RR 22 Highpass Lowpass ECF


We now know that two identical audio signals, one with inversed polarity, will mix to nothing in mono. But what would happen if we altered the original signal after we have created an inversed one?

Let’s say we equalize the signal. Now the two people pulling the rope are not of equal strength. One person is pulling the rope harder and the rope moves. The result in our world of audio would be an audible signal that is equal to the alteration we just made.

For example: Let’s say we add a little equalization to the original drumloop to enhance it’s hihats. When we mix this equalized drumloop with the inversed original, we would only hear a bit of the hihats: only the bit we just enhanced.

If we attenuate the hihats a bit using equalization we would still only hear hihats and again only the bit we just attenuated, only this time the polarity is inversed. Once again: The result is an audible signal that is equal to the alteration we just made.

The patch we are about to explore will make things hopefully clearer and if not, it’s at least useful and fun to play with.

The Combinator patch HighLow_ECF.cmb is a filter effects patch, best used as an insert effect. It is an expansion of the ECF_env_inv.cmb patch. Please read the tutorial on this patch first in the Automatic Envelope section.

The patch HighLow.cmb uses the ECF to filter audio and has an envelope for the Filter Frequency. By using inversed polarity it can also function as a highpass and notch filter.

When we toggle the rack when can see the only difference between this patch and the ECF_env_inv.cmb. In this patch there’s an extra split from the Spider Audio into Thor. The output of Thor is merged with the output of the ECF using the same Spider Audio before it is routed into the Maximizer.

When we toggle the rack back to front we can see what Thor does. Like in the patch PolarityPan.cmb only the Modulation bus of Thor is relevant. And again inputs 1 and 2 are simply routed to outputs 1 and 2. The polarity of the audio is inversed. The difference with patch PolarityPan.cmb is we now use the scale parameter. This time Button1 on Thor scales by an amount of 100. Or in plain audio-English: Button1 toggles the output of Thor on and off.

So with the outputs of Thor disabled, Button4 on the Combinator, this patch functions the same as ECF_env_inv.cmb. But with Button4 enabled it combines the inversed version of the original signal with the filtered signal from the ECF.

Once again, the result is an audible signal that is equal to the alteration we just made. With the ECF set to LP12 the alteration is a lowpass, or highcut, or the removal of the high frequencies. So the resulting sound is only these high frequencies, the low frequencies are cut of.

By combining the inversed original with the original through a LP12 the result is a HP12: a highpass filter with the same 12dB slope and resonance as the LP12.

With the ECF set to LP24 the result is a highpass filtered sound with a slope of 24dB.

And with the ECF set to BP12 the result a notch filter with slopes of 12dB.