Guide to Electronic Music Techniques (Part 2)
Synth Performance Controls
Last month we looked at some of the various performance control hardware which manufacturers provide for us on their machines in order that we may exploit the full potential of the synthesiser as a musical instrument. The performance control is a device that can be used to modify a certain parameter of the instrument's sound whilst it is actually being played, thus it has to be ergonomically positioned on the instrument's control panel, and it must afford the right 'resolution' of control - what's more, it must feel comfortable and natural to use. This month we shall continue to look at some more types of performance controls.
The devices were developed by ARP for their range of monophonic synthesisers. Essentially these consist of small rubber pads that cause an element to change its electrical resistance when pressed. So it is a simple matter to design a circuit which will turn this varying resistance into a control voltage. ARP generally incorporated three pads onto their instruments - one for bend up, one for bend down, and the third for introducing modulation. ARP were rightly proud of their pads as they were very pleasing to use and by applying different amounts of pressure to specific areas of the pad you could very accurately determine the amount of effect you were going to induce.
Roland are the main supporters of this form of control mechanism, which is essentially the same as the sprung wheel, only mounted from left to right, not up and down. Again this device suffers from the problem of not being able to be left in a 'bent' position, but otherwise most people quite like it.
This is probably the most obvious existing control device that could have been incorporated into the early synthesiser designs, however very few manufacturers saw it as a good idea (except for Korg, Polyfusion, and a few others). On the surface a joystick seems like a perfect mechanism to control the various parameters - you can use one plane (normally the X-plane) for pitch-bend, and have it centre sprung, if desired, and use the Y-plane for modulation - one manufacturer (Jeremy Lord) on his ill-fated Skywave synth utilised a 3-way joystick, with rotation of the stick being the third movement, and he used this as a volume control (see Figure 4). The joystick is one of the best devices to go for if you are building your own synth, and aren't too keen on mechanical engineering - you can easily obtain ready made mechanisms, and you can decide as to which control pots you want to use.
Some musicians, however, tend to dislike the joystick, possibly because it is too clever - preferring to have separate controls for pitchbend and modulation in order to see exactly what they are doing, even if it does mean doing the splits with their fingers.
We've now looked at the most popular control mechanism. However, there are one or two more worth mentioning, if only for the reason that they should be, if possible avoided. The rotary pot with a deadband, but no centre detent, is an undesirable control. It is often used as a pitchbender and operates either side of a central deadband - anticlockwise rotation lowering the pitch etc. This device has two problems - first it is awkward to use, and has no 'feel' to it; and second you can find yourself not returning the control exactly to its central deadband, so that consequently the synth goes out of tune.
The slider with the centre-detent is also very tricky to use effectively - sliders operating about a central position are clumsy to control. A slider can however be used as a pitchbender 'up' only from its stop position; in this mode it is perfectly easy to control (this might also appeal to the home constructor).
There is a long running debate concerning these types of keyboard as to whether they should be called touch 'sensitive' or touch 'responsive'. It would seem more logical to class them as being responsive to touch, but the general consensus seem to classify a keyboard whose action can be used to produce a modifying control signal as being touch sensitive. A touch responsive keyboard is considered to be one with no moving parts - i.e. one whose keys consist of a series of touch pads, that respond to 'fleshy contact', e.g. as on the Wasp and Gnat synthesisers. That then is the authoritative statement on the subject, but be prepared to find some musicians who use the terms round the other way.
Let's start by examining the velocity sensitive keyboard. This is the electronic design engineer's method of producing a keyboard that will generate a control signal that varies with the strength the note is struck. In effect the keyboard is responding to the speed at which the note is depressed rather than to how hard it is to play, but most of the time there is little difference between the two. The way in which this device functions is, like all good ideas, simple. There are two contacts provided for each key, arranged such that one contact breaks before the second one makes. As the first contact breaks, a capacitor is allowed to discharge until the second contact makes and the decay is halted. It therefore follows that the faster the key is played, the less time the capacitor has to discharge, and thus the greater the charge, hence the control signal, remaining on the capacitor. This is the basic principle behind the velocity sensitive operation. However, various manufacturers do have variations on this theme and certain electronic piano manufacturers (e.g. Yamaha and Crumar) have their own LSI chips incorporating touch sensitive circuitry.
The second type of touch sensitive keyboard is of the force sensitive variety. Here there is a sensor of some type present at the bottom of the key's travel. So, having played the note, by pressing harder against this sensor, a control voltage or signal can be generated. Unlike velocity sensing, force sensing is generally a monophonic effect, i.e. there is just one sensor for all the notes of the keyboard. This is generally due to reasons of cost. In many cases you will find that the application of this force, or second touch as it is sometimes called, causes the entire keyboard to move, which can be a bit disturbing if you aren't used to it. This movement is because of the arrangement of the force sensor and makes it somewhat easier to incorporate into the instrument's overall design. One such example of this dipping keyboard can be found on the new CE20 from Yamaha.
One of the main advantages of the touch sensitive keyboard is that it doesn't require the use of your second hand - you are still controlling the sound - pitch and performance effects, so you still have a hand free to make further modifications to the sound, or to use to play another instrument.
Pitchbending up and LFO modulation are the most common uses for the force sensing keyboard. Sometimes the velocity sensor is generally routed to the envelope generator/voltage controlled amplifier so that the output level of that note can be determined by the speed at which it is struck. This isn't always the case, as the velocity signal is often used to open the voltage controlled filter's cutoff frequency, This leads to the sound becoming brighter, which in turn makes it sound louder to our ears, therefore this effect has a similar result in terms of amplitude to routing it to the VCA, but with the added 'bonus' of enabling the player to simultaneously emphasise a note by brightening it. By sending this velocity control signal to both the VCF and VCA we can get as close an approximation to the action of an acoustic piano's keyboard that is financially realistic (i.e. without spending thousands).
The velocity control signal can also be used in conjunction with the filter to produce those characteristic overblow effects that are associated with woodwind and brass instruments; these can be further enhanced by using the force sensor to add filter and oscillator vibrato (if your synth is fitted with both features).
So, you can see that there is more to touch sensitive keyboards than you may realise. Their role in synthesis is most important, especially if you consider you've still got that spare hand if you are using a mono synth. It is more natural to have just one hand controlling the various parameters of the sound than, as with the more conventional performance controls, splitting the functions so that one sets the pitch and the other is used to modify it; a pianist has control over each note with each finger, and you don't hear him complaining!
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