Equalize Phase Modes

The mixed phase mode in Acon Digital Equalize 2 is a new and unique feature and the difference between the available phase modes are described in detail in this chapter.

The effect of a clean equalizer that doesn't emulate analog artifacts like saturation or noise can be fully described by the amplitude and the phase response. The amplitude response is what Equalize visualizes in the curve editor in the upper part of the plug-in window.

The phase response is not visualized because its impact on the processed signal wouldn't be visualized intuitively in a spectrum analyzer. However, the importance of the phase response is revealed when we measure the so called impulse response of the equalizer. The impulse response is measured by feeding the equalizer with a Dirac impulse. In the digital world with sampled time series, a Dirac impulse is nothing more than a zero signal consisting of one single full scale sample. The output is what we describe as the impulse response.

The illustrations below visualize the impulse responses obtained from a steep band pass filter (100 - 200 Hz) using the three phase modes that are available in Equalize along with the input signal at the top:

Above is the input test signal consisting of a single pulse at 200 ms (Dirac impulse).

Minimum phase mode. Notice that the peak amplitude is shifted backwards in time.

Linear phase mode. The extensive non-zero signal prior to the peak is noticeable and disturbing. Another disadvantage is the long latency introduced by the filtering.

Mixed phase mode. The pre-ringing is limited to fit within the temporal masking of the human auditory system and the latency introduced is short. The amplitude peak is perfectly time aligned.

Traditionally, only the minimum and the linear phase modes have been available. The minimum phase mode is the only mode that ensures zero latency and is the way analog equalizers work. The downside is that the time alignment between different frequency components is lost.

The linear phase mode was introduced with the advent of digital processing, but the pre-ringing effect can distort transient rich material severely. Linear phase filters show advantages when processing tracks separately and there is crosstalk between the tracks. In such cases, linear phase processing has predictable results whereas minimum phase processing can alter the frequency response in unintentional ways.

The new mixed phase mode is a unique feature in Equalize and allows you to set the latency and hence the maximum pre-ringing time. The human auditory system masks signals before and after an impulsive noise, something which is called temporal masking. The masking is stronger after the impulsive noise than before, which is why extensive pre-ringing is undesirable.

By choosing latencies around 20 ms in Equalize, the pre-ringing is masked by our hearing. At the same time, the maximum peak is perfectly time aligned.