Notch Filter Node

Standard notch filter, also called band-stop or band-rejection filter. It is the opposite of the Bandpass Filter Node: frequencies outside the give range of frequencies pass through, frequencies inside it are attenuated and muted.

You can use this Node to remove a specific frequency range from an audio signal.


How to Use

The Notch Filter Node needs a source audio signal connected to its audio input, to which it can apply the effect, and then output the result. The input audio signal can come from anywhere, as long as it's audible: an Audio File Node, Oscillator Node, Noise Generator Node all work. Click their output, then click the audio input of the Notch Filter Node to establish a connection.

Connect to an Audio Destination Node to listen to the output, and play your project if your audio source needs it.


Inputs

Audio Input

An audio signal to filter. You can also connect a control signal, but results will vary based on whether the control signal changes in a way that conveys meaningful frequency information, or not.

Frequency-Control Input

A control signal to control the Frequency setting. You can also connect an audio signal, but keep in mind that it can cause the Notch Filter Node to become unstable and reset. See the technical details section below.

You can enable this input from node properties, or by connecting to it.

Sharpness-Control Input

A control signal to automate the Sharpness setting, from 0.0001 to 1000.

You can enable this input from node properties, or by connecting to it.


Settings

Frequency

The filter’s center frequency, in Hz.

Sharpness

How sharp/narrow the frequency bandwidth is around the center frequency. Greater values result in a smaller frequency band, and a sharper effect.

Filter Order

The order of the differential equation describing the filter. In simpler terms, when you increase the filter order, it’s essentially the same as chaining multiple Notch Filter Nodes.


Outputs

Audio Output

The filtered audio signal (or whatever is left of the control signal if that’s what you connected).


Technical details

The Notch Filter Node is a digital biquad filter, and as such, it's possible for it to become unstable, e.g. in response to rapid frequency modulation. When this happens, the Notch Filter Node will attempt to reset itself every few seconds, to avoid remaining unstable forever. This auto-reset is done in real-time mode (i.e. when you play your project), but not during exporting. That is, if the filter becomes unstable while exporting, it may remain unstable until the end of the exported audio.

Note: the behavior of this auto-reset is mostly non-deterministic, and depends on several undocumented factors. Don't rely on it to produce "glitchy" audio. Instead, use a Gain Node with an LFO-controlled gain, or a similar amplitude modulation technique.