All Pass filters in Synthedit.

I can hear those of you who have never heard of an All-Pass filter thinking “hang on a filter that passes everything? What possible use is that, how can this be a filter?”
Well they are very useful items. They do indeed have an almost flat frequency response (see below), and technically they are a filter when connected up in the correct manner.

But if we put a 1kHz sine wave through the filter, and compare the output with the 1kHz tone direct from the oscillator something is happening to the signal passed through the filter:

Now we change the filters “pitch” control (below)

You can see that the phase of one signal has changed with regard to the other, so although the frequency response is flat, the phase of the signal passed through the filter is changing with frequency. This is useful, it will create a well known musical effect for us, the phaser. A phaser is quite a simple effect to create.

Creating a phaser in Synthedit:

If we connect an All-Pass filter to a noise source, then mix the direct noise signal with the signal passed through the filter you can see there’s a pronounced dip in the signal level at roughly 1kHz. This is where the filter has shifted the phase by 180 degrees relative to the input, so when the signals are added in the Level Adj module we get a cancellation effect causing a “notch” at that frequency.

Adding a second filter in series

By adding the second filter we get a second notch, at roughly double the frequency, so we can change the sound once again just by adding a second filter coupled to the first.

The resonance control adds a degree of feedback into the filter module.

Two stage Phaser for SynthEdit.

The structure below gives us a Phaser module like you might find on a guitarist’s pedal board. Adding the TD LFO A allows us to automatically sweep the notch frequencies up and down at a rate set by the speed control, and a frequency range set by the sweep control.
The mix control on the X-Mix control allows us to go from the “dry” unfiltered signal through to the fully filtered “wet” signal. The LFO type on the TD LFO A is set to “triangle” which is the sweep waveform on most phasers, and the modify plug has a value of 0 Volts to give us the triangle shape. The speed slider can be set to a minimum of 0.01 volts, and a maximum of 5 volts giving a useful LFO range of 0.01Hz to 5Hz.
Note: The All Pass filter module has two frequency to control voltage settings:
1 V/Octave and 1 V/kHz. The latter 1 V/kHz is perfectly good for a phaser, and uses less CPU power.
Note: When you connect two All Pass filters in series, you won’t create a deeper “notch” effect, as the second filter adds it’s own phase shift onto the shift introduced by the first filter, hence you get a second notch at a different frequency, and by adding a third filter we get yet another new notch frequency.
Adding filters always adds new notch frequencies.

Getting a better “resonance” sound from a SynthEdit phaser.

However the resonance control as it stands, won’t sound quite right due to the way synthedit handles feedback, but we can remedy this by adding a peak filter like so with its control voltage to pitch response set to 1 V/kHz and a variable resonance control added instead of being connected to the second phaser stage. The resonance we get when the SV filter is set to single stage band pass will sound much better than we can ever get from the stock All Pass filters.