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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 phase value contains two components, frequency independent delay, and frequency dependent delay. It is only the second component, frequency dependent delay, which alters the shape of the audio waveform thus altering 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. This is not a new subject or discovery, but one which needs more of our audio industry's attention. RF, microwave, and acoustic phase measurements have routinely incorporated delay corrections, but conventional phase measurements of audio electronic devices have not corrected for frequency independent delay which we propose must be done to make phase specifications meaningful.

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AES - Audio Engineering Society