What is a filter?
A filter is a device, or object, that removes unwanted or selected parts of what it acts on. In the case of making coffee, a filter removes the used grains from the delicious tasting liquid that you will eventually drink. In the case of music production, it removes frequencies from an audio signal, not necessarily muddy frequencies, but frequencies that you don’t need at that moment in the track. Filters for your DAW come in many forms; high pass, low pass, band pass, band stop, notch, double notch and more weird and wonderful variations. A filter cut will generally have a control for selecting the frequency you want to remove, or the point at which you want to make a cut, and by how much you want to cut it. They will also usually have a resonance control, which allows you to boost a frequency at the cutoff point. Filters can move, or remain static, and it’s the filters that move is what this blog post focuses on, filters we like to call ‘envelope filters’.
What is an envelope filter and what does it add?
An envelope filter is a simple concept. It’s a filter, for example a low pass or high pass filter which has envelope controls like a compressor or a dynamic eq does. Envelope filters are not one of the most well known plugin types but their uses are far reaching, especially as some plugins of this type come with additional controls like LFO’s and sequencers. These plugin types will give you an incredible edge to your sound design capabilities, allowing you to create grimey resonant screeches, sci-fi type wobbles or classic filter type movements. As a mixing tool, envelope filters can also provide an interesting edge to your mix as well. As they respond to an envelope, it can remove unwanted frequencies as amplitude and time of a signal goes on.
The History of filters
Electronic filters emerged in the 1910’s by an inventor called George Campbell and were made of either resistors and capacitors or resistors and inductors and we’re originally designed for AT&T, the American telephone company. Using a ladder network of inductors and capacitors in appropriate configurations he produced low-pass, high-pass and band-pass filters. These filters could be designed to pass frequencies in any specified range and reject those in other ranges. Fast forward 50 years and in ‘66 Robert Moog (of Moog synthesiser fame) submitted a patent for an electronic filter entitled “Electronic High Pass and Low Pass Filters Employing the Base to Emitter Diode Resistance of Bipolar Transistors”. It’s the addition of transistors which changed the future of filters for synthesisers. What made the Moog filter special was their simplicity. The Moog sound came from the fact that, even with a higher cutoff slope, you could drive the resonance really high without losing too much of the low end, at the same time hear some of those typical transistor type harmonic peaks.
What practical uses do they have?
- Can be used as an alternative to a wah-wah pedal.
- Some of these plugins have side-chain input, so it can be used as an alternative to side-chain compression which provides more of a precise, frequency specific cut.
- Removing unwanted noise – a good alternative to noise gates as well as side-chain compression.
- Shaping drums, by adding a resonant boost that moves with the shape of drum hits these plugins can help give the hits more presence.
What is technically happening with envelope filtering?
An envelope filter vst is a type of vst or audio unit that sets its frequency based on the loudness of the source signal or the side-chain signal. This could be a direct signal as a channel insert, or from a side-chain input. There are a few plugins that adopt this plugin model, Waves Metafilter, Fab Filter’s Volcano and The Sweeper are some examples. These plugins are almost a transient shaper for audio, but have many creative uses at the same time. They work in the following way:
- The source signal is copied, after which this copy – or alternatively a side chain signal – is used as the effect’s detection signal.
- This detection signal is turned into an amplitude signal by rectifying it, after which an attack and release envelope is applied.
- This envelope signal is used to calculate and set the frequency of the filter.
- The filter is applied to the original source signal, creating the output signal.
What famous songs use envelope filtering?
Disclosure ‘When A Fire Starts To Burn’
Disclosure are well known for employing these plugins in their productions. The duo make a classic UK Garage sound, but with a very modern touch and the envelope following filter adds a nice resonant movement to their bass line throughout the duration of the track.
Squarepusher ‘Dark Steering’
We’re big fans of Squarepusher here at denise, and we’re happy to be able to reference the artist once again like we did in our blog post about recreating bitcrushing used in famous songs. The mid range stabs in this track are really brought to life with a small addition of enveloping filter.
Pro tips (nrec)
1) Recreate a 303 style filter.
Here we’re using the resonance from an envelope filter to add a sharp edge to an over-driven 303 style filter. The added automation on the filter cutoff really makes the filter effect come to life at certain parts of the sound.
2) Shape a kick drum.
In some bass heavy genres the kick should always sound huge and fat. But, problems can arise during faster parts of your song where the decays stack up with adjacent kicks, ruining the mix and the clarity of the kick itself. In this djent/metal drums, we used the Sweeper to remove the low rumble when the kick plays 16th notes by using a long attack time and short release.
3) Remove hiss
In this recording the drums, bass, synth and clavinet all have a lot of noise, and by using an envelope filter we can get rid of the unwanted noise in the less intrusive way. This helps to preserve the quality of sound and maintains the dynamics very well.
4) Alternative to side-chain compression
In this track the low end is overpowering, so we side-chain the kick to the bass. In this case, using traditional side-chain compression would push the low frequencies to the back of the mix and cause unwanted changes in the mid range. Listen to what happens when we use The Sweeper to only cut the low frequencies we want to: The bass stays clear and natural, keeping its power and presence in the mix.