Large Delay Changes

When implementing large delay length changes (by many samples), a useful implementation is to cross-fade from the initial delay line configuration to the new configuration:

• Computational requirements are doubled during the cross-fade.
• The cross-fade should occur over a time interval long enough to yield a smooth result.
• The new delay interpolation filter, if any, may be initialized in advance of the cross-fade, for maximum smoothness. Thus, if the transient response of the interpolation filter is samples, the new delay-line + interpolation filter can be ``warmed up'' (executed) for time steps before beginning the cross-fade. If the cross-fade time is long compared with the interpolation filter duration, ``pre-warming'' is not necessary.
• This is not a true ``morph'' from one delay length to another since we do not pass through the intermediate delay lengths. However, it avoids a potentially undesirable Doppler effect.
• A single delay line can be shared such that the cross-fade occurs from one read-pointer (plus associated filtering) to another.

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written by Julius Orion Smith III
Julius Smith's background is in electrical engineering (BS Rice 1975, PhD Stanford 1983). He is presently Professor of Music and Associate Professor (by courtesy) of Electrical Engineering at Stanford's Center for Computer Research in Music and Acoustics (CCRMA), teaching courses and pursuing research related to signal processing applied to music and audio systems. See http://ccrma.stanford.edu/~jos/ for details.