal to the difference 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 amplifier you will get 100,000 volts out. Well you would if you powered it with a 100,000
volt power supply and you could find an op amp that would work at that level. What happens
in practice is that the output will only go as high as the power supply. Also the open loop gain
is too high to be useful most of the time and so when you design a circuit that you want to
use as an amplifier some negative feedback is applied, as shown in Figure 3-6. Don't confuse
this with positive feedback, sometimes known just as feedback or howl around, when an
amplifier's output is fed into an input, like a microphone picking up the amplified sound.
Figure 3-6:
A non-inverting
amplifier.
Negative feedback means feeding a proportion of the output back into the - (negative) input
so that the input in effect gets turned down. Consider the circuit in Figure 3-6 and assume
there is zero volts on Vin, and also zero volts on the output. Also imagine that the two
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