ARP2600 - Fundamentals of Music Technology - Cyborgstudio.com

048 - SECTION SIX: VCFreason for this will become clear in a moment. For now,just notice what is normalled to each of the control inputjacks. The VCF’s five audio inputs and three control inputscan be seen in Figure 6-5.The left most jack is normalled to the keyboard CV (thiswill be explored in a moment). The middle control jack isnormalled to a module that has not yet been discussed. Theright most jack has been normalled to VCO-2’s sine output.Again, it is important to note the normals, as they indicatewhat will be connected to the control jacks mostfrequently. Also notice that the filter’s output is normalledto the mixer’s input.KEY TRACKINGFigure 6-5: The VCF’s audio and control inputs As mentioned before, when the fundamental frequency ofa sound gets higher than the set Fc, some higher harmonicswill be attenuated changing the patch’s timbre as it gets higher. If the fundamental frequency is toomuch higher than the Fc, no sound will be heard at all, since even the fundamental will be entirelyfiltered. So it would seem that one must be careful to set the Fc higher than the highest harmonic oneintends to create. This can quickly become more trouble than it is worth. No one wants to stop to decidewhat note is the highest they might play, since this really does not invite spontaneous or creativeplaying. Fortunately, the ARP’s designers came up with an inventive way to get around this problem.Key tracking works by sending a copy of the keyboard control voltage to the left control input on thefilter. As higher notes on the keyboard are played, more voltage flows into the control input. As wasmentioned a moment ago, the more voltage flowing into the filter’s control inputs, the higher the Fc.Thus, the problem of the fundamental tone of different sounds is solved. Note that key tracking is arather fine adjustment, and not anything as drastic as moving the filter’s initial frequency slider even asmuch as a centimeter. It is easy to observe key tracking in action by using a dummy plug to break thekeyboard CV normal to the filter’s control inputs. When this normal is broken, sounds become dullerand duller as higher and higher notes are played on the keyboard. This can be heard on CD track 22.HIGHPASS FILTERSAlthough the ARP 2600 has onlya lowpass filter, it is important tounderstand the usefulness of theother three filter types, as they willcome up time and time again in thestudy of music technology. Thesecond most common filter typeis the highpass filter. As its nameVolumeFundamental20 Hz FrequencyFigure 6-6: The highpass filter in action20 kHz