In Sound Design 4.1 (Chapter 4) we will begin with a square wave to approximate a square wave. On its own it doesn’t sound much like a clarinet, in part because of the intensity of the high frequencies. To help get us closer we used the ubiquitous modifier: the low-pass filter (LPF). Although a confusing name at first glance, its meaning is quite literal: a filter that allows low frequencies to pass through while blocking high frequencies that exceed a specified point, a location referred to as the cutoff frequency.
The LPF is of utmost importance to the circuit design of analog synthesizers in that its characteristics are largely what gives a particular instrument its unique sound. Moog, ARP, and Oberheim synthesizers (just to name a few) all were distinguishable by and revered for their unique filter qualities. Digital synthesizers therefore often allow the user to select from a variety of filter options to further customize this effect or choose a design that emulates a classic.
In this screenshot of Logic’s ES1 (Figure 3.10) the filter is currently set to 24 dB/octave “fat,” which is said to emulate the Oberheim filter. One step counterclockwise shows a 24-dB “classic,” which is modeled after the Moog filter. Below are lower-order 12- and 18-dB filters emulating the Oberheim SEM and Roland TB-303, respectively.
Figure 3.11
Audio Example 3.1
Why the differences in filter sound among manufacturers? An analog LPF requires the audio signal to be combined with itself at a slight delay through the use of a resistor–capacitor (RC) circuit that introduces a phase shift that increases the amount of attenuation as the frequency increases. The untouched area is known as the passband and the area being attenuated is called the stopband. Modifying the circuitry enables the steepness of the filter’s slope to shift in increments of 6 dB: 6 dB/octave is “first order,” 12 dB/octave is “second order,” 18 dB/octave is “third order,” and 24 dB/octave is “fourth order” (see Figure 3.11).
The introduction of a phase shift into the audio signal produces a distortion of the original wave in the transition area where the passband and stopband intersect. As the “order” increases, so does the intensity of this distortion. The contrasting circuit designs alluded to earlier generate
different artifacts in this region that can be quite musical.
Figure 3.12
Pink noise with LPF no resonance and 100% resonance (Sound Source: Logic’s ES1 synthesizer).
Figure 3.13
If this were the end of the LPF story, analog synthesis would be pretty boring. The behavior of the different analog filter designs is fairly consistent when simply attenuating high frequencies (see Figure 3.12). Where things start to get very interesting is when you turn the resonance control (sometimes called emphasis), and the true character of designs by Robert Moog, Alan R. Perlman (ARP), Tom Oberheim, and others, becomes evident.
Turning up the resonance control increases the intensity of frequencies around the crossover point (see Figure 3.13. At low levels this can add some bite and color to the sound; at higher levels the crossover area is far more prominent than the passband region resulting in a dramatic shift in timbre—when pushed far enough, the filter resonance may even begin to self-oscillate! A curious side effect here is that the filter may self-oscillate even if you turn off the VCOs feeding into it! Some great sounds are possible this way. Let’s look at how to use resonance in Sound Design 3.1 by using a free software instrument, TAL NoiseMaker. Figure 3.14 HPF orders from Spectrograph.
Sound Design 3.1—Exploring Resonance and Self-Oscillation Using Pink
Noise
High-Pass Filter
The high-pass filter (HPF) is essentially the opposite of the LPF in that it allows the high frequencies to “pass” through while blocking the low frequencies (see Figure 3.14). This is particularly useful for sounds that you want to emanate from the background of your aural soundscape or appear as a light texture. When you are layering sound, the HPF can be useful in segregating a timbre into a segment of the spectrum that spans from the cutoff frequency of the filter up through the highest overtones of the patch. The resonance characteristics on the HPF are different than those of the LPF but equally as important as a creative element.
As with the LPF, interesting results can be achieved by modulating the HPF with an envelope or a LFO. Many synthesizers do not include a HPF, but they are readily available in any digital
audio workstation or digital mixers and many analog mixers as well.