Matti Otala

Conversion of amplitude nonlinearities to phase nonlinearities in feedback audio amplifiers

It is shown that by the application of feedback, an arbitrary open-loop amplitude nonlinearity of an audio amplifier is converted into a corresponding closed-loop phase nonlinearity. This effect manifests itself by the amplitude of the audio signal phase-modulating the high-frequency components of the signal.

Audibility of transient intermodulation distortion

The threshold of audibility of TIM was determined for 68 subjects representing all categories of listeners from musicians and sound engineers to the average man on the street. Three different music samples were used, and controlled amounts of distortion were produced with a digital stereophonic TIM generator. Two basic experimental methods were used to obtain the approximate threshold, after which the reliability of the detection of the distorted passages was verified with a time localization test. The results show that the audibility varies very much depending on the music sample, listening media and person. The most sensitive group of listeners could reliably perceive 0.5 % of momentary TIM. Low values of TIM were generally perceived only as slight changes in the tonal character of the sound, and not as distortion. In a number of cases, a preference was found for the slightly distorted sound.