Technology Institute for music educators ti: me course 2a Advanced Sequencing, Second Edition
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- Appendix G: Subtractive Synthesis Basics
| (2) Other multi-output virtual instruments are capable of receiving multiple MIDI signals (on one or more MIDI channels), which are routed to multiple sounds or presets and, in turn, routed to independent outputs. Examples would include virtual samplers like Kontakt and Structure. The setup for this type of device is a bit more complex.
A) Install the virtual instrument on an instrument track. B) Load the desired sounds or presets. C) Set each sound or preset to a separate MIDI channel and output channel. D) Create MIDI tracks for the additional MIDI parts. Set the track MIDI outputs to the corresponding MIDI channels. E) Create aux tracks for each necessary output. Set the aux track input to the corresponding virtual instrument. Some DAWs allow for a somewhat less complicated setup for this type of device. The first three steps would mirror the above example, but would substitute instrument tracks for MIDI tracks. A third variant should be mentioned but is not a multi-output device: Some virtual instruments can receive multiple MIDI signals and assign them to play different patches simultaneously—they are multi-timbral—but have only one stereo output path for those sounds. When using these, the individual levels and signal processing of the different sounds must be adjusted in the plug-in. Appendix G: Subtractive Synthesis BasicsSubtractive synthesis employs oscillators that output a prototypical “audio waveform” (sine, sawtooth, square, triangle) from which filters act to remove (or “subtract”) frequencies from the original waveform to create the desired result. Synthesizers based on subtractive synthesis principles typically have one to three oscillators that, in combination, allow the user to create extremely rich and complex sounds. Oscillators may output the following types of waveforms: Sine (Sinusoidal) wave: Sawtooth wave: fundamental plus all harmonics. The harmonics are in inverse proportion (2nd harmonic is ½ as loud as the fundamental, 3rd harmonic is ⅓ as loud as the fundamental, 4th harmonic is ¼ as loud as the fundamental, etc.) Square wave: fundamental plus only odd-numbered harmonics; again, in inverse proportion. Triangle wave: fundamental plus all odd-numbered harmonics. Here, the harmonics are proportional to the inverse square of their number in the series (the 3rd harmonic is 1/32 or 1/9 as loud as the fundamental, 5th harmonic is 1/25 as loud as the fundamental, etc.).
Synchronized: an oscillator outputting a waveform is slaved to a master oscillator. Each time the master oscillator completes a cycle; the slave oscillator is reset and starts the beginning of its cycle. The master oscillator’s pitch is not heard and the slave oscillator’s pitch is not changed. However, the waveform of the slave oscillator is altered in a way that often results in aggressive and expressive “sounds.” Cross Modulated: Similar concept to the sync type, but the pitch of a sawtooth wave is modulated by a triangle wave. Pulse Width Modulation: A square wave in which the positive and negative portions of the wave are not equal. In a 20% pulse wave, for example, the positive section lasts only 20% of the wave’s period, while the negative section occupies the remaining 80%. Sub-oscillator: generates a second waveform one octave below the pitch of the oscillator being processed. Noise generator: sets the amount of white or pink noise added to a signal Audio Filters Audio filters are devices that divide the frequency spectrum into two or more regions and then allow some frequencies to pass through the device uneffected while others are attenuated. The frequency regions that are uneffected are said to be in the pass band and the frequency regions that are attenuated are said to be in the stop band. The dividing point between a pass and stop band is called the cutoff frequency. The example below shows a diagram of a low pass filter. Example: High Pass Filter 
The example above shows that at the cutoff frequency, frequencies in the stop band are not immediately attenuated to a zero output. Instead the output level of frequencies in the stop band is gradually reduced the further you move into the stop band frequency range. The rate at which the frequencies are attenuated is referred to as the filter slope, commonly stated as a negative number of decibels per octave. Common filter types include: Low pass (high cut): passes frequencies below the cutoff frequency, attenuates frequencies above the cutoff frequency. High pass (low cut): passes frequencies above the cutoff frequency, attenuates frequencies below the cutoff frequency. Band pass: combines a low and high pass filter to create a frequency range in the “middle” that is allowed to pass, while attenuating frequency below the low-end cutoff frequency and above the high-end cutoff frequency. Band reject (notch): again combines low and high pass filters but, this time creates a frequency range (usually narrow) that is attenuated, while frequencies below and above that range are allowed to pass. All pass (phase shift): an interesting variant that allows all frequencies to pass, but not at the same rate. The small amount of time delay introduced to some frequencies results in phase cancellation, which is perceived as a “whooshing” sound (phase shifting or flanging). Other common filter parameters: Cutoff frequency: sets the frequency that divides the filter’s pass and stop bands Filter slope: sets the rate of attenuation. Typical settings are 3 to 24 dB per octave. Resonance: accentuates the frequencies that surround the cutoff frequency to create a more noticeable effect. Modulation By themselves, oscillators and filters often create a sound that is static and sometimes not very interesting. A good example is a violinist who plays without any vibrato or dynamics. So, to create a sound that is interesting or more “human,” a method or process is needed to alter the sound in real time. Synthesizers typically use two additional devices to manipulate sound in this manner—low frequency oscillators (LFOs) and envelopes. A low frequency oscillator is a device that outputs a very low frequency (.1 to 20 Hz). The LFO frequency is not added to the resulting audio signal but, instead, is used to change or modulate a sound in real time. Because the LFO frequency is very low, it vibrates at a rate slow enough to mimic vibrato or other human performance characteristics. An envelope is a device used to mimic or manipulate the shape of a note. Envelopes typically have four sections—the attack, decay, sustain and release (ADSR). In a synthesizer the envelope can be used to shape the time-variant characteristics (amplitude, filter settings, etc.) of a note. LFOs and envelopes can be used individually or in combination. For example:
Filter Envelope: An envelope that modulates the filter cutoff frequency Amplifier Envelope: Controls the dynamic shape of a note or sound Velocity envelope: modify envelope parameters by MIDI velocity. The harder a note is struck the more it increases the loudness of a sound, the intensity of modulation, the attack time, envelope time, etc. Download 3.6 Mb. Share with your friends:
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