ARP2600 - Fundamentals of Music Technology - Cyborgstudio.com

SECTION SIX: VCF - 043This analogy is a good one because as the water passes through the handkerchief, some of it is absorbedby the handkerchief, so not all of the water is allowed to pass through. This holds true when passingsounds through a filter. While the unwanted harmonics can be removed, some of the other harmonicsend up being filtered out as well.When learning about filters, it is important to understand that there are many types of filters. Earlysynthesizers usually only offered the user one type of filter, but sometimes had as many as four. Modernsynthesizers, however, may offer as many as 36 different types of filters! The study of some ofthese more esoteric filters is, for the moment, beyond the scope of this first volume, but they will betaken up at a future time. It is also worth noting at this time that most older synthesizers had only onefilter on them, but modern synthesizers sometimes have as many as 128 independent filters!WHAT DO FILTERS DO?The exact electronic workings of a filter are unimportant at this time. It is very important, however, tounderstand the function of a filter. As stated above, a filter removes unwanted harmonics, along with afew of the wanted harmonics. However, one cannot pick and choose exactly which harmonics onewants to remove. To take the next step in understanding, one must first understand that there are fourbasic types of filters, each of which performs a specific job.The four basic types of filters found on older synthesizers are lowpass, bandpass, band reject andhighpass. Of the four, the lowpass filter is by far the most common. The ARP 2600 has one lowpassfilter, but has no other filters. (The noise generator has a lowpass filter on it, but it is dedicated to thenoise generator’s output, and cannot be used for general purpose filtering.) Each type of filter is capableof filtering different ranges of harmonics. Some filters have the circuitry for some or all of thesedifferent types of filters. When a filter can operate in more than one mode (for example highpass andlowpass) it is said to be a multimode filter. Some synthesizers have filters that can actually perform allof these types of filtering simultaneously!LOWPASS FILTERSLowpass filters filter out high harmonics. At first, ‘lowpass’ may seem like a strange name for a filterthat attenuates high harmonics, but it does make some sense. Filters are named by the information theyallow to pass through rather than by the information they remove. So, a lowpass filter will allow allinformation below a certain frequency to pass through, while a highpass filter will allow all informationabove a certain frequency to pass through.Although filters have several parameters, the most important is the cutoff frequency. Cutoff frequencyis abbreviated Fc. Cutoff frequency is the frequency at which the filter will begin to attenuate thevolume of harmonics. This attenuation is actually how a filter filters out harmonics. On the ARP 2600,Fc is set using two controls. There is an INITIAL FILTER FREQUENCY slider, and a FINE TUNEslider, which function much like the corresponding sliders on the VCO’s. Instead of determining thefrequency of an oscillator, however, here they determine the Fc. This is yet another example of amaster-submaster relationship. These controls can be seen in Figure 6-2 on page 44.