Working with digital logic may seem counter intuitive to the idea of sound synthesis. Audio is an analog phenomenon and as such we typically steer towards analog electronics to produce or manipulate it. That being said digital electronics can be incredibly valuable to us as synth enthusiasts. We can use them to breath new life and depth into humble square wave oscillators, control analog devices and interface with micro-controllers/processors. For this reason I wanted to provide an introduction to some of the more common digital logic devices you may encounter.
Digital Vs. Analog
Before going too far I’d like to reiterate the difference between analog and digital signals. An analog signal is a signal which varies across a range of voltages. Think of a sound wave, the wave is a continuous signal which rises and falls based on the intensity of the sound being captured.
A digital signal on the other hand varies between two discreet states. The signal is either high or low (0 or 1). Electronically speaking these two states are typically (though not always) 0V for low and 5V for high. Any values between these two states will either be assumed to be one of the two or ignored depending on the components used. The square waves we have produced using 555 or 40106 oscillators could be considered as digital signals as they oscillate between two distinct values.
Truth Tables
Truth Tables are a tool we use to clarify the function of digital logic. The tables show a listing of all possible inputs to the system and the output which they would provide. In the inverter example above (and most of the components I will discuss today) these tables are quite straight forward however as you get into progressively more complicated digital circuits these truth tables will also become more complex.
Building Blocks
Once you have a digital signal whether its from a humble square wave oscillator or something more complex like a microprocessor, there are a number of operations you can carry out on it using digital logic chips. There are hundreds of different chips available but most of them fall into a few basic categories.
NOT Gate:
NOT Gates (commonly called inverters) are probably the most basic digital component you will encounter. As the name suggests they invert whatever signal you send to them. This means if you send in a 1, you will receive a 0 from the output. Alternately if you send in a 0 you will receive a 1. The 40106 I have used in previous projects is a special type of inverter which uses threshold voltages to interpret analog inputs.
AND Gate:
An AND Gate is a digital logic gate which takes two inputs (notice there are now two input columns on the truth table). The output of the gate is only high if both these inputs are high.
NAND Gate:
The NAND gate is actually a combination of the AND gate and the Inverter. Notice the circuit symbol is that of the AND Gate with the small circle from the inverter added. This small circle is used in circuit symbols to indicate that an output is inverted. This means the output of the NAND gate is opposite the output of the AND gate (as seen in the truth table). A NAND gate will output high at all times except when both inputs are high.
OR Gate:
The OR Gate is a gate which will output high when either (or both) of the inputs are high. This can be very useful for combining signals or triggers from multiple sources. There are also a few modified versions of the OR gate which I will look at next.
NOR Gate:
Similar to the NAND Gate this is an OR Gate with an added inverter. This means the NOR Gate will output high when no inputs are sensed. It will go low when Either (Or both) Inputs go high.
XOR Gate:
The XOR Gate is another special type of OR Gate. This time the output will go high when either input is high however if both inputs go high the output will go low.
This is by no means an exhaustive list but should give us a starting place to work from. We can also create progressively more complex gates by combining these different building blocks together. I hope this has given you some ideas about what you can accomplish using digital logic. Keep an eye out for more projects using these gates coming soon!