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Back to Basics (Part 5)

More on filters and envelopes from the educational pen of Steve Howell.

Our beginner's guide to synth programming continues with a quick peep into the world of envelope shaping and filter sweeping.

Last month we saw how a synthesiser can be used to 'shape' a sound, simply by the user applying a rising and falling Envelope Generator (EG) voltage to a Voltage Controlled Amplifier (VCA) to vary a sound's amplitude during the course of a note.

Now, it so happens that in addition to a sound's amplitude altering during the course of its existence, its tone usually varies to a greater or lesser degree in accordance with those amplitude changes. More often than not, the louder a sound is, the brighter it will be. Why? Well, the reasons boil down to a question of energy. Play a musical instrument with vigour, and you supply more energy not only to the overall level but also to the harmonics, especially the higher ones. Result: a sound with a significant amount of extra top end information.

Another acoustic phenomenon, also connected with principles of energy, is the way these higher harmonics die away more rapidly than lower ones; they simply don't have enough energy to sustain for as long a period of time as the lower harmonics and cannot, therefore, survive as long.

Well, I'll be quite frank with you and admit that the above is a highly summarised version of events as they occur in real life, but it's as much as you'll ever need to know in the field of synth programming, so don't panic.

As we've already discovered, an EG is nothing more than a voltage generator whose output voltage is shaped using a synth's ADSR (or whatever combination of initials you happen to have) controls. Normally, this module does nothing whatsoever without receiving a gate/trigger pulse of some description, this being commonly derived from a keyboard. It's this pulse that initiates an envelope transient which, in turn, is applied to the VCA so that the sound can be given an envelope shape.

So, given that the EG is nothing more than a voltage generator, it follows that its output can be applied to any voltage-controllable module within an analogue synthesiser for any number of programming purposes - so long as your synth has the sort of routing versatility such operations demand. For the moment, though, we'll confine our activities to the application of the EG to the control input of a Voltage Controlled Filter (VCF).

Figure 1. Block diagram of simple synthesiser, with one EG being used for both amplitude and tonal shaping.

Envelopes & Filters

Let's kick things off with a good ol' diagram. Figure 1 shows the layout of a simple synthesiser such as a Roland SH101 or any of its Juno series polysynth derivatives. The important point to note is that the EG is connected not only to the VCA but also to the VCF. It's this arrangement that allows the creation of the strangled duck effects made (in)famous by the unimpeded musical tastelessness, of a few mid-seventies keyboard players.

Figure 2. Response slope of a typical VCF - the shaded area represents the sound we actually hear.

Just in case you missed E&MM March, the response slope of a typical VCF is given in Figure 2. As you can see, the higher harmonics are drastically attenuated above the cutoff point, allowing only the lower harmonics to be heard. The Cutoff Frequency control on the front panel of your synth lets you choose the exact location of that point, giving rise to all manner of timbral manipulations (tone changes, dummy!). If you want a graphic illustration of what sort of effect varying the Cutoff Point actually has, place the left-hand edge of a fag packet at an angle of 45 degrees over the diagram and move it from left to right - you should see harmonics being lopped off at intervals as you move the packet back and forth. The very fact that a synth's filters are voltage-controllable means that the cigarette packet can be replaced by a device that generates a voltage of some form to do the job automatically. Enter the EG, a wondrous piece of electronic wizardry that accomplishes all the tasks required of it, and as a bonus, happens to be cheaper (in the long term), healthier and less anti-social than 20 Benson & Hedges will ever be.

Given that as a sound dies away into oblivion, its upper harmonics are first against the wall, it follows that if we sweep the VCF with the same voltage that's opening and closing the VCA, the upper harmonics will fade along with the sound's amplitude, as the voltage sweeps downward through the filter's cutoff range. Thus, the sound becomes 'softer' as well as quieter.

Well, that's the theory anyway - in practice there are a couple of things you'll need to bear in mind if you want your filter-sweeping activities to be the envy of all and sundry...

Sweeping Statements

Back in March, we mentioned the fact that a filter has a summing amplifier at its control input that allows you to mix voltages together. The Cutoff Frequency control does nothing more than route a DC voltage through this summing amp, and all other voltages are then added to it so that, in effect, the control sets the working area of the sweep. And in the case of the EG, it sets the lowest level of cutoff, whilst the EG Modulation Amount sets the upper limit of the sweep.

Let's say the uppermost limit of the filter is set by pumping five volts into it. Setting the Cutoff Frequency control to maximum will do just that, and any sound you choose to route into the VCF will pass through unaffected. Now let's say you want to sweep through the filter - simply turning up the EG Modulation Amount will have no effect, because no matter how much additional voltage you ram into the filter's control input, its effect will be ignored by a filter that's already at its maximum setting courtesy of the DC voltage from the Cutoff Frequency control. So if you want some kind of sweep, you've got to decrease the setting of the Cutoff Frequency control to give a point from which the EG can take over.

What we end up with as a result of all this is a couple of rules of thumb for those aiming to get the best sweeps from their synth. Very wide sweeps are best achieved by setting the Cutoff control quite low and the EG Modulation Amount relatively high, but the procedure for more subtle sweeping effects is a mite more involved. You're best off setting the Cutoff point fairly high and then adjusting the EG Mod Amount to give the exact degree of sweep you're after. Remember that you can set the Cutoff control higher than the EG Mod Amount and still hear a sweep; an arrangement comprising the former control on full and the latter on minimum will result in the filter being almost fully open, with the application of the EG's voltage opening it still further.

Sustaining Interest

Now, unfortunately, we come to another complication. Whilst we've discovered that the Cutoff control governs the lowest range of an EG's sweep and that the EG Amount control governs the upper regions, there is in fact a further level control to consider.

Imagine you've set the Sustain control on your EG to halfway. The tone of the sound will come to rest at that point as you keep your finger(s) on the keyboard. But maybe that sustained sound isn't what you're looking for: and if it isn't, there are three possible ways of altering the status quo. You could adjust the Sustain level directly, but that won't necessarily give you the amount of sweep you're looking for. Alternatively, you could alter the Cutoff Frequency control, but this could alter the character of the sound quite drastically, as could adjustment of the EG Mod Amount parameter. Unfortunately, I can't supply any concrete answers to this predicament - all I can say is 'listen for yourself, and bear in mind you might need to alter a couple of other parameters in addition to the three mentioned above to get exactly the effect you're after.

Figure 3. Inverting an envelope shape.

OK, time for yet another complication. Many of the more recent synth designs have a feature that allows you to invert the EG voltage, a process that simply turns a normally positive-going voltage upside down so that it's going to a negative value - see Figure 3. In fact, all this means in practice is that you have to turn all my words of wisdom (!) upside down: set the Cutoff Frequency control high so that the EG has a point from which it can sweep downwards (remember the summing amp!). If you set the control too low, adding the negative-going voltage will take the filter out of its lowest range and the whole sound will be filtered out, leaving you with one hell of an embarrassing silence...

Set the controls correctly, however, and inverted envelope sweeps can give a huge range of extremely effective sounds - especially on polysynths.

Figure 4. Layout of more complex synthesiser, with two EGs for independent shaping of VCF and VCA.

Extended Options

Thus far, I've been looking only at budget synths that have just one shared Envelope Generator built into them. But when it comes to more upmarket (though these days, that doesn't necessarily mean they're all that expensive) designs, a pair of EGs is more the norm - a typical layout is shown in Figure 4.

Not unexpectedly, this arrangement offers a good deal more in the way of versatility than the shared shaping option, because although having just the one EG to cater for both amplitude and tonal shaping sounds like a reasonably logical idea, really complex envelope structures (such as those exhibited by most acoustic instruments) are well-nigh impossible to achieve using such a simple bit of design. The crucial factor is that many acoustic instruments display tonal changes that take place separately from changes in amplitude - and sometimes, the difference between the two is quite marked. A gong, for example, has a distinct harmonic composition when it's first struck, but an increase in upper harmonics as its sound dies away.

Recreating this sound on a synth with only one EG would be distinctly tricky, but relatively straightforward on a machine equipped with two. All you need do is set the attack on the VCA-linked EG to instantaneous (for the initial part of the acoustic sound) and adjust the VCF's EG attack to a fairly slow setting (for a subsequent build-up of harmonics as the EG sweeps upwards through the filter's range).

And, of course, these principles can be applied to completely 'new' sounds as well as recreations of acoustic timbres. It could be, for instance, that you're looking for a sound that has a downward filter sweep but whose amplitude shape has a soft attack, in which case the gong patch above is reversed: the filter EG's attack time should be instant, while the amplifier EG's attack time is slowish. Thus, the filter will be swept downwards, the VCA going through its attack cycle when the filter is going through its decay period.

If you've sat through the last few paragraphs in the knowledge that your synth has only one Envelope Generator, don't despair. A lot of people with dual-EG synths set the two envelopes up in a similar fashion anyhow, since there's still a lot you can do with a single module. We haven't exhausted the EG's possibilities, either, so stay tuned till next month...


Read the next part in this series:
Back to Basics (Part 6)

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - May 1985


Synthesis & Sound Design


Back To Basics

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 (Viewing) | Part 6 | Part 7

Feature by Steve Howell

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