right|thumb|An example of the frequency magnitude of a room response before and after digital room correction
Digital room correction (or DRC) is a process in the field of acoustics where digital filters designed to ameliorate unfavorable effects of a room's acoustics are applied to the input of a sound reproduction system. Modern room correction systems produce substantial improvements in the time domain and frequency domain response of the sound reproduction system.
History
right|thumb|Digital room correction may involve [[minimum phase algorithms, to maintain wavefront coherence over the intended frequency range.]]
The use of analog filters, such as equalizers, to normalize the frequency response of a playback system has a long history; however, analog filters are very limited in their ability to correct the distortion found in many rooms. Although digital implementations of the equalizers have been available for some time, digital room correction is usually used to refer to the construction of filters which attempt to invert the impulse response of the room and playback system, at least in part. Digital correction systems are able to use acausal filters, and are able to operate with optimal time resolution, optimal frequency resolution, or any desired compromise along the Gabor limit. Digital room correction is a fairly new area of study which has only recently been made possible by the computational power of modern CPUs and DSPs.
Operation
The configuration of a digital room correction system begins with measuring the impulse response of the room at a reference listening position, and sometimes at additional locations for each of the loudspeakers. Then, computer software is used to compute a FIR filter, which reverses the effects of the room and linear distortion in the loudspeakers. In low performance conditions, a few IIR peaking filters are used instead of FIR filters, which require convolution, a relatively computation-heavy operation. Finally, the calculated filter is loaded into a computer or other room correction device which applies the filter in real time. Because most room correction filters are acausal, there is some delay. Most DRC systems allow the operator to control the added delay through configurable parameters.
Implementation
The most widely used test signal is a swept sine wave, also called chirp. This signal maximizes the measurement's signal-to-noise ratio, and the spectrum can be calculated by deconvolution, which is dividing the response's Fourier transform with the signal's Fourier transform. The spectrum is then smoothed, and a filter set is calculated, which equalizes the sound pressure levels at each frequency to the target curve. To calculate the delays and other time-domain corrections, an inverse Fourier transform is performed on the spectrum, which results in the impulse response. The impulse peak's distance from the start of the signal is its delay, and its sign is its polarity. The delay is corrected by subtracting each channel's delay from the system's peak delay, and applying this result as additional delay for the channel. This correction is sometimes provided to the user as distance from the speaker, which is calculated by multiplying the delay with the speed of sound. Inverse polarity (most likely caused by switching a speaker's + and - wires) could be fixed by multiplying each sample with -1 or swapping the speaker wire ends on one side of the cable, but this result is usually shown as a warning, as some speakers (e.g. Focal Kanta) do this intentionally.