For me, Reason's most appealing feature is the ability to cable devices together in almost limitless combinations to create new sound sources, modulation assignments and signal-processing chains. Reason 4's new synth, Thor, promises to take this concept to a new level, featuring audio inputs and a routing matrix so you can process audio through its many modules. However, there's plenty we can do while we wait for our upgrades, and this month we'll look at several techniques for filtering audio signals using both the ECF filter unit and the Malström synth. We'll work towards creating a new effect device: an envelope-controlled filter, complete with all the controls you'd expect to find on this effect.
The ECF42 is one of the original Reason 1.0 effects devices, and is a simple external filter unit. We're going to use it to filter and shape individual drum channels from a Redrum drum machine. This is an ideal application for an external filter, as the drum channels have no filters of their own, and are monophonic in the sense that they only use one 'voice' at a time and only need one filter and envelope each (see the 'Mono Vs Poly' box for more on this consideration). In the screen above you can see that each Redrum channel has its own individual output, which is generally unused when you use the Redrum as a complete instrument. I've connected the channel 1 outputs (kick drum) to the inputs of an ECF filter unit.
With just the audio connected, you'd have a simple, static filter which could process the sound of the kick drum in three filter modes: band-pass, low-pass 12dB/octave, or low-pass 24dB/octave. However, for the ECF's envelope to function it needs to be triggered, and for that we need a gate signal. The best gate signal for the job is the same one that is triggering the kick drum. Luckily, each Redrum channel has a gate output that transmits the gate signals generated by Redrum's internal sequencer. In the picture, the kick drum's gate signal is connected to the Env Gate input on the ECF's rear panel.
With this configuration, every time Redrum's pattern sequencer triggers the kick drum, it also fires off the ECF's filter envelope. The front panel provides controls for Envelope Amount, Velocity Sensitivity and the familiar ADSR stages. By setting the Filter frequency to minimum, you can use the envelope to sculpt the transient nature of the sound.
The audio signals from any ECFs you use can go to separate channels on the main mixer, but a better arrangement is to connect them, along with the Redrum's main mix outputs, to a line mixer. You can then control the submix of the Redrum and any filtered sounds on a single channel of the main mixer. Connecting a drum's individual output to a filter automatically cuts it from the Redrum's main mix, ensuring the sound is not doubled up.
A less obvious, but more versatile way to filter signals is to pass them through the Malström synth. This gives you access to different filter and effect types, such as Comb and AM (ring modulation) and also Malström's Shaper module for grunging up your sound. Malström also has two filters that can be accessed and controlled separately, allowing you to create, amongst other things, true stereo effects.
We'll start by doing the same as we did with the ECF: routing a single Redrum channel through the Malström. In the bottom screen on the previous page, Redrum channel 5 has been connected to the Audio Input labelled Shaper/Filter A, and the gate signal is connected to the Filter Env input. Looking at the front of Malström (as in the screen above), signals coming into it follow the same path as the signals from the oscillators. This means that our mono signal to Input A goes through the Shaper module and then through Filter A.
Malström's dual filters and flexible routing system provide a number of different signal processing possibilities. Next we'll look at a stereo example: in the screen at the top of the next page, I've routed the main output of the Redrum through Malström, with the Left signal going to the Shaper/Filter A input, and the right signal going to Filter B. The earlier individual channel filters have been removed to make the cabling clearer to follow. No gate signals are present, so the filter is now static. Interesting rhythmic effects can be created by connecting the gate signal from one of the drum channels, or by employing a Spider CV merger to use the gate signals from several channels.
On Malström's front panel, the Spread control sets the separation of the outputs. When fully clockwise, Filter A's output goes to the left output only, and Filter B goes to the right, so this is the setting to choose for stereo operation. When working in stereo, you should also make sure the button above Filter B, which routes the B signal to the Shaper and Filter A, is in the 'off' position. When hard panning the filters, you may also want to link the Left and Right cutoff and resonance controls to keep the stereo image stable (see the 'Combis & Control Linking' section at the end of this article).
With the ECF, modulating the filter cutoff frequency requires an external CV signal to be connected to the unit. However, Malström has its own on-board modulation generators, one of which (Mod B) has a dedicated knob for applying modulation to the filters. A switch lets you apply the LFO to Filter A, Filter B, or both. A wide range of modulation shapes are available, and you can sync the mod rate to the song tempo, providing the opportunities for traditional LFO sweeps or more radical rhythmic gating effects. In the screen below, a fast, tempo-sync'd impulse-type shape has been applied to Filter A, resulting in the sound being chopped up into a sequence. You can also use Mod B in Single Shot mode to create envelope effects on either filter.
Although Malström provides a direct assignment from Mod B to the filters, you don't have to stick with this arrangement if you have something more complex in mind. Both Mod A and B have CV outputs on the back panel. You can cable either of these into the Filter CV input and assign this to A, B, or both, and scale the modulation with the input trim knob. This can be useful for creating panning effects when using the filters in stereo. Alternatively you might be using the filters serially, with one as a comb or AM effect, and the other as a filter. You can then have different types of LFO modulation assigned to each effect.
An envelope-following filter (sometimes called auto-wah) tracks the amplitude of an audio signal and applies it to the cutoff frequency. This adds movement to the filter, and can be used in place of, or in addition to the filter envelope. A particular advantage is that you are no longer reliant on a separate gate signal, as the audio appears to gate the filter magically by itself. Famous examples of envelope-following filters are the Moog/Big Briar Moogerfooger Low-pass Filter and the Mutronics Mutator. Reason does not have a direct equivalent — but we can build one.
There are two ways to create an envelope-following filter or auto-wah from Reason devices. The simplest is to start with the Scream distortion unit. The Body section of Scream has an envelope follower that can be used to modulate the Body Scale parameter, which can itself create a kind of comb-filter auto-wah effect. The envelope follower is applied using the Auto knob. Thoughtfully, Propellerheads have provided us with a CV jack on the back of Scream that outputs the 'voltage' generated by the follower. This can be plugged into the Filter CV input of your filter of choice (ECF or Malström) and straight away you have yourself an envelope-following filter. In order for this side-chaining to work, the signal you are processing must be fed both to the Scream and the filter itself. The audio output of the Scream doesn't need to be connected, because it's only there to generate the control voltage.
Using a Scream's Auto CV output is a very simple trick, but you have limited control over the effect. The only adjustment you can make is to set the depth of the filter modulation using the CV input scale knob on the filter's rear panel. More sophisticated envelope-following filters feature controls such as Sensitivity, Depth, and Attack and Release times. This gives you access to a range of results similar to using a traditional triggered filter envelope, but with its own special character (think Stevie Wonder's clavinet sound on 'Higher Ground'). Rather astonishingly, it's possible to build an effect with all the features we require.
Mono Vs Poly Filtering
An external filter unit has one main difference from the onboard filters used in Reason's synths and samplers: it is one filter and envelope shaping the final mixed output of an instrument or instruments. In the synths and samplers, each voice is an individual audio stream and has its own filter and filter envelope. This means that each note playing can be at a different point in an envelope. With an external filter, all the voices (or instruments, if you are processing a mixed signal) share one envelope, producing different results. For example, if you are triggering the filter envelope from a gate signal, or from MIDI key-presses in a Combi, each gate will retrigger the envelope, even if the previous envelope cycle has not finished. When playing a polyphonic synth through a single filter, earlier notes that had died down as the filter closed can reappear unexpectedly when you press another key. This is not what you normally want, but can be pleasing in its own right. It would be great if one day Reason featured polyphonic audio connections (like Reaktor), allowing you to build entirely new synths from separate devices, but for now we have to accept that Reason is more of a 'synth studio' than a 'synth creator'.
The key to building an advanced envelope following filter in Reason is the MClass Compressor. This has a connection on the back that outputs the compressor's gain reduction as a CV signal. Now, if you consider that a compressor is something like an envelope-following gain control, you will see how we can take advantage of this CV signal. As long as the signal coming into the compressor is above the threshold, the gain-reduction output carries a signal that is proportional to the audio level. So if we plug this CV output into the CV input of a filter (as shown in the screen below) the filter frequency will track the audio level.
But what about the other controls? With the Scream-based filter, the CV is always directly analogous to the audio level. This is not the case with the compressor, and you can control how the compressor's gain reduction (and therefore the CV output and the connected filter) responds to the audio input. By adjusting the threshold you can adjust the response to suit the overall level of the signal, and 'tune in' to the level changes that are interesting; this acts like a Sensitivity control. The Ratio scales the amount of gain reduction, and therefore the amplitude of the CV output, giving you a Depth control. The Attack and Release controls adjust how fast the CV responds to changes in the input level, and translate to the Attack and Release characteristics of your filter.
With a little adjustment of all the MClass Compressor's controls you can achieve the unique smooth filter response that is so characteristic of auto-wah effects and is a mainstay of funk music. The finishing flourish is to pack your creation into a Combinator, either with your instrument of choice or as a stand-alone effects unit.
As we've seen many times before, the Combinator device provides a neat way to package and save your multi-device configurations. With the building projects we've looked at this month, the Combi actually provides some important additional functionality that can't be achieved with devices left loose in the rack.
If you are working with both of Malström's filters to create a stereo effect, it soon gets frustrating having to adjust separate controls for the left and right filters. With the Malström inside a Combi, it's easy to link controls by assigning them both to the same Combi knob. In my example, I set the first Combi front-panel knob to both the Filter A and Filter B cutoff knobs, to give a single control over filter frequency. You can also use buttons to switch between two filter modes, change the signal routing, switch in the Shaper, and so on.
In the examples earlier in the article, the filter envelope is either triggered by a dedicated CV gate signal, or an envelope follower is used to create filter movement. If you place a Malström filter into a Combinator, another powerful option presents itself: triggering the envelope via MIDI notes. I recently made a configuration that placed a sampler patch and Malström inside a Combinator. The sampler's output was routed through the Malström's filters, as in the examples in this article. The important difference is that multiple devices in a Combinator can receive notes from a MIDI keyboard (or the sequencer) simultaneously. This means that each note can trigger the Malström's filter envelope, as well as play the instrument(s) being fed into it. In my configuration, a completely different sound is created compared to the original patch that used the NNXT's onboard filter. As I was sending MIDI notes to the Malström, I had to switch off both of its oscillators, to prevent them being added to the sound. This trick works best with monophonic instrument patches, for the reasons described in the 'Mono Vs Poly' box on the right.