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Circuit and Method for Correcting Distortion in a Digital Audio System (Anti-Aliasing Filter)

US Patent: 4,764,938
Date Filed: October 25, 1982
Date Issued: August 16, 1988
Inventors: John D. Meyer
This patent is an offshoot of research John Meyer had undertaken to test the audibility of non-linear phase distortion. When he became aware of high-frequency phase anomalies produced by high-order anti-aliasing filters in early digital recorders, he was concerned that such phase distortion could nullify the benefits of the advanced phase correction circuits then under development for the HD-1 studio monitor. Meyer saw the digital recorder problem as an opportunity to apply simplified variants of those phase alignment techniques.

The circuit in this patent compensates for the effects of the recorder's anti-aliasing filter by introducing varying amounts of time delay in different frequency ranges to obtain constant group delay, that is, a time delay that remains relatively constant with frequency across the recorder's operating bandwidth.

In 1982, Meyer Sound introduced the MS-8201 dual time correction filter module, designed to be placed before the input of early digital recorders such as the Sony PCM-F1. Several hundred were sold, and response was enthusiastic from users who reported substantive improvement in overall sonic characteristics.

The patent application was originally filed in October of 1982, and by the time it was finally issued, nearly six years later, new filter techniques utilizing oversampling had greatly improved the phase response of digital recorders. Although some of these circuits apparently incorporated elements covered by the patent, Meyer Sound opted not to file for infringement. as this specific application of the technology was outside the mainstream of the company's product development.