Tags
amplifier, Coursera, envelope, filter, LFO, music, oscillator, synthesizer
This blog post is written for assignment #6 for the Introduction to Music Production course at Coursera.org.
In this post I will describe the function and use of the five key synthesizer modules: the oscillator, the filter, the amplifier, the envelope, and the LFO.
We begin with the oscillator, since it it the module that generates the signal that will, when processed, output synthesized sound. The oscillator converts direct current (DC) electrical energy into alternating current (AC). Oscillators meant for sound synthesis produce AC current with a frequency within the range of human hearing (20 Hz- 20,000kHz). Software oscillators found in digital synthesizer programs can produce signals with various waveforms: sine wave, square waves, triangle waves, or sawtooth waves, depending on which partial tones are produced along with the fundamental frequency. The fundamental tone is usually set by a MIDI program or by an external device such as a MIDI keyboard.
After the signal is generated by the oscillator, it passes into the filter. Filters modify the signal, which allows the timbre of the generated signal to be adjusted. For example, sawtooth waves have a very bright, harsh sound that can be unpleasant. This is because sawtooth waves have every partial frequency included in the signal, including many very high-frequency harmonics. The sound can be made much more pleasant by applying a low pass filter, which cuts off harmonics above a set frequency threshold, modifying the timbre of the resulting sound. Other filters, such as high-pass filters (where frequencies above a specified threshold) and band-pass filters (where frequencies near a specified threshold) may be used.
After modification by filters, the signal is modified by the amplifier, which is used to modify the amplitude of the signal. This may be done, for example, as a pre-amp to boost the signal to an appropriate level.
After modification by the amplifier, the envelope is applied to the signal. The envelope is a set of parameters that control how the signal plays. Specifically, the four parameters are attack, decay, sustain, and release (ADSR). Attack indicates the time required for the signal to reach its maximum once the signal is applied (such as when a key is pressed). This, a higher attack value indicates a slower onset of the sound. Decay is the rate at which the sound’s amplitude declines from its peak. Sustain is the amplitude where the signal stops decaying and maintains itself (while the key is pressed). Release is the time required for the sound to decline from the sustain amplitude to zero. Modification of these parameters is particularly useful when using a synthesizer to model the sound of a real instrument. For example, a piano would have a short attack and delay, a fairly high sustain, and a quick release.
The final module is the low-frequency oscillator, or LFO. The LFO is an oscillator, which we saw above generates a signal wave. The LFO, however, generates a waveform with a low frequency, usually (but not always) below the threshold of human hearing. The limit of human hearing is at about 20 Hz. LFO’s usually generate waves between 0 Hz-100 Hz, but generally between 0-20 Hz. The purpose of the LFO is not to generate audible sound, but to create a signal that modifies the carrier signal we’ve put through the other four modules. The waveform generated by the LFO adds to the carrier signal, by cancellation of waves that are out of phase and by addition of those waves that are in phase. The LFO can be applied to the amplifier, which results in LFO amplitude modulation. When applied to the oscillator, the LFO creates frequency modulation. (As an aside, these are the principles behind how AM and FM radio work. Radio signals are waveforms with frequencies above the threshold for human hearing-100 kHz-100GHz). Amplitude modulation creates a tremolo effect- a rapid oscillation of amplitude. Frequency modulation creates vibrato- minute oscillations in pitch, similar to the sound I make when I shake my pressed-down finger when playing a note on my double bass or guitar. LFO’s can also be applied to the cutoff frequencies of filters or even applied to other LFO’s for increasingly complex, interesting sounds.
These five modules are the fundamental tools applied in the form of synthesis described above, known as subtractive synthesis. Subtractive synthesis is synthesis done by creating a signal, then modifying it through the above modules. Other forms of synthesis are done, but are beyond the scope of this post.
Thank you for taking the time to read this review. I hope you’ve enjoyed this brief tour.
Radical Totality 