Ask the Doctors! Drs. David P. Berners and Jonathan S. Abel Answer Your Signal Processing Questions.
"Look-ahead allows the limiter to do level detection on a signal which is advanced relative to the audio path."
For dynamic range control applications, signal detection should result in an estimate of the incoming signal's envelope. The detector should be relatively insensitive to modulation at audio rates, but should react quickly enough to accurately detect the short-term dynamics of the signal. Generically speaking, the detector should behave as a lowpass filter operating on the absolute value or square of the incoming signal. For proper signal tracking, the filter should be normalized to have a DC response of unity. Traditionally, the lowpass filter used for detection is a first- or second- order IIR filter with different time constants for upwards-tracking (attack) or downwards-tracking (release) behavior. The historical reason for implementation as an IIR filter is that many detection algorithms are motivated by analog circuit designs. For peak limiting with look-ahead, it is necessary to use an FIR filter for tracking signal level if the detected output is to reach the signal level by the time the look-ahead period expires. Lowpass FIR filters can be designed using the window method, and normalized for unity gain at DC. The goal of the tracking filter is to have as
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short a length as possible (to minimize necessary look-ahead time), while having as small a bandwidth as possible to prevent excessive modulation on the audio path. In digital-domain applications, it is especially important to minimize modulation, because of aliasing concerns. Because of the many applications, which demand time-limited, low-bandwidth windows, there is a vast array of window designs appropriate for detection in this situation. Choice of a reasonable window results in the ability to have true limiting of peaks while preventing the large amounts of distortion that can accompany fast changes in gain.