ВУЗ: Не указан
Категория: Не указан
Дисциплина: Не указана
Добавлен: 27.09.2024
Просмотров: 377
Скачиваний: 0
PRESET PROGRAMMING
A Notch Filter is just the opposite of a bandpass filter and is used to eliminate a narrow band of frequencies.
Another control found on traditional filters is called Q or resonance. A lowpass filter with a high Q would emphasize the frequencies around the cutoff frequency. The chart below shows how different amounts of Q affect the low pass filter response. In terms of sound, frequencies around the cutoff will tend to “ring” with high Q settings. If the filter is slowly swept back and forth, with a high Q, various overtones will be “picked out” of the sound and amplified as the resonant peak sweeps over them. Bells and gongs are real world examples of sounds which have a high Q.
Amplitude
Low Q Med Q High Q
Frequency
Another parameter on a filter is the number of poles it contains. Traditional synthesizer filters were usually either 2-pole or 4-pole filters. The number of poles in a filter describes the steepness of its slope. The more poles, the steeper the filter's slope and the stronger the filtering action. The tone controls on your home stereo are probably one-pole or two-pole filters. Parametric equalizers are usually either two-pole or three-pole filters. In terms of vintage synthesizers, Moog and ARP synthesizer filters used 4-pole filters, Oberheim synthesizers were famous for their 2-pole filter sound.
Amplitude
4-pole |
2-pole |
Lowpass |
Lowpass |
Frequency
86 Morpheus Operation Manual
PRESET PROGRAMMING
Using a filter, we now have a way to control the harmonic content of a sampled sound. As it turns out, even a simple low pass filter can simulate the response of many natural sounds.
For example, when a piano string is struck by its hammer, there are initially a lot of high frequencies present. If the same note is played softer, there will be fewer of the high frequencies generated by the string. We can simulate this effect by routing the velocity of the keyboard to control the amount of high frequencies that the low pass filter lets through. The result is expressive, natural control over the sound.
If an envelope generator is used to control the cutoff frequency of a low pass filter, the frequency content can be varied dynamically over the course of the note. This can add animation to the sound as well as simulate the response of many natural instruments.
PARAMETRIC FILTERS
A more complex type of filter is called a parametric filter. A parametric filter gives you control over three basic parameters of the filter. The three parameters are: Frequency, Bandwidth, and Boost/Cut. The Frequency parameter allows you to select a range of frequencies to be boosted or cut, the Bandwidth parameter allows you to select the width of the range, and the Boost/Cut parameter either boosts or cuts the frequencies within the selected band by a specified amount. Frequencies not included in the selected band are left unaltered. This is different from a band pass filter which attenuates (reduces) frequencies outside the selected band.
Amplitude
+18 dB |
Freq. |
|
|
|
Boost |
Parametric
Filter
0 dB
Bandwidth
Cut
-18 dB
Frequency
The parametric filter is quite flexible. Any range of frequencies can be either amplified or attenuated. Often times, several parametric sections are cascaded (placed one after another) in order to create complex filter response curves.
Chapter 7: Preset Programming |
87 |
|
|
PRESET PROGRAMMING
If four parametric filter sections were cascaded, it would be possible to create the following complex filter response.
4 Parametric Equalizers
dB Magnitude
20
15
10
5
0
-5
500 |
10,000 |
15,000 |
20,000 |
Linear Frequency - Hertz
Many natural instruments have complex resonances which are based on their soundboard or tube size. The resonance shown above would be impossible to create using a normal synthesizer filter.
THE MORPHEUS FILTER
The Morpheus filter is actually much more complex than the four parametric sections described above. As an example of its power, the diagram below shows one of the possible ways that the Morpheus filter can be configured. This amount of filtering is unprecedented in all of electronic music history. (Especially when you consider that we are only talking about one of the 32 channels.)
|
1 Low Pass |
|
|
|
|
6 Parametric Equalizer Sections |
|
|
|
|
|
|
|
|
|
||||||||||||||||
|
|
Section |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|||
In |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Out |
||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|||||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
||||||||
|
|
Fc Q |
Fc Bw |
|
|
Fc Bw |
|
Fc Bw |
|
Fc Bw |
|
Fc Bw |
|
Fc Bw |
|
||||||||||||||||
|
|
|
|
|
|
|
Gain |
|
|
Gain |
|
Gain |
|
Gain |
|
Gain |
|
Gain |
|||||||||||||
Right away you can see that we now have 20 different parameters to control. Ah, there's the catch. How can all these parameters be effectively controlled? 20 envelope generators? We don't think so.
88 Morpheus Operation Manual
PRESET PROGRAMMING
Consider, as an example, the human vocal tract, which is a type of complex filter or resonator. There are dozens of different muscles controlling the shape of the vocal tract. When speaking, however, we don't think of the muscles, we just remember how it feels to form the vowels. A vowel is really a configuration of many muscles, but we consider it a single object. In changing from one vowel to another, we don't need to consider the frequencies of the resonant peaks! You remember the shape of your mouth for each sound and interpolate between them.
THE Z-PLANE FILTER
In a simple Morpheus filter, we would start with two complex filters and interpolate between them using a single parameter. Refer to the diagram below.
Morph
B Filter
Amplitude
A Filter
Morph
Frequency
The Morpheus Z-plane filter has the unique abilty to change its function over time.
Filters A and B represent two different complex filters. By changing a single parameter, the Morph, many complex filter parameters can now be changed simultaneously. Following along the Morph axis you can see that the filter response smoothly interpolates between the two filters. This is the essence of the Z-plane filter. Through the use of interpolation, many complex parameters are condensed down into one manageable entity.
This Z-Plane filter sweep can be controlled by an envelope or function generator, an LFO, modulation wheels or pedals, keyboard velocity, key pressure, etc. In fact, any of the modulation sources can control the Z-Plane filter.
Chapter 7: Preset Programming |
89 |
|
|
PRESET PROGRAMMING
••• See the Step-By-Step chapter for more information on the Z-plane filters.
Because creating the complex filtering is difficult and very time consuming, we have created hundreds of filters and installed them permanently in ROM for your use. You simply select and use the filters in a manner similar to choosing an instrument. Because there are so many types of filters to choose from, the number of possible permutations is staggering. For example, you could play a guitar sound through a vocal tract filter and create a talking guitar sound. Or you could have an acoustic guitar whose body shape changes as you play.
In the current example, two complex filters were created and the Morph parameter was used to interpolate between them. This is the simplest way to visualize a Morpheus filter
Morph
The next logical extension of this two filter model would be to add yet another filter pair. The diagram below shows an example using four filters. Now we have two parameters that can be controlled.
Morph
(Envelope,
Wheel, LFO, etc.)
Frequency Tracking
(Set at Note-On Time)
The diagram above, adds a Frequency Tracking parameter which varies the frequency of the filter. If we had set the frequency tracking and morph parameters as shown in the diagram above, the dot would represent the resulting filter position: a mixture of all four, but closest in nature to the left rear filter.
In the Morpheus filter there is only one parameter that can be continuously varied in realtime and that is the Morph parameter. Other filter parameters, such as Frequency Tracking, can only be changed at noteon time.
90 Morpheus Operation Manual
PRESET PROGRAMMING
Suppose we added yet another dimension to the filter model. We could have the realtime Morph parameter, the Frequency Tracking parameter (set at note-on time) and one more parameter, perhaps controlling the amount of the filter peaks with key velocity. A way to visualize a three-dimensional filter model is shown by the diagram below.
Morph |
Morph |
|
|
Frequency Tracking |
|
|
|
|
2 |
|
2 |
|
Velocity |
Transform 2 |
Transform |
Transform 2 |
Transform |
|
|
Frequency Tracking |
|
|
|
|
Morph |
Morph |
-Time |
|
|
Frequency Tracking |
Real |
|
|
|
|
|
|
|
|
Key Number |
|
|
••• If the frequency graphs at the corners of the cube are confusing you, think of them as the settings on a graphic equalizer.
The Cube filters are actually constructed of eight different complex filters.
Each axis of the three-dimensional cube changes the filter in a different way. In the example above, key number is being applied to the Frequency Tracking parameter in order to make the filter frequency track or follow the notes played on the keyboard. Assigning the Keyboard to frequency is called Key Tracking and is used to keep the timbre of the sound constant as you play up and down the keyboard. Without key tracking the sound would get duller as you move up the keyboard and could sound completely different at opposite ends of the keyboard. Because Frequency Tracking is so important it has been assigned its own parameter which is used in many of the filters (there are exceptions).
In the filter model above, there is another note-on (defined at the time the note is pressed) parameter, Transform 2. Unlike the Frequency Tracking parameter, the effect of Transform 2 may vary from filter to filter. In the example above, Transform 2 is being used to vary the size of the peaks and notches in the filter. The frequency plots in the upper plane of the cube have sharper peaks. Key velocity could be used to control Transform 2 and the sharpness of the filter peaks.
••• Note: The Frequency Tracking parameter is Transform 1.
Chapter 7: Preset Programming |
91 |
|
|
PRESET PROGRAMMING
••• On some filters, the Frequency Tracking parameter DOES NOT control the filter frequency. See the filter descriptions in the Reference Section for details.
• Another View
Another way to look at the Z-Plane filter is simply as a “black box”. You don't really need to think about all the possible filter permutations in order to use it. You just need to know what the controls do and listen to the sound. Each Z-Plane filter is described in the Reference Section of this manual.
|
|
Filter Type |
|
||
|
|
|
|||
Filter |
|
|
Reverse |
|
|
Level |
|
Morph |
Freq. |
Transform |
|
|
|
|
|||
|
|
Offset |
Track |
2 |
|
The Z-Plane filter can change its function in time.
Imagine a Lowpass filter transforming into a Bandpass filter. The Morph parameter (Morph Offset) typically handles this type of function.
Lowpass Filter Morphing into a Bandpass Filter
|
Amplitude |
|
Amplitude |
|
Amplitude |
|
|
|
|
|
|
|
|
Frequency |
|
Frequency |
|
Frequency |
The Morph and Transform 2 parameters vary in function from filter to filter. About half of the Z-Plane filters are not Cubes and therefore DO NOT use Transform 2. The Frequency Tracking parameter usually controls the frequency of the filters, moving peaks, shelves or notches up and down in frequency. Refer to the Reference Section of this manual for specific information on each filter.
Many of the filter types in Morpheus are models of traditional acoustic instruments such as electric piano or guitar. When an electric piano sample is played through an electric piano filter, the harmonics of the sample can now be controlled using the filter parameters.
Of course, you can put any instrument through the filter and the filter will shape the basic character of the instrument. This “building-block” approach to synthesis is only a part of what makes Morpheus such a unique and powerful synthesizer.
92 Morpheus Operation Manual