Ask the Doctors: The SPL Transient Designer
by Dave Berners

Q: How does the SPL Transient Designer relate to the “sound enhancement” processors described in the May, 2007 "Ask the Doctors?"

SPL Transient Designer

The SPL Transient Designer falls into the category of transient shapers--in fact the Transient Designer invented this category--but is different from many transient modification tools. The SPL circuit uses several different kinds of signal envelope detectors working on a decibel-encoded signal. The decibel encoding allows the processor to work effectively without the need for a threshold control. For example, if a signal has a transient that reaches above the base level of the signal, the transient will be processed without regard to the overall level of the signal. Transient processing is instead determined by the relative levels of the transients as compared to the steady-state amplitude of the signal.

The Transient Designer has two controls, "Attack" and "Sustain," which allow for two independent dimensions of transient control. The attack control modifies the portion of the signal envelope during, and immediately following, a transient; the sustain control alters the evolution of the entire signal between transients. The controls are additive/subtractive, and allow the dynamic range of a signal to be increased or decreased. Change in dynamic range is accomplished via a time-varying gain, which is computed on the basis of the detector outputs. Both the attack and sustain controls are designed with psychoacoustics in mind; the response times of the envelope detectors correspond to human perception. Because of this, there is no need for "time" controls for the attack and sustain behavior.

The Transient Designer can be thought of as philosophical relative of the LA-2A.

The attack circuit uses a composite of two envelope detectors in connection with an ingenious feedback mechanism to produce an “attack wavelet.” This wavelet has a duration short enough to be perceived as a transient, but long enough to preserve the spectrum of the original signal. The snappiness of enhanced transients produced by the SPL device thus relates to the spectrum of the signal content, so that transients retain their original character, but are amplified or decreased by application of the effect.

The sustain circuit comprises a high-order filter and a nonlinear feedback path to produce an estimate of a signal’s decay. The nonlinearities in this circuit, together with the filter characteristics, produce an envelope estimator that is responsive enough to track quickly decaying signals, but highly immune to modulation distortion produced by low-frequency signals. Distortion immunity is further aided by pre-processing the signal on the way in to the detector. Range of the sustain control is calibrated so that short-term decay in the signal can be virtually nullified by the effect, producing almost arbitrary lengthening of individual envelopes following onsets of transients.

The Transient Designer can be thought of as philosophical relative of the LA-2A: control complexity is minimized, and circuit parameters are chosen in such a way that a pleasing effect is obtained for a wide variety of source materials. The natural quality of the effect is deceptive in the face of the drastic amount of signal processing that can be performed with this device.