Exploration
Technique : Effects / Processing
PART 4: PAUL WHITE looks at how MIDI can be used to turn static effects into dynamic ones. This is the fourth article in a five-part series. Read Part 1, Part 2, Part 3 and Part 5.
Last month, I examined the ways in which the various effects blocks found in a modern multi-effects processor can be connected to produce different results, and I also touched upon the idea of effects being modulated by envelopes or LFOs (Low Frequency Oscillators). This month I'd like to expand further on the modulation theme, and see what sort of effects can be achieved using MIDI control.
There's a tendency to think of effects as being pretty static -- you plug them in, and straight away you have delay, chorus or reverb that you can add to your sounds. However, modern devices offer much more scope: the modulation and/or MIDI control facilities built into some of the more sophisticated units enable effects to be altered in real time, either under the control of MIDI or an LFO, or in response to the dynamics of the input signal. In many ways, there's a close analogy between how this type of modulation works and how analogue synths produce their sounds, so if you have some experience in synth programming, effects boxes should hold few mysteries.
Just as a synth can have its filter opened or closed according to an envelope, a digital effects unit might be able to produce a chorus effect that slows down as the input signal level decays, or perhaps the chorus might also get deeper as it gets slower. A lot depends on the number of modulation sources and destinations a machine provides, and how many may be used at one time, but even with a little imagination and a fairly basic effects unit, it's quite possible to produce morphing-style effects, where you hear one type of effect when the input is loud, and this then changes into a quite different effect as the input level decays. Such effects are probably best suited to use with instruments such as guitars or other natural sounds -- with synths, we're so used to hearing impossible things that even an effect of world-shattering proportions could go completely unnoticed! However, effects units are now starting to include resonant filters as standard, and by linking these to a MIDI triggered envelope, or by making them track the input signal's envelope, you can add synth-like filter sweeps to instruments that have no filters of their own.
It may also be possible to use an external volume pedal to control some effects parameters in real time, and though the most obvious things to control are wah-wah and volume effects, you can produce some fairly sophisticated results by making the pedal change several parameters at once. There is an important point to make about modulation, though: it's not usually desirable to make a parameter shift over its complete range in response to your control stimulus. More often than not, you'll want to be able to set upper and lower limits between which the parameter will change in response to the full travel of a pedal, or to the sweep of an input-tracking envelope follower. Fortunately, any effects unit with worthwhile modulation facilities will include a facility to let you define your own limits.
MIDI
The majority of effects units, other than the very cheapest models, come with some form of MIDI control, but you'll have to check your manual to see exactly what that allows you to do. At the very least, you should be able to change patches or mute the effects output via MIDI, but you can usually do a lot more. Some effects units will have a full set of MIDI In, Out and Thru connectors, whereas others may have a combined Out and Thru. Most machines will let you do a SysEx (System Exclusive) dump of individual or global patch data, and this is easily saved on a sequencer, so it's a good idea to back up any of your own patches in this way, just in case the unit fails or the internal memory battery dies on you. You can also transfer patches directly to another effects unit of the same type via SysEx, providing both machines are set to the same SysEx ID number.
Another common feature is the patch map or allocation table, which allows you to decide which patches will be called up by which MIDI Program Changes, rather than patch 1 always being tied to Program 1 and so on. The idea behind this is that for live use you can team your effects unit with a MIDI keyboard, then create a table so that whenever you call up a new patch on the synth, an appropriate effects patch is loaded to complement it. If you're working from a sequencer this isn't so important, as you can specify effects patches in the same way as you set up synth patches. I have to say that I've never yet had the need to set up a Program Map, but at least you know it's there now!
Things start to get more interesting when you begin to control effects parameters themselves over MIDI. Depending on the effects unit you're using, this may be done using SysEx or MIDI controllers, the latter being by far the easiest to manage, as you can set up a page of controller faders in your sequencer, or use a keyboard controller such as a joystick or wheel. The number of controllable parameters available depends entirely on how the effects unit is designed -- some budget units may give you only a couple of MIDI-variable parameters per patch, whereas some of the more sophisticated boxes let you access virtually any user-adjustable parameter over MIDI. Whether MIDI control of all of these makes sense is another matter, but at least you won't find your way barred by a limited operating system!
"Just because a parameter can be varied in real time, this doesn't mean that it will sound OK."
If your machine uses only SysEx to control effects parameters, don't panic, because there's a fair chance that if you can adjust the desired parameter using a data wheel on the front of the unit, you'll also get real-time SysEx transmission via the MIDI Out socket at the same time. In other words, you make the changes on the effects unit front panel and record the changes directly to your sequencer via MIDI. Note that with most machines you have to go into a MIDI setup page and switch on SysEx transmission.
Because SysEx messages can get quite complicated, there's a limit to how many parameters you can control at the same time before the data stream gets clogged up, in which case the result will become pretty jerky. Two or three simultaneous changes is probably as good as it gets using SysEx, whereas MIDI Controllers are less greedy. However, you may be able to map several parameters to change in response to a single controller, and that doesn't take any more MIDI data.
When using controller data, it can sometimes be tedious to map the required controller to the parameter you want to change. Some units have a so-called MIDI learn mode, which makes this job a lot simpler. All you do is choose the parameter you want to change, then move the physical controller you want to use to control it, and the effects device will recognise the controller and assign it for you. The exact procedure varies from machine to machine, but it doesn't get much more complicated than that.
Because MIDI controllers can leave your parameters set to non-standard values, most sensibly designed effects units respond to the MIDI command 'Reset all Controllers', enabling you to set any patch to neutral after having made changes to it via MIDI. Parameters will also be reset when you change to a new patch.
In addition to using MIDI to make continuous parameter changes, you may also find that you can use MIDI Clock to synchronise time-related functions such as LFOs, delay times, and so on. This is a very useful feature to have, as you can create stock effects that will always work in time with a track, no matter what the tempo is. It also means that you can include tempo changes in your songs and the effects will stay in sync. Typical sync effects would include tremolo, vibrato, panning and delays.
MAKING CHANGES
If you've ever tried to switch to a new effects patch during a mix, you'll almost certainly have noticed that most units don't switch patches very smoothly, but leave you with a rather clumsy gap at the changeover point. If you have a suitable gap in the track, that's the best place to change patches, but in some instances it's possible to use MIDI parameter changes to make the current effect change into a suitable new effect without having to switch patches, and without glitching. For example, if your effects unit lets you create patches using two parallel effects blocks, you could use MIDI to control the level of each block. To create a change from delay to chorus, you'd set up chorus in one block and delay in the other, then use MIDI to turn down one block at the same time as turning up the other. This could be done quickly or as a smooth crossfade. You may also find it useful to use the MIDI master volume facility in your effects unit to automate effects level from your sequencer.
SUMMARY
The concept of using modulators or real-time MIDI control to add dynamic automation to effects is reasonably straightforward, but to get the most out of some of the more sophisticated effects boxes, you'll have to spend as long learning to use them properly as you would a synth. When creating a brand-new effect, it helps to draw it out in block diagram form on paper, then see if you can create a routing within your effects processor that will let you do the same thing. You may also have to verify that your unit can run all the blocks you need at the same time, because a great many devices use shared processing resources to generate the individual effects. If you happen to use a couple of processor-intensive effects in your patch, there may not be enough processing capacity left to help you achieve your aim. Sometimes you can compromise by using a less elaborate reverb, or if you need delay and pitch-shift at the same time, you may find there's enough delay available within the pitch-shifting algorithm to do the job without you having to use a separate delay block at all.
If you're going to get into effects programming in a big way (or synth programming, for that matter), I'd recommend you consider using a computer editing system if one is available for your machine. Even if you can't get a dedicated editor (or universal editor module) for your specific effects unit, you might find that you can go part of the way by creating a MIDI controller map in your sequencer to provide access to the main parameters. All you need to do is create one patch with the desired MIDI controller assignments, then use that patch as your starting point when making edits. Some tasks, such as assigning effects blocks and changing routing, may still have to be done from the front panel, but you should still be able to use controllers to fine-tune the effects.
ZIPPER NOISE
Just because a parameter can be varied in real time, this doesn't mean that it will sound OK. There's a phenomenon known as 'zipper noise' that occurs to a greater or lesser extent in all digital systems where real-time parameter change is involved, and it comes about because, unlike analogue equipment, digital devices can't produce perfectly smooth changes -- everything happens in a series of small steps. If the steps are small enough, the change sounds smooth enough to fool the ear, but if the steps are large, you can hear them change with a kind of rasping or ticking sound -- hence the term zipper noise. To change some parameters requires the processor to calculate a lot of data in order to work out what to do, so you may find that trying to change chorus depth very rapidly produces audible and unpleasant side effects. This is because the time needed for calculation means the steps can't be as close together as perhaps we'd like them to be. Usually the processor's manual will warn you which parameters are most susceptible to side-effects, and you may also find that there's a lot of difference between different models of machine due to the different algorithms used, and whether or not they include something called interpolation, to smooth out the steps.
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