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High Frequency Phase Response Specifications-Useful or Misleading?

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Conventional high frequency phase response specifications of audio electronics, although easily acquired with standard available instruments, are misleading. The absolute (total) phase angle contains two compoonents, frequency independent delay and frequency dependent delay (1). It is only the second component, frequency dependent delay, which alters the shape of the audio waveform which may in turn alter the characteristic sound of the processed signal.: To be meaningful, phase data must be presented as two separate values or graphs, deviation from linear phase and group delay. The first is a modified phase graph, where the frequency independent delay is subtracted out, leaving only that part of the phase transfer function which alters the shape of the audio waveform. The group delay graph shows delay vs. frequency for the spectral components of a complex waveform.

JAES Volume 36 Issue 12 pp. 968-976; December 1988
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AES - Audio Engineering Society