SouthPole Expedition Part 1: Low-pass Filter Basics

AudioMulch's SouthPole contraption is capable of a wide range of modulated filter effects, ranging from analog-synth style filtering and enveloping, auto-wah effects controlled by an audio input, to enveloping incoming audio according to a sequenced pattern. With all these possibilities SouthPole might seem a bit complex to begin with. Perhaps the SouthPole contraption isn't one of my most intuitive designs, but people who "get it" find it to be one of AudioMulch's most powerful contraptions.

In future posts I'll cover each SouthPole feature in detail: modulation mixing, the envelope followers, the envelope generators and their various trigger modes, and the LFOs. But I can't cover everything on one post, so this week I'm going to start at the beginning and discuss the basics: resonant low-pass filtering and the primary SouthPole parameters Base Cutoff, Resonance and Gain. It's important to understand these parameters because everything else in SouthPole builds on them.

If you really want to skip Low-pass Filter 101, scroll down to Filtering with SouthPole to check out the screencast and schematics.

Why SouthPole?

The name SouthPole was a logical choice given that the contraption was inspired by NorthPole – a free plugin from Prosoniq (apparently it's Mac only these days, but you can download version 1 for PC here ). A quick comparison of the NorthPole and SouthPole GUIs should give you some idea that SouthPole is not exactly a NorthPole clone.

User interface comparison: NorthPole 0.91b vs. AudioMulch 2.0 SouthPole

In any case, the basic idea is the same: a resonant low-pass filter with some modulation sources. (Actually NorthPole also includes a delay effect, you could easily add that in AudioMulch with a separate SDelay contraption.)

Resonant Low-pass Filter 101

Resonant low-pass filters have been a staple of electronic sound synthesis since the birth of the analog synthesizer. As the name suggests, a low-pass filter is one which passes low frequencies (and doesn't pass high frequencies). "Low frequencies" might mean the bass parts of a sound but it's all relative – the question is "lower than what?" The frequency where the filter stops passing and starts cutting is called the Cutoff Frequency (actually this is usually defined as the point where the gain drops 3db below the pass band gain). Often the frequency response is depicted by a graph like this:

Low-pass filter frequency response graph

Higher gains run upwards, higher frequencies run to the right. The graph shows that the gain below the cutoff frequency is unity (unchanged) whereas for higher frequencies the filter progressively cuts out more and more sound.

Low-pass filters are good for creating something melow out of a bright sound source. For example filtering a bright and fizzy sound with bass in it, like a mixed pair of sawtooth oscillators from an Arpeggiator contraption:


An AudioMulch patch showing an Arpeggiator connected to a SouthPole connected to SoundOut

This "dual oscillators into a low-pass filter" is the kind of thing you'd see in an analog synthesizer. That's what I'm going to use here, but really you could filter anything: drum loops, granulated vocals, whatever. For reference, here's what the output of the Arpeggiator sounds like without filtering:

Two oscillator sawtooth Arpeggiator output

When you filter a broad-band sound with a low-pass filter with a low cutoff frequency it will sound dull and muffled like this (you might need headphones to hear this if your speakers don't reproduce much bass):

Sound of two detuned sawtooths filtered with a low-pass filter

If you sweep the cutoff frequency from low to high, the sound will get brighter as the cutoff frequency gets higher. Here's what an upwards filter sweep sounds like:

Upwards low-pass filter sweep, no resonance

A low-pass filter passes frequencies below the cutoff frequency. But in the case of a resonant low-pass filter, frequencies around the cutoff are boosted – by how much depends on the filter's "resonance" parameter. Often you'll find a knob on analog synthesizers to control resonance (sometimes its called "Q" – an engineering term for more or less the same thing). With resonance the low-pass filter frequency response looks like this:

Resonant Low-pass Filter Frequency Response Graph

Compare this to the first frequency response graph above, notice how the gain is boosted around the cutoff frequency. With a bit of resonance, the filter sweep sounds like this:

Upwards low-pass filter sweep with resonance

Filtering with SouthPole

I don't want to scare you with a full SouthPole schematic just yet, here's the abridged version:

Simplified schematic diagram of AudioMulch's SouthPole contraption

The input sound (which enters SouthPole through the contraption's left input) is filtered through a low pass filter and has its gain (volume) controlled before coming out the audio output. You can see the three parameters Cutoff frequency, Resonance, and Gain marked along the top.

SouthPole offers many ways to control Cutoff, Resonance, and Gain, but the best place to start is with the "base values". These are the three knobs at the lower left of the contraption, handily labeled, Cutoff, Resonance, and Gain. I've circled them in red on the screenshot below:

SouthPole user interface highlighting the Base Cutoff, Resonance and Gain knobs

Try this out yourself: Gain is like a volume control. If its turned down you won't hear anything. So at first make sure the Gain is set to 0.0dB. Then you can play with the Cutoff knob to hear how it changes the sound. Finally, try adjusting the resonance. Just tweaking the resonance isn't as interesting as changing the Cutoff frequency, but sweeping the cutoff frequency with different amounts of resonance will sound quite different.

I've automated all three parameters in the screencast below. You can watch the Base Cutoff, Resonance and Gain knobs moving to see how each parameter affects the sound. The video starts with the filtered sounds you heard above, then uses faster automation to articulate a rhythmic pattern.

Screencast showing automated Base parameters in SouthPole

As you can see, there's quite a bit you can do just by automating the SouthPole's Base parameters, although often you might use SouthPole's envelope generators for this kind of rhythic sequencing.

You can download the AudioMulch document here (compatible with AudioMulch 1 and 2). Press the play button to hear the automated sequence. Make sure you hit the stop button if you want to tweak the knobs otherwise automation will override them. I also recommend that you check out HappyPenguins and the MulchOnly example files for some examples of other uses of SouthPole.

Until next time...

As I said at the beginning, SouthPole can do a lot more than I can write about in one post. But I want to leave you with one final thought: all of the other capabilities of SouthPole are concerned with providing various means to modulate the base parameters of Cutoff, Resonance and Gain. Each modulation source can be mixed to control any or all of these parameters as shown in the following diagram:

SouthPole schematic showing modulation source routing

Once you get your head around it it's not so complicated. The next step is understanding how each of these modulation sources works and how they can be mixed together. I'll write more about it later, or if you can't wait, check out the help file :-) Until next time, Happy Mulching! -- Ross B.