Filters in action – Part 1

Let’s apply these filters in Clearsynth and hear how they sound.

Load up Clearsynth in your DAW and set it up as follows –

  1. Make sure only one oscillator is generating the sound. (Mix knob is dialed all the way to the left).
  2. Filter is turned off (cutoff is turned all the way to the right and res is turned to the left).
  3. Choose the saw waveform for Osc 1 (the second button from the top).

Alternatively, you can choose the saw preset. That automatically achieves these 3 steps.

Now, play a long note on your keyboard, or arrange a note sequence and loop-play them in your DAW.

While the notes play, turn the cutoff knob gradually to the left. Do this slowly and observe the change it makes to the sound you hear.

You’ll notice three things as you gradually reduce the knob’s value to zero:

  1. The sound gets less bright and more dull.
  2. The volume of the sound decreases gradually.
  3. At around 40 – 45 degrees from zero, the sound stops completely. There is no audible change in sound when you turn the knob between zero and 40 degrees.

Let me explain what the cutoff knob does, and the reasons for the above three behaviors will become clear.

We’ve learnt in the previous post that a filter cuts off frequencies beyond a particular point. In most filters, we get to control what that cutoff point is. And that’s exactly what we do when the move the cutoff knob.

You must have noticed that the first filter button, the low-pass, or high-cut filter is selected. It means that the filter allows all the frequencies to go through, that are less than the cutoff frequency. So, when we turned the cutoff knob all the way to the right, what we did was make sure it allows all the frequencies to pass through, thus effectively turning off the filter.

If we had selected the second filter button, the high-pass filter, what should be the cutoff knob value to make sure all the frequencies would pass through? Yes, the knob should then be turned all the way to the left.

Okay, back to low-pass filter. What’s happening as you sweep the cutoff knob from max to min? Well… the saw waveform generated by Osc 1 is holds a bunch of frequencies, doesn’t it? As you reduce the cutoff value, more and more of the high frequencies are getting removed from the output sound.

High frequencies in any sound make the sound seem bright. Now since we are gradually chiseling away the high frequencies,  the resultant sound seems duller. And of course, since some sonic content is being removed, the energy, or the volume of the sound gradually decreases too.

Any guesses on observation 3? Why is there no difference in the 0 – 40 degrees knob positions in the cutoff knob? It’s because there are no frequencies in that range! By the time you’ve reached the 40 degree position, you’ve removed all of the audible frequencies that the saw waveform generates. So, any changes there do not result in any audible difference. (As you aren’t really hearing anything at all there!)

Let’s try the same thing with the second button – the high pass filter. This time, start the cutoff from zero, gradually dialing it up to the max. Notice that the sound get’s shriller and sharper, until it disappears.

The band-pass and notch filters also result in audible differences as you sweep the cutoff, but the effect isn’t as pronounced as the high and low pass filters. We’ll try those two filters out again in a different scenario later.

Before I wind up the first part of filters in action, let me address a question some of you may ask:

Are these the only four types of filters?

If subtractive synthesis is all about sound chiseling, these filters seem to be very basic chiseling tools. What if I need to do some really fine surgical removal of frequencies?

To answer that question, let me ask another question. What are the ways you can remove frequencies? The most surgical way is to say something like “I want XYZ frequency removed”. But that wouldn’t do! There are so many frequencies. Not to mention they are continuous, not discrete values. Also, in order to make an audible difference, you have to remove a bunch of frequencies.

And that’s what these filters do. They remove a range of frequencies based on the values you specify. And yes, theoretically, you can achieve any filtering you want by chaining different types of filters one after another. Clearsynth is a simple synth, and it provides only one filter. However, there is a lot we can achieve using just this.

In my next post, I’ll talk about the other knob in the filter panel, res, and the keyboard track button, so that you’ll then understand all the controls in the filter panel!


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