Allpass Filter User Guide

[utdgrant]

Is there a user guide for the Allpass Filter included in VM Core?

I'm particularly interested in what the 'Width' control does.

[UrbanCyborg]

There's a Manual for all the CA modules, but the Allpass Filter doesn't appear to be in it. I'm only guessing, but I'd suspect that it refers to the width of the Q spike, inasmuch as both the width and height of the resonance spike at cutoff are functions of Q.

Reid

[utdgrant]

I think I've sussed it out, now.

In order to get one 'notch' in the output spectrum, you need to have two cascaded all-pass stages (which are then mixed with the original, dry signal). This is what the CA Allpass Filter does 'under the hood'. The Freq slider sets the centre frequency of the 'notch'. The Width slider separates the corner frequency of each all-pass stage away from the centre frequency in an upwards and downwards direction. When both corner frequencies are the same, you get a narrow and deep 'notch', and when they are spread apart, you get a wide and shallow 'notch'.

AllPassNarrow.png (566.86 KiB) Viewed 5619 times

AllPassWide.png (586.21 KiB) Viewed 5619 times

[utdgrant]

I set up two Smol RATs as single all-pass stages by subtracting the 6dB HPF output from the 6dB LPF output. I used one DC knob to control the corner frequency of both stages, and used another knob to separate them in upwards and downwards directions. The results are very similar to the CA Allpass Filter output.

AllPassNarrowSmolRAT.png (707.48 KiB) Viewed 5618 times

AllPassWideSmolRAT.png (736.61 KiB) Viewed 5618 times