# Need help on the Two Notch Filter module.

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edited August 2023

Hey guys, They didn't give us any mathematics for this. How can we calculate the spread of the two notches and does one of the notches stay fixed at F while the other moves? This is the module info:

2-notch filter.

[F] - cutoff of the underlying allpass filters. Defines the "middle" frequency between the notches.

[Fbk] - controls the internal feedback of the entire filter, affecting the notch width. Range [-1..1]. Positive feedback values increase the width, negative values decrease the width.

[Spread] - controls the internal feedback of the underlying allpass filters, affecting the spreading of the notches. Positive values increase the spreading, negative values decrease the spreading. Typical range -1..1.

[Inv] - should be set to 0 (non-inverted mode) or 1 (inverted mode, swapping the notch and peak positions). Intermediate values can be used during parameter smoothing.

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I have a feeling this is not a dual notch filter that has two adjustable notch filter center frequencies. I ran a sweep thru it and played with the variables but never did see two notches. By writing about the notch filters spreading in the info above; I think they meant the width centered around the notch spreads and not the frequencies. I guess you could say the spread is the Q of the notch. If anyone else would care to clarify it's behavior, that would be great.

• Member Posts: 27 Helper
edited September 2023
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i see two notches and the module info seem to be correct.

no idea how to calculate the exact distance of the notches from the center (F). the 'spread' parameter changes the resonance of the allpass filters, which affects the position of the notches.