044 SECTION SIX: VCF

As the Fc is raised, the filter is ‘opened’ and more harmonics are allowed to pass through. The more the filter is opened, the brighter the sound. As the cutoff frequency is lowered, the filter is said to ‘close.’ One might think that the filter would completely block all harmonics above or below the Fc, (de- pending upon the type of filter) but this isn’t how filters really work. Remember that the Fc is the frequency at which the filter begins to attenuate harmonics.

In the Figure 6-3, the lines on the graph represent a harmoni- cally rich waveform. The vertical black lines show the har- monics of a made-up waveform. Their height represents the volume of each harmonic. The area shaded gray represents all of the possible harmonics which could pass through unfiltered. If the sound is unfiltered, then all harmonics of the waveform are allowed to pass through the filter unchanged, no matter how high or low they are. Remember that a single sawtooth wave has high harmonics which may fall just about anywhere along this graph, de- pending upon the fundamental.

Now, in an ideal world, a low- pass filter would entirely eliminate all harmonics above the Fc. This is not how filters work, however. If the signal shown above is put through a lowpass filter, the volume will begin to gradu-

ally decrease once the fre-

quency of the harmonics get higher than the Fc. It is important to note that if the fundamental frequency is too much higher than the Fc, no sound will be heard at all. This is the first thing a synthesist should check when troubleshooting a patch which uses the filter. If the Fc is completely closed, no sound will get through the filter at all.

In Figure 6-4, harmonics which fall partially outside the gray shaded area will have their volume reduced significantly (taller lines represent more volume). Harmonics which fall entirely outside the gray area will not be heard at all as their volume level will be so greatly reduced. So, a saw wave would sound less buzzy than normal, since it is the high frequencies found in a saw wave that give it its Figure 6-2: The filter’s controls

Figure 6-3: The harmonic content of a waveform

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Figure 6-4: The effect of a lowpass filter

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buzzy sound. As the Fc is lowered, more and more harmonics will be removed and a saw wave will begin to sound more and more smooth until finally, it will sound almost exactly like a sine wave. This is because as the lowpass filter’s Fc is lowered, more and more harmonics are removed. CD track 19 The difference between a sine and a saw wave is that the sine wave has no harmonics other than the fundamental, while the saw wave has lots of harmonics. This brings up an interesting point: What happens if a sine wave is put through a lowpass filter?

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Assuming that the sine wave being fed into the filter is pure and truly has no overtones, there should not be any change in sound at all until the Fc is moved so low that the cutoff slope is over the fundamental tone the sine wave is producing. Then the sine wave will gradually decrease in volume as the Fc is moved lower and lower until it cannot be heard at all. However, connecting VCO-2’s sine output to the filter and changing the Fc results in a surprising occurrence: One can hear upper harmonics being attenuated as the Fc is swept lower. This is because none of the waveforms that the 2600’s VCOs produce are perfect, and when the shape of the waveforms change, their harmonic content changes slightly as well. It is very difficult to produce a true sine wave using technology that was available at the time the 2600 was built, so manufacturers came as close as they could while staying within budget. How does a lowpass filter sound in general (when used with a signal other than a sine wave)? As the Fc is moved lower, the sound becomes duller as harmonics are attenuated and finally eliminated. In this respect, a filter is like the tone controls on a stereo or boom box. As the treble is decreased, the sound becomes duller. This sound can be heard on CD track 19

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As different synthesizer companies started working on different filter designs, they changed something about the filter. The rate at which a filter attenuates frequencies is called the cutoff slope. Most synthe- sizers use either a -24 dB per octave slope or a -12 dB per octave slope (sometimes written -24 dB/8va and -12 dB/8va respectively). Decibels are a measure of volume, which means that for every octave higher the sound is, a filter with a -24 dB/8va slope would attenuate the sound 24 decibels. This is a steeper cutoff slope than a filter with a -12 dB/8va slope.

Sometimes, filters are referred to by their poles. A pole is a measure of attenuation, or how much the filter can reduce the volume over a given frequency range. A pole is -6 dB/8va of attenuation. Thus, filters which employ the -24 dB/8va slope are called 4-pole filters while filters which employ the -12 dB/8va slope are called 2-pole filters. While this is certainly something which differentiates different filters, cutoff slope will not be considered to be a parameter at this time, as it is not possible to change the cutoff slope on the filter on the ARP. On a few synthesizers, it is actually possible to change the cutoff slope. Some filters even offer 1-pole filters for extremely subtle and gentle cutoff slopes.

It is also possible to chain filters together, and their effect is cumulative. One can just add the cutoff slope amounts together to find the cutoff slope of the combined filters. For instance, if two -12 dB/8va or 2-pole filters are chained together (the output of the first is fed to the input of the second) they will have the same sound as a single -24 dB/8va or 4-pole filter.