This section describes miscellaneous digital audio effects which the author has seen applied in practice. For much more about signal processing for digital audio effects, see, e.g., [#!MUS424-04!#].
Doubling Simulation
Doubling is a studio recording technique often used to ``thicken'' vocals in which the same part is sung twice by the same person. In other words, doubling is a ``chorus of two'', where both parts are sung ``in unison'' by the same person. As an example, the Beatles used doubling very often, such as on the track ``Hard Day's Night''. A single variable delay line can simulate doubling very effectively.
The term slap back refers to the use of a single echo on a recorded track. The echo may be placed in a different spatial location in the stereo mix. Normally the echo delay is just large enough to be heard as a discrete echo on careful listening (e.g., on the order of tens of milliseconds). Slap back is very popular in 1950s-style recordings such as ``rockabilly'' tunes.
In summary, slap back can be regarded as a simplification of doubling in which the second voice is kept at a larger, fixed delay relative to the first voice.
