From the April 1989 issue of Radio And Production

The MIDI Page

by Todd Albertson

Well friends, I've got some good news and some bad news... (Don't you hate that?) The bad news is that if you did not read last month's MIDI PAGE article, chances are good that this month's article will leave you up that well known tributary, without means of locomotion. The good news is that if you stuck with me through last month's article, we are about to hand you a paddle!

As I mentioned last month, a synthesizer is best understood as a collection of independent modules, "black boxes", if you will. The sound of the synth originates in its source module or oscillator and winds its way through all the other modules which serve to add, subtract, or alter the sound in some way. These modifying modules are the topic of this month's discussion.

The easiest to understand of course, is the Voltage Controlled Amplifier (often Digitally Controlled Amplifier). The VCA/DCA modules are usually, but not always, the last modules in the series. The sound passes through this module so you, as the adept synth programmer, can compensate for all the volume changes that have taken place in the other modifiers. Without this simple "black box", all your patches would end up at different volumes, and switching from one patch to another would mess up your carefully set recording levels. Think of this module as a fancy volume control. A real toughie, that one! Other applications of this module are discussed in the following paragraph.

Next, we come to the LFO or Low Frequency Oscillator. It simply generates a low frequency wave, quite often lower than human hearing would recognize. We never hear this module directly however, and this one may confuse you at first, because it does not modify the sound itself. Rather, it is used to control other modifying modules. For instance, by applying the output of this module to the controlling input of the VCA (volume control), a programmer can produce a vibrato much like the Fender guitar amps of the late 1960's. When the LFO wave is cresting, the volume will be loudest, and conversely, the LFO wave troughs will produce lower volumes. The RATE of this rising and falling is controlled by the LFO's frequency (sometimes called "Speed"), and the AMOUNT of change is controlled by the LFO's amplitude (usually called "Depth"). Please do not start to think of LFO's as applicable only to volume. Like most modifiers, this one may be applied to several other modules, such as filters, Q, and even envelope parameters.

Another simple module is the Filter (older synths Voltage Controlled Filter). This module often seems to be something like a treble control on your stereo system, and indeed it is, but beware, for it is also much more. The filter's purpose in life is to remove high (usually) frequencies from whatever sound passes into it. The "cutoff point" is determined by the programmer. Usually, higher settings allow more high frequencies to pass, while lower settings remove them. The tricky part is that the CUTOFF POINT CAN ALSO BE CONTROLLED BY OTHER MODULES! Uh oh! Here's where I usually see the light start to dim! The best way to illustrate this concept is to look back again at our LFO. Suppose we apply the output of the LFO to the Filter. What sort of sound do you imagine it would produce? Try to conceptualize this before you run off and implement this on your own machine. The result is very much like moving a treble control up and down rapidly (depending on LFO speed), over and over again.

I mentioned that the filter was much more than a treble control. The reason for this is the filter is only one half of a dynamic duo. Batman and Robin are in this case known as Filter and Resonance (or Q). Resonance is able to affect the sound very little (if any) without the simultaneous use of a filter. Trying to explain this particular black box turned into such a nightmare that I actually ended up calling specialists for technical information on the circuits involved. Ron Desantis, a Detroit area synth service technician, and Brian McLaughlin, engineer at Ensoniq, were gracious enough to provide some of the additional technical data needed. Fortunately for you, "Q" is much easier to use than to explain! To the modern human ear, the addition of resonance causes the output to sound more "electronic". One way to accomplish this is by using a "feedback circuit" from the filter output back to the filter input. If you "cutoff" everything above say, 500 hz., then send the filter's output back into itself, you would think that this would increase all frequencies below 500 hz.. Surprise! There's a twist to this little scheme in that the lower frequencies tend to be out of phase on their return to the filter. This means that they work against the original signal and diminish the low frequencies while the volume of the frequencies near the cutoff point of the filter INCREASES. The effect is to accentuate the frequencies near the filter cutoff point. In these "feedback circuits", it is possible to increase the amplitude of these "Q" frequencies to the point of self regeneration. In other words the feedback circuit provides enough amplitude to the filter to become an oscillator. This is sometimes useful in producing special effects. Once again, the humble Juno 106 by Roland is an excellent instrument for such procedures. Nowadays of course, there are several other ways to produce the non oscillating effect of "Q". Many synths (Ensoniq's included) are software based, and all this magic is accomplished with numbers and computer chips. The effect is still the same: A "peak" is formed near the frequency cutoff point of the filter, and the sound is often more "electronic".

Ayyiieee! I can hear the curses now. What is this guy trying to do to our heads?

Now suppose we were to go ahead and set up a nice strong Resonance in our Filter and then allow the LFO to modulate the frequency cutoff point in the Filter. Ayyiieee! I can hear the curses now. What is this guy trying to do to our heads? Sorry... and welcome to the world of modern music! In the example above, you will (with a little adjustment) achieve a very passable "sweeping" sound from your synth! AHAH! Your first sweeper!! The cutoff point moves up and down, modulated by the LFO with the "Q" accentuating the frequencies near the cutoff point (which as I said is constantly moving). The effect is much like the old "Wah Wah" pedals isn't it?

I'm afraid I'll have to cover the other modifying modules (envelopes, etc.) next month. I was hoping to get through them all here, but this article is going to get me in trouble with the editor already (too long). We will get into MIDI computer code the month after next, I swear! Questions about synths, and MIDI, can be directed to me through RAP, or you may call my computer Bulletin Board Service (if you have a modem) 10pm 6am, nightly, at 313 544 0405, and leave a message.