Signal Processing 102: An Introduction to Time and Modulation Based Effects
Time Based Effects
Also refered to as parallel effects due to the way in wich the processed signal is mixed together with the original signal, time based effects extend the duration of audio signals relative to the original signals. The two main time based effects are reverberation, and delay.
Reverb is simply a simulation of a natural reverberation of a room. The ambient sound waves that bounce around a room based on the rooms dimensions are reproduces by digital signal processing (DSP) either modeled from a convolution algorithm or a sampled impulse response. Prior to DSP recording studios had to use an actual room to record reverbs by reproducing an audio signal such as a vocal with a loudspeaker and recording it again with a microphone in the desired environment. These resulting track was then mixed back in the original signal. Reverbs are categorized by the way in which they are made. Smaller sounding reverbs are considered room or chamber reverbs while long echoing reverbs are refereed to as halls. Spring and plate reverbs refer to the manner in which the reverb is artificially created.
Delays are echo effects that are mixed back in with the original signal to give space and dimension. Delay parameters include: time, (the actually delay time from the orignal signal to the affected signal), feedback, (the amount of affected signal that is reintroduced into the delay circuit). The individually instances of delay resulting from feedback are refered to as taps.
Modulation effects are composed of audio signals that are split into two or more signals, then mixed back together creating an effect based on the relationship between the two or more identical audio signals. The effects are created by phase anomalies resulting from the varying the time and amplitude relationships between the multiple signals. they can be across the entire frequency spectrum or select bands of frequencies. Modulation effects include: flanger, chorus, and phasers. Modulation effects are some of the more spectacular and dramatic processing effects that can be done to an audio signal but they can be reproduced with very simple circuitry and even simpler DSP (digital signal processing). Flanger effects are a very common and popular effect for guitars and basses. The term flanger comes from the original way in which the effect was created. By syncing together two reel to reel tape machines and recording the same signal to both, a flanger effect can be produces by playing back the tape machines in sync and dragging your finger with varying pressure across the flange of one of the tape machines. The resulting varying speed producing a phase anomaly that sounds like a jet fly by.
Imaging is an element of audio signals that are in stereo and is simply where in the stereo image the audio signal is placed and how it behaves. For example a tremolo is a popular guitar and organ effect that raises and lowers the amplitude of an audio signal to a set rate and depth. If you were to incorporate an imaging effect such as autopan, the signal will seem to dance back and forth, left to right in the stereo image while seeming to appear and disappear.