Electronic Music Techniques Used to Enhance Introductory Circuit Analysis

To provide not only an interesting challenge but also experience in teamwork and communication skills, honors students in my introductory circuit analysis course are assigned a project involving electronic music synthesis devices. The students are teamed up into pairs, with each pair having responsibility for one of several modules which together compose a working voltage-controlled sound synthesis system. In addition to their individual tasks, each pair of students must collaborate throughout the semester with the others to resolve system integration issues. A typical set of modules built by the students includes voltage-controlled amplifiers, power supplies, envelope (or transient) generators, and simple low frequency oscillators. Basic circuit designs are provided which the students must analyze, construct, integrate into a functional synthesizer, and explain to the class during their final oral report and demonstration. Taken together, these modules demonstrate practical applications of most of the major concepts and components covered in the standard introductory circuits course. They also introduce several more advanced ideas and devices. An audio amplifier and speakers as well as the more complex functions (voltage-controlled oscillators and filters) necessary for a fully functional system are usually provided for the project due to the complexity of these circuits for students at this level. INTRODUCTION When I was assigned to teach the introductory circuit analysis course (ECE 202), I was informed that I would have to provide an “honors component” for those students taking it as an honors course, ECE H202. (There is insufficient demand to offer a stand-alone honors course, so ECE H202 is taught as an “add-on” course, with the honors students attending a standard section.) I decided to adapt the work I was involved with during the late 1970’s in electronic music synthesis as a project for ECE H202. HISTORY In the mid-1960’s, Robert Moog began marketing the first commercially successful voltage-controlled electronic music synthesis equipment. Following the release of Walter Carlos’ ground-breaking album Switched On Bach, sales of Moog’s synthesizers soared. Several competing companies jumped on the bandwagon, and by the mid-seventies, most popular music groups used such equipment in addition to many avant garde and classically oriented musicians. With the personal computer revolution of the eighties, sound synthesis moved inexorably toward the digital domain. As microprocessors and memory became faster and cheaper, real-time digital synthesis became feasible. By 1990, analog voltage-controlled music synthesis was essentially obsolete. With its dozens (or hundreds) of potentiometers and patch-cords for “programming” the sounds, the voltage-controlled devices simply could not compete in terms of cost and ease of use. THE BASICS Voltage-controlled music synthesis equipment is by its very nature modular. The general idea is that every parameter of all sound generation and processing devices should be controllable by a voltage, and that the output of every device should be a voltage, thus (theoretically) any module can control the operation of any other module. In reality, of course, there are some types of interconnections which are never used. The interconnections traditionally were accomplished by means of patch cords (the usual type used 1/4” phone plugs). As an example, if vibrato (a slight periodic (a few hertz) variation of the frequency of a sound) is desired, the sinusoidal output of a low-frequency oscillator (LFO) would be connected to the frequency control input of a voltage-controlled oscillator (VCO). The output of the VCO would then vary in frequency sinusoidally at the rate set by the LFO. Although there are many of types of modules that can be constructed to generate and modify sounds (or rather audio frequency electrical signals) there are only a few which are used to implement most sounds. Out of this group, some are beyond the scope of an introductory circuits course, so the problem was to determine a subset of modules which would be sufficient to illustrate the basic principles of voltage controlled music synthesis without going into too much depth with more advanced concepts such as electronics or control theory. A connection of modules for a very simple sound is shown in Figure 1. The VCO generates an audio waveform (sine, pulse, sawtooth, etc.) with the frequency being determined by a voltage from a user interface device, usually a standard organ keyboard. The output of the VCO goes to the signal input of a voltage-controlled filter (VCF) which modifies the harmonic content (the timbre) of the sound. Note that the cutoff frequency of the VCF is also controlled by the keyboard to maintain a constant timbre. The output of the VCF goes to the signal input of a VCA and from there to an audio amplifier and speakers. To create individual notes, the keyboard must in some way control the VCA, turning it on when a key on the keyboard is depressed, and setting the gain to zero when the key is released. If the VCA is turned on or off suddenly, a distinct pop will be heard (this follows from basic Fourier analysis), so somehow the gain of the VCA must be increased and decreased gradually (over a few milliseconds or more). The device usually used for this purpose is called either a transient generator or an envelope generator. Although most envelope generators are a bit more complex, in the simplest case it will accept a Gate signal (a binary signal indicating whether or not a key is depressed). When the gate goes high (key depressed) the envelope generator will gradually increase its output voltage from zero to some maximum (typically 5V) at a rate set by the user. The output remains high until the gate goes low (key released), at which point the output of the envelope generator gradually decreases back to zero, also at a rate set by the user and usually different from the rate of increase. Connecting the output of the envelope generator to the control input of the VCA then allows the amplitude of the sound to increase from zero up to a maximum without the pop, then die away gradually when the key is released.