Previous research has proven that phase distortion in audio signals is audible. The real question then is not the existence, but the significance of the audibility. A psychoacoustic experiment is proposed to ascertain permissible levels of phase distortion in audio signals. This thesis research is not meant to be exhaustive by any means, but to ascertain some permissible levels based on careful experimental design and analysis. The Kwalwasser-Dykema Music Tests format will be used in the implementation of the thesis research with other considerations. These permissible levels may be beneficial in the design and application of audio equipment, especially in the area of transducer and loudspeaker system engineering.
Chapter 1 introduces the mathematical foundation of this thesis research. Phase distortion and group delay is defined in mathematical terms. Frequency- and time-domain characteristics of a minimum-phase second-order low-pass and first-order all-pass systems are investigated. Finally, an example of phase-intercept distortion is presented.
Chapter 2 introduces the audibility of phase distortion in audio signals. The physiology of the human ear is examined. The time-based processes of sound perception, such as timbral sensation in the human auditory process, are discussed. The phase-locking property of the auditory process is presented. Finally, previous research investigating the audibility of phase distortion is presented.
Chapter 3 will introduce the all-pass filter and audio test signals used in the listening test. The steps in formulating the all-pass filter are outlined. This is followed up by a formulation of a tunable second-order all-pass filter. The bilinear transform is introduced to perform the analog to digital transformation for the all-pass filter. Finally, test signals used in the listening test are introduced and discussed.
Chapter 4 introduces the listening test formulation process. A study is shown which compares the validity of various psychoacoustic listening test implementations. Test equipment used in this thesis research is presented. Finally, the listening test implementation used in the thesis research is outlined.
Chapter 5 presents the results and discusses them. The statistical analysis method of equalizing type 1 and 2 error is presented and implemented. Average correct responses of all test subjects for both the headphone and loudspeaker listening tests are presented. Finally, the results are discussed and implications are brought forward.
Chapter 6 finalizes this thesis research by presentation of the conclusion.