Advanced Music Synthesis
Voltage Controlled Filters Part 2
In the last Advanced Music Synthesis column we looked at the basic theory behind voltage controlled filters, in particular, voltage controlled lowpass filters. We saw how a basic sound is made up of harmonics and how these harmonics can be affected by filters. This month we're going to try a few patches in an attempt to put that theory into practice.
In Figure 1 the filter is being used as an elaborate tone control and is not being controlled by any other device except the keyboard. This will allow us to get a wide range of sounds by simply altering the cutoff frequency and resonance controls. Using this particular technique many impressive organ sounds, from pipey church organs to cheap, electronic organs, can be obtained. For more fruity Hammond organ sounds it would benefit to overdrive a Leslie speaker or alternatively, use a chorus or mild flange to duplicate that effect. Vocal sounds, in particular soprano voices, can be imitated by selecting a pulse wave and varying the cutoff frequency control and resonance to suit. If you find the voice effect too 'shrieky' further up the keyboard try backing off the keyboard track control. Vocal sounds also benefit from a touch of portamento, vibrato and reverb.
Both organ and vocal sounds utilise a fairly high resonance setting but things like strings usually have the resonance set at minimum and the cutoff frequency control sets the 'brightness' of the string sound. String sounds benefit from either multiple VCOs or from a touch of chorus and vibrato. If your synthesiser allows it, try mixing in a pulse wave whose width is being modulated by a low frequency sine or triangle wave as this will thicken the ensemble effect.
Filter sweeps are also easy to achieve using a patch such as in Figures 2a or 2b. The example shown in Figure 2a is, in fact, how most small prewired synths such as the Roland SH101, SH09, ARP Axxe etc., are internally patched whilst larger synths such as the Pro-One, MiniMoog and Moog Source are patched together as in Figure 2b where one envelope generator is devoted solely to filter sweeps and the other to the VCA for amplitude shaping. Whatever type of synthesiser one has, a few things should be borne in mind when setting up the filter controls.
If you want the sound to have a wide filter sweep, set the cutoff control fairly low (practically at minimum) and set the envelope generator (EG) modulation amount control fairly high. This will allow the voltage output of the EG to sweep through the full range of the filter cutoff frequency.
For more subtle tonal variations set the cutoff frequency control to any given point (say about halfway or whatever) and adjust the EG amount control so that there is just a hint of harmonic movement. In this example, the position of the filter cutoff frequency control sets the lowermost limit of the filter, so that no harmonics will be filtered out or affected below that point, whereas the EG amount control sets the uppermost range of the filter modulation. In this way quite a wide range of subtle tonal changes can be obtained which are ideally suited to muted brass, vocal, flute and various synth effects. Just listen to the work of Wendy Carlos or Larry Fast to see how effective this more delicate approach to filter modulation can be when compared with the more 'bombastic' tactics of other synthesists who go for wider filter sweeps.
The setting of the keyboard amount and resonance controls depend on the exact nature of the sound you are trying to create, as does the setting of the EG controls. Remember, also, that if you have the filter cutoff control set at maximum there will be no change in sound regardless of how much you change the EG amount control. This is because the filter is at its maximum frequency capabilities and is allowing all the harmonics to pass through and so no amount of modulation from the EG will affect the sound. Unless, that is, you have your synth patched up as in Figure 3.
Here, the EG is routed via an inverter which turns the otherwise positive going voltage output from the EG into a negative going voltage. Now, whenever a note is played, the output voltage goes down instead of up and so you must set the filter cutoff control fairly high. This allows the cutoff frequency to be swept 'downwards' and then 'upwards' to the point set by the cutoff frequency control during the decay/release portion of the note. This particular technique can yield some exciting effects and is available as a standard feature on some synths such as the Roland Juno 6/60 and Korg Polysix but sadly doesn't appear on some of the smaller (or even larger) synths currently available. If I had my way, all synthesisers would have this feature on them as it can be extremely useful.
Filter sweeps with a low frequency oscillator (LFO) as shown in Figure 4a operate on the same principles except for one important difference. The output from LFOs, in particular sine and/or triangle waves oscillate around 0 Volts — that is, one half of their cycle is positive going and the other half is negative going (see Figure 4b). What this means in practical terms is that you will have to set the cutoff frequency control to a point around which the modulating waveform of the LFO can sweep.
In other words, imagine that the cutoff point is to be the 0 Volts line; when the voltage increases the harmonics will be swept above that point but when the waveform goes into the negative half of the cycle the harmonics below that point will also be affected. Therefore, if you set the filter cut off frequency control too low you may find that you lose the sound altogether, as the sweep moves below the fundamental of the note during the negative sweep. So the settings of the cutoff control and the LFO modulation amount control are crucial, but with patience and experimentation the filter and all its associated controls can allow you to create some very exciting sounds and effects. This may all sound somewhat complicated but it looks worse on paper than it is in practice. Filters can be difficult to understand fully as there are so many variable factors which can affect their operation, but hopefully by bearing these few tips in mind when you are setting up a sound things might become a little easier.
Finally, let's have a look at some of the other things one can do with voltage controlled filters.
On many synthesisers it is possible to mix the outputs of two or more controlling devices for an even greater range of sounds. The voltage controlled filter is no exception. One could, for example, mix the outputs from two LFOs to create interesting rhythmic effects or mix the output from an EG and several LFOs.
One technique of which I am particularly fond is that of using a VCO in the audio range as the modulation source. By patching up as in Figure 5 it is possible to obtain a wide range of textures that would be impossible to create by any other means. By experimenting with the frequency of VCO 2 one can create curious vocal sounds, very realistic percussion sounds such as kalimba, glockenspiel and so on, as well as many interesting concordant and discordant bell and clang sounds. Many of the textures produced in this way sound very digital and complex. In this example of filter modulation, as well as subtracting harmonics and sweeping through them, VCO 2 is also adding harmonics and, for want of a better word, distortion in a variety of ways which makes for some highly original sounds.
Most of the filter effects described above can be obtained on even the most simple of synthesisers but are difficult to achieve on elaborate digital instruments such as the Fairlight or Synclavier. As such, it is these filter effects that characterise many analogue synthesiser sounds and even though they have become somewhat overused and a trifle cliche, they can still be very exciting and very effective if tastefully employed.