Plan de Mantenimiento Después del Cierre

1. Which parameters of the noise generator can be modulated?

2. Can the noise generator be a carrier? Can it be a modulator?

3. Name the parameters of the noise generator.

4. State why white noise sounds like it does. Include in your answer a discussion of harmonic content and level as well as human hearing.

5. State why the noise generator will be of great importance later on.

6. Draw a picture showing a patch in which the noise generator frequency modulates VCO-2. Number and list the parameters of this patch.

7. Draw a picture showing a patch in which the noise generator is modulating the pulse width of VCO-2. Number and list the parameters of this patch.

8. What sorts of sounds generally benefit most from the addition (or use) of noise?

9. State which parameters in a PWM and FM patch involving VCO-2 and the noise generator have a master-submaster relationship.

10. To which inputs on VCO-1, 2, and 3 is the noise generator normalled?

11. Give an example of the noise generator’s output being used as a control signal and being used as an audio signal.

T

T

K

:

Filter

Low Frequency Noise Noise

Noise Generator Pink Noise White Noise

6

SECTION

042

VCF

A B

I

S

S

The filter is unlike the rest of the modules mentioned thus far in that its primary function is not to produce sound, although it can be used for this purpose. Rather, it is used to change and shape other sounds being made by the other modules of the synthesizer.

The Voltage Controlled Filter (VCF) is perhaps the single most important part of a synthesizer, because it determines the overall sound of the synthesizer and opens the doors to a new method of synthesis: subtractive synthesis. Subtractive synthesis is a method of synthesis which starts with one or more harmonically rich waveforms from which some harmonics are then removed. An oscillator usually produces this harmonically rich waveform, (usually a saw or pulse wave, but sometimes a triangle wave) but it is the filter that performs the task of eliminating some harmonics. The filter has several audio inputs, so the outputs of several oscillators can be connected to just one filter using patch cords. The oscillator’s output is filtered, and then comes out of the filter’s output jack. A block diagram of a subtrac- tive synthesis patch can be seen in Figure 6-1. Notice that every VCO has an output nor- malled to the filter.

It is important to under- stand that filtering is not like frequency modulation. The two can easily be confused since the FM jacks on the VCO’s are aligned perfectly with the audio inputs on the filter. Both also have attenuation sliders above them, which only adds to the confusion. However, in FM, an incoming control signal modulates the frequency of the oscillator, but does not pass through the oscillator. On the filter, signals coming into the audio inputs are actually modified by the filter and then passed through to the speakers or another module as shown by the dotted line in Figure 6-1. Notice also that both modules are outlined with green, indicating that there is no modulator in this relationship. Notice also that the signal flowing from the VCO to the VCF is an audio signal, not a control signal.

B

P

F

Filtering, by definition, means to remove certain elements from others. A filter on a synthesizer is a device which removes some harmonics. As an example, if one fills a glass with water and then places marbles into the glass, we have a perfect analogy of a harmonically rich waveform. Say the marbles are the harmonics that one wishes to remove, and the water represents the harmonics one wishes to pre- serve. When the contents of the glass are poured through a handkerchief, the marbles are not permitted through. They are filtered, while the water is allowed to pass through, mostly unchanged.

Audio Input Output

N

Any VCO

VCF

Figure 6-1: A basic subtractive synthesis patch

To amplifier and speakers

This analogy is a good one because as the water passes through the handkerchief, some of it is absorbed by the handkerchief, so not all of the water is allowed to pass through. This holds true when passing sounds through a filter. While the unwanted harmonics can be removed, some of the other harmonics end up being filtered out as well.

When learning about filters, it is important to understand that there are many types of filters. Early synthesizers usually only offered the user one type of filter, but sometimes had as many as four. Mod- ern synthesizers, however, may offer as many as 36 different types of filters! The study of some of these more esoteric filters is, for the moment, beyond the scope of this first volume, but they will be taken up at a future time. It is also worth noting at this time that most older synthesizers had only one filter on them, but modern synthesizers sometimes have as many as 128 independent filters!

W

D

F

D

?

The exact electronic workings of a filter are unimportant at this time. It is very important, however, to understand the function of a filter. As stated above, a filter removes unwanted harmonics, along with a few of the wanted harmonics. However, one cannot pick and choose exactly which harmonics one wants to remove. To take the next step in understanding, one must first understand that there are four basic 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 and highpass. Of the four, the lowpass filter is by far the most common. The ARP 2600 has one lowpass filter, but has no other filters. (The noise generator has a lowpass filter on it, but it is dedicated to the noise generator’s output, and cannot be used for general purpose filtering.) Each type of filter is ca- pable of filtering different ranges of harmonics. Some filters have the circuitry for some or all of these different types of filters. When a filter can operate in more than one mode (for example highpass and lowpass) it is said to be a multimode filter. Some synthesizers have filters that can actually perform all of these types of filtering simultaneously!

L

F

Lowpass filters filter out high harmonics. At first, ‘lowpass’ may seem like a strange name for a filter

that attenuates high harmonics, but it does make some sense. Filters are named by the information they allow to pass through rather than by the information they remove. So, a lowpass filter will allow all information below a certain frequency to pass through, while a highpass filter will allow all informa- tion above a certain frequency to pass through.

Although filters have several parameters, the most important is the cutoff frequency. Cutoff frequency is abbreviated Fc. Cutoff frequency is the frequency at which the filter will begin to attenuate the volume 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 TUNE slider, which function much like the corresponding sliders on the VCO’s. Instead of determining the frequency of an oscillator, however, here they determine the Fc. This is yet another example of a master-submaster relationship. These controls can be seen in Figure 6-2 on page 44.