Preemphasis

Preemphasis is a signal processing technique used to boost higher frequency components of a signal before further processing or transmission. The aim is to compensate for the natural tendency of communication channels and recording systems to attenuate high-frequency content, and to improve the performance of subsequent processing steps such as feature extraction or decoding by whitening the spectrum and improving signal-to-noise ratio.

Implementation is typically a first-order high-pass filter in digital systems. The common form is y[n] = x[n] - a x[n-1], where a is the preemphasis coefficient, usually in the range of 0.95 to 0.99. In analog form, simple RC high-pass filters can be used with a chosen time constant to approximate the same effect. De-emphasis circuits at the receiver apply the inverse filter to restore the original spectrum.

Applications include telephony and FM broadcasting, where preemphasis is applied before transmission to improve listening quality and noise performance. In speech and audio processing, preemphasis is used as a preprocessing step for spectral analysis and feature extraction (for example, in MFCC computation). In broadcast and recording workflows, standardized de-emphasis curves with fixed constants (such as 50 microseconds or 75 microseconds) are employed at the receiver to recover a flat frequency response.

Considerations involve a trade-off between boosting high-frequency noise and preserving important signal content. The choice of coefficient or time constant depends on the channel characteristics and downstream processing goals. Appropriate de-emphasis is typically applied at the receiver to maintain overall, unbiased frequency response.