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Basics of Synthesis: Filters

Hello again, it’s Tom here! Welcome to the fourth instalment of our Basics of Synthesis blog series. Today, I’ll be shedding light on filters! More specifically - what they are and how we can apply them in synthesis, plus some tips and tricks to finish off.


If you haven’t already, I recommend checking out the previous blogs in this series on waveforms, envelopes, and LFOs. They have valuable information that will make your life easier as we continue down this synth rabbit hole.


To kick things off today, you might be wondering...


What Do Filters Do?

A filter is a tool we use to attenuate frequencies, which means to remove frequencies from a sound. There's some terminology that we have to understand in order to effectively appyl synth filters, and I'm going to dive into that right now!


Filter Terminology

Cut-Off Frequency

The cut-off frequency is the frequency at which your filter begins attenuating. Your filter may cut above, below, or around this frequency.


Slope

The Slope of a filter dictates how quickly it attenuates once we pass the cut-off frequency. This is measured in dB reduction per octave - for example 6 dB/oct (mild), 12dB/oct (moderate), or 24 dB/oct (steep).


Resonance

Lastly, Resonance is a tool that emphasises the frequencies immediately around your cut-off frequency, giving you a boost in that localised area.


Now of course, boosting frequencies is the opposite of attenuating them, which is what I originally said filters do - so I did tell a half-fib. But I promise, the purpose of a filter is almost always to attenuate! Note that this is the main difference between filters and other tools like shelves or bell curves, which are able to either boost or attenuate the frequencies you choose.


Types Of Filters

There are four filter types commonly used in audio production. I'm going to give you a quick rundown of each of the filter types, along with some of their practical uses in music production before we dive into how you can use them specifically to shape your synth sounds.


Low-Pass Filter

The low-pass filter removes all frequencies above your cut-off frequency. With a low-pass filter applied, your source audio will sound muffled and dead. You could use a low-pass filter to isolate the bass in a track, remove hiss from a recording, or perhaps even build yourself a nice lush pad sound.


Low-pass filter example

High-Pass Filter

The high-pass filter, on the other hand, removes all frequencies below your cut-off frequency. If you apply a high-pass filter to a track, you lose the low frequencies as you raise your cut-off, making your source audio sound thinner. Practical applications for a high-pass filter include removing bass from a track, removing kick drum bleed from another drum microphone, or removing hum from a recording.


High-pass filter example

Band-Pass Filter

The band-pass filter removes frequencies above and below the cut-off, leaving us with a thin band of frequency content. This effect can be pretty intense, and is used much less often than low-pass and high-pass filters. You might want to utilise a band-pass filter if you’re trying to emulate the sound of a phone call, or just trying to isolate a specific set of frequencies in a recording.


Band-pass filter example

Notch Filter

Lastly, we have the notch filter (also known as the band-reject or band-stop filter). The notch filter is the inversion of the band-pass filter; it removes frequencies around the cut-off frequency, allowing all other frequencies to pass though. It’s the least common of the four filters listed here, and the only real use for it is to get experimental and creative. You can apply modulation to the cut-off frequency to get some cool psychedelic effects. Or, you might want to completely hollow out the mid-range of your mix… for some reason? We don’t judge here at Island Studios.


Notch filter example

Using Filters In Synthesis

Filters are imperative in building synth patches. To demonstrate this, load up the pad sound you created through the previous blogs in this series, or feel free to create a new one.


Firstly, apply a low-pass filter to the sound (you may need to do some research into how this works on your particular synthesiser). Next, drop the cut-off frequency down to minimum, and slowly start raising it. Continue until you have a warm lush pad sound… I recommend around 800hz as a starting point, but depending on your taste, you might pick anywhere from 250hz up to 2.5khz. You might even go beyond that if you feel the context of your song demands it.


If you feel like it, try swapping the low-pass filter out with the other filter types we discussed. Pay attention to how they sound, and start to familiarise yourself with those sounds.


Filter Tips And Tricks

To finish off, let’s swap back to the low-pass filter and go through some techniques to add more character to your sound.


Firstly, we can use our LFO knowledge (covered in my previous blog post) to add movement to our pad. Start by applying an LFO to the filter cut-off frequency. Once again, do some digging online if you need help setting this on your synth. With the LFO applied, set the LFO rate to around 1.5 cycles per second (1.5hz). Max out the depth parameter so your LFO modulates the entire frequency spectrum - you should be hearing a repetitive ‘wub’ or ‘wow’ sound as the filter opens and closes… it’s a bit excessive, isn’t it? Drop the depth down a lot, so that the modulation is subtle, but still enough to add noticeable movement. With a bit of tinkering you should be able to find the sweet spot - it’ll likely be close to the minimum depth value.


Another idea is to apply an envelope to the filter cut-off frequency. Set the envelope parameters for a fast attack, medium decay, minimum sustain, and minimum release. Now drop that directly onto your filter cut-off frequency parameter, and you should have a nice little lead pluck sound. Mess around with your envelope values, and experiment with some reverb - see what sounds good!


We’ll leave it there for today. If you’re just getting started with synthesis, I hope this blog was helpful in one way or another - keep an eye out for more soon. Until then, adios!


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