Time & Phase

O neM oreT ime

The subject of time and phase rears its ugly head time and again among audiophiles. The arguments for time and phase coherency are logical. Some manufacturers make a BFD out of it by basing their entire line of products on absolute phase and time alignment. They do it despite the logic’s relation to reality doesn’t exactly line up. They do it despite the fact that with first order crossovers the alignment only occurs at one point in space. They do this despite stressing the drivers by forcing them to work outside their optimal bandwidth. They do it despite increasing harmonic and intermodulation distortion. They do it despite using a port to extend the bass response, and in doing so, creating significant phase shift and group delay between the woofer and the port, which is not at all time and phase coherent. They do it despite the evidence that time and phase “incoherency” has been proven inaudible when listening to music. Yet the real reason for time alignment is that first order crossovers have a 90° phase shift between the high and low pass filters. This causes partial cancellation in the overlapping frequencies produced by both drivers. To fix this, the drivers must be positioned on a sloped or adjustable baffle to time align the center frequency of the crossover thus avoiding a dip in the frequency response.

But the proponents of time & phase coherency try to make more out of it, as if any phase or time shift will alter the sound we hear. Their arguments stress time alternation, suggesting that it “smears” the sound, “causes the loss of” directional imaging cues, without which we can’t “completely preserve the unique character of each sound.” Yet studies indicate we can’t hear any change from the time smearing, cue losing, character deteriorating time shift. I had thought the reason was simply that human ears cannot distinguish these exceptionally short time intervals. And that may be part of the story, but yesterday I ran across an article on harmony and waveforms. Something I read in it struck me. “Real instruments are close to periodic, but the frequencies of the overtones are slightly imperfect, so the shape of the wave changes slightly over time.” (emphasis mine) I already knew that musical instruments and human voices do not produce harmonics in perfect whole number ratios. What I hadn’t thought about was how aperiodic overtones do not perfectly line up with the fundamental. Each cycle is different because the overtones, not being exact multiples of the fundamental, shift slightly with each cycle. No wonder small time and phase shifts go unnoticed by the ear—it’s a natural part of music. Shifting the phase or time position of individual overtones does not change the character of the sound. Only the following matters; the shared frequencies through the crossover region must be in phase to sum properly. Frequencies outside the crossover can be shifted without consequence.

As long as the individual frequency and amplitude components of a waveform are correct, that is, no added harmonics and no frequencies played back louder or softer than they were produced by the original voice, it will sound exactly like the original voice.

Link to : [Harmony and Waveforms]  Sorry, this link is no longer functioning.

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