Why does eq cause phase shift

A linear phase EQ is a type of equalization that does not alter the phase relationship of the source. Achieving linear phase is not possible with analog circuits and has been made possible with computer coding.

What is audio equalization? EQ is the process of adjusting the balance between frequencies within an audio signal. This process increases or decreases the relative amplitudes of some frequency bands compared to other bands with filters, boosts and cuts. Using EQ, we can effectively alter the frequency content of a signal to best suit the mix. EQ is used in mixing, tone shaping, crossovers, feedback control and many more applications. It is easily one of the most important audio processes there is.

Equalization largely relies on filters to adjust boost, cut or completely eliminate frequencies within the audio signal. Typical audio filters in analog and digital systems will introduce some amount of phase shift to the signal.

With typical minimum phase EQ plugins, the latency or delay of the frequencies affected by EQ filters causes phase shift. Many EQ plugins are designed to emulate specific analog minimum phase EQs, which would require phase shift to be part of the programming if the emulation was to be precise. This can be visualized in the following graphs:. The cutoff frequencies will mark the halfway point of the phase shift. Below is a basic second-order band-pass filter essentially a first-order high-pass filter combined with a first-order low-pass filter.

Here, we see two reactive electrical components: the capacitor for the high-pass portion C HP and the capacitor for the low-pass portion C LP :. Remember that, by increasing the Q, we would narrow the bandwidth and steepen the roll-offs. If we increased the gain boosting or cutting , we would increase the phase shift.

Here are examples of amplitude-frequency and phase-frequency graphs for first-order low shelf boost and low shelf cut filters.

The phase shift is at a maximum negative at the centre frequency of the low shelf boost and a maximum positive at the centre frequency of the low shelf cut. The phase shift is at a maximum negative at the centre frequency of the high shelf boost and a maximum positive at the centre frequency of the high shelf cut. Before we move on, I should note that, in some cases, phase-shifting may produce sonically pleasing effects on the audio.

However, in many other cases, it has the opposite effect. Linear phase equalization is EQ that does not cause any phase shift in the signal it processes.

If we drag it far enough, the peak of one track would align with the trough of another. When we play the track now, the signals, being out of time with one another, will cancel one another out or nullify each other — this is called destructive interference. Any equalizer that is zero-latency will inevitably alter the phase of the signal. To understand why this is, we need to delve into something called the Fast Fourier Transform.

Like digital audio in general, the overall signal is broken up into samples. Using the Nyquist Frequency Theorem we know that this means we can have frequencies. If our session has a Because only frequencies of this Hz range can be represented, we end up with about 1 band every 10Hz. Each of these bands is referred to as a minimum-phase filter since they each very subtly alter the timing and in turn the phase of the signal. The greater we amplify this band, the larger this phase discrepancy becomes.

Notice that the only point that is in phase, is the center of the band — but the frequencies around this center are pushed out of phase. This becomes even worse when we start to affect the low-frequency range, especially when we create a high-pass filter.

It's the comb filtering - a series of many peaks and dips in the frequency response - that drastically changes the sound, not the audibility of the phase shift used to create that effect. The "problems" caused by phase shift have been repeated so many times by magazine writers and audio salespeople that it's now commonly accepted, even though there's not a shred of truth to it.

Some people claim they can hear phase shift in equalizers because when they boost the treble they hear a "phasey" sound. So they wrongly assume what they hear is the damaging phase shift everyone talks about. In truth, what they are hearing is comb filtering that was already present, but subdued. When a microphone is near a room boundary like a wall or ceiling, or when placed near the open lid of a grand piano, the delay between the direct and reflected sound creates a comb filter acoustically in the air.

When the treble is boosted by EQ the comb filtering becomes more apparent. But the EQ did not add the phasey sound, it merely brought it out. The same thing can happen when mixing tracks recorded with multiple microphones in a room.

For example, a mike near the snare drum picks up the snare sound as well as sound from the nearby kick drum. So when the snare and kick mikes are mixed it's possible for the low end to be reduced - or boosted - because of acoustic interference caused by the arrival time difference between mikes.

So while phase shift is indeed the cause of the response change, it's the response change that you hear, not the phase shift itself. For even more compelling proof that phase shift alone is inaudible, see this gem I recently discovered:. All rights reserved. Funny Names. Magazine Articles.