1. Field of the Invention
The invention relates to audio peak limiters and other audio control systems, more particularly to the cancellation of lower sideband modulation distortion in such systems.
2. Prior Art
Fast audio peak limiters include simple peak clippers, radio frequency clippers, Hilbert transform limiters as disclosed in U.S. Pat. No. 4,495,643, delay-line limiters, and others. Some of these operate directly on an input audio signal such as the simple peak clipper, which replaces any positive or negative instantaneous level exceeding a preset positive or negative threshold with the corresponding threshold value. Other peak limiters operate by multiplying a control signal by the input signal. All systems that operate directly on the signal have equivalent systems (producing the same outputs) that multiply the input signal by a control signal. U.S. Pat. No. 5,168,526 discusses how such an equivalent system can be synthesized.
Any system in which the input signal is multiplied by a control signal can be considered an amplitude modulator, where there are multiple "carriers" corresponding to each Fourier frequency component of the input signal and the "modulating signal" is the control signal. It is well known that amplitude modulators produce a pair of symmetrical, mirror-image sidebands on each side of the carrier. The frequencies in the upper sideband are the sum of the carrier and each Fourier frequency component in the modulating signal. The frequencies in the lower sideband are the difference between the carrier and each Fourier frequency component in the modulating signal. In the case of the peak limiting systems, the modulating process adds to each Fourier frequency component of the input signal a pair of sidebands corresponding to the frequency components in the control signal. This is true of all prior art processes enumerated above.
Psychoacoustic research has established that strong frequency components in an audio signal have the ability to "mask" weaker signals so they cannot be heard in the presence of the strong signals, although they would otherwise be audible in the absence of a strong signal. As the weak frequency becomes more distant in frequency from the strong signal, the strong signal must become stronger and stronger to mask the weak signal.
This effect is asymmetrical with frequency. Strong signals are more able to mask weak signals "x" octaves above a strong signal than to mask weak signals "x" octaves below a strong signal. Therefore, in any of the prior art peak limiting systems discussed, the lower sideband (representing distortion induced by the peak limiting process) is much more likely to be audible than is the upper sideband because the undistorted input signal is less able to psychoacoustically mask the lower sideband. This is particularly true because the lower sideband is arithmetically symmetrical to the upper sideband, occupying many more octaves than does the upper sideband.