All filters in Synthedit will by their nature exhibit some phase shift at some point in their frequency spectrum. Some filters exhibit more phase shift than others.
(Even analogue filters will introduce phase shift)
Phase shift and Analogue filters.
A basic analogue resistor–capacitor (RC) low-pass filter, as an example, will shift the output sine wave by up to 90° compared to the input sine wave. The “up to” is important—the actual phase shift generated in the circuit also depends on the frequency of the signal passing through the filter. As you can see this quickly becomes a complex subject.
Moog filters and phase shift.
A Moog ladder filter, introduces a significant phase shift as the signal frequency approaches the filter cutoff frequency, with each stage of the filter adding about 45 degrees of phase shift, which gives us a total phase shift of nearly 180 degrees at the cutoff frequency for a typical 4-stage Moog filter; it is this phase shift that contributes to the unique “warm” and resonant sound of Moog synthesizers when filtering sounds at, or near to the cutoff frequency.
SV Filters and phase shift.
An SV filter, which stands for “State Variable Filter,” introduces phase shift by design, meaning it alters the timing of different frequency components within a signal as it filters them, with the amount of phase shift varying depending on the frequency and filter settings; (this is a common to most filters, not just SV filters), but the design of an SV filter can make this phase shift far more pronounced at certain frequencies.
Key points about SV filters and phase shift:
The phase shift in an SV filter arises from the internal circuitry and the way it interacts with different frequencies, causing different time delays for various components of the signal.
The Impact on the filtered sound:
When filtering audio with an SV filter, a very noticeable phase shift can manifest as subtle changes in the sound quality, particularly affecting the timing of transients or the overall “naturalness” of the filtered sound.
“Korg MS20” filters, otherwise known as Sallen-Key filters.
The Sallen-Key filter, is a second-order active filter, with a phase shift that approaches -90 degrees at its cut-off frequency, with its phase response smoothly transitioning from 0 degrees at lower frequencies to -180 degrees at very high frequencies.
Its key characteristics include:
1) High input impedance,
2) Low output impedance,
3) Good stability, with the ability to readily adjust its cut-off frequency and quality factor (Q) by adjusting the values of its resistors and capacitors, allowing for a flexible filter design.
The Sallen-Key design used in the Korg MS20 also had the ability to go into saturation giving some unique “screaming” sounds.
Summary.
As you can see phase shift in filters is normal, and is inherent in their design and the resulting unique “flavour” imparted to the output filtered sounds. Without their built in phase response there would be no “Moog filter” sound.
Digital filters.
Most filters exhibit a phase shift of some sort, with DSP the shift is always in the form of a small delay in the signal due to the large number of complex calculations. The more efficient the computation method used, the lower the degree of phase shift introduced by the filter, unless phase shift is called for to recreate (for example) the sound of a Moog ladder filter.
For a simple 1-pole hp filter, the phase response at the cut-off frequency will be about 45 deg. As a general rule the lower the number of poles in the filter, the less the phase shift will be. All of the filters in Synthedit will introduce some degree of phase shift, this is largely unavoidable.
The degree of phase shift is dependent on the type of filter, number of poles or steepness of cutoff slope.
You can see below the phase shift that even a simple 1 pole HP filter introduces:
At 50 Hz, the two ‘scope traces are superimposed- we have an almost zero phase shift between input and output.
However at 500 Hz you can see there is a marked shift in phase (Ignoring the difference in amplitude) between the input and output signals.
Cumulative phase shift.
If you cascade (or stack) filters the phase shift is also cumulative (adds up), so if one filter gave you 45 degrees at 1kHz, two filters will give you 90 degrees phase shift at 1 kHz.
Why and when is phase shift important?
Often you wouldn’t notice that a filter was introducing phase shift, however it soon becomes apparent when you mix the filtered signal with the original “dry” signal. As the filter frequency changes you would likely notice a “phasing” effect on the sound in addition to any timbral changes from the filter itself.
You can see this in the sequence of screenshots below:
100% Low pass Moog filter.
With a mix of 50% Low pass and 50% dry signal, you can see there is a pronounced notch at 2kHz.
And with 100% dry signal we get a more or less flat white noise spectrum.
Introducing feedback (resonance) into the filter introduces further phase shifting effects as raising and lowering the resonance level will alter the phase shift quite markedly at mid range filter frequencies.
0 Resonance at a middle range frequency gives this pronounced phase shift.
Whereas with a resonance of 8.3 the signals are approaching being in phase again.
As you can see phase shift in filters is not a simple and easily predictable subject.
The only filters that I have tested in Synthedit to give a constant phase shift over their frequency range are the high and low pass SINC filters.
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