ben m
Joined: 15 Sep 2002
Posts: 337
Location: UK |
| Week 5 - Synthesis 2 |
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Sorry for the lateness of this - I've been recording all day and I've only just got off!
Anyhow, questions here and a practical in another post on the forum. As always questions take priority over practicals.
Good Luck
a) Which method of additive synthesis had widespread commercial success in the 1980s?
b) Describe the basic principles of this method of synthesis.
c) How this was implemented in the synthesisers that were commercially available.
What are the ADSR characteristics of the following instruments;
Piano
Guitar
Snare Drum
Violin
Describe the effect of the following modulations:
a) A VCO is going into a VCF and to a VCA. The VCO is being modulated by another VCO producing a square wave with a frequency of 15 Hz.
b) A VCO is going into a VCF and to a VCA. The VCF is being modulated by an ADSR.
c) A VCO is going into a VCF and to a VCA. The VCA is being modulated by another VCO producing a sine wave with a frequency of 10 Hz.
In early analogue subtractive synthesis, describe the difference between a voltage controlled oscillator and a conventional oscillator.
In subtractive synthesis, describe the function of the resonance control in a low-pass filter.
Describe three common ways an LFO might be used in analogue synthesis. Include references to other components of analogue synthesisers. Do not use abbreviations in your answer.
Give four terms used in FM (Frequency Modulation) synthesis and not in analogue synthesis.
Describe the waveform source in a wavetable synthesiser.
What technical problem would you expect to encounter when using an old analogue synthesizer for a lengthy live performance, even if the synthesizer is working to its original specifications?
What would be the effect of modulating the VCF of an analogue synthesizer with the LFO?
On the ADSR of an analogue synthesizer, how would you make a note decay to complete silence, no matter how long the key is pressed?
In an FM synthesizer, how would you quickly make a major (but probably random) change in the sound produced?
In an FM synthesizer, what would you adjust to change the high frequency content of a sound?
Which would you expect to sound best on an FM synthesizer: a tinkly electric piano sound or an imitation of string instruments?
Describe how frequency modulation can be achieved with a traditional analogue synthesizer, or modern equivalent. How will the end result differ compared to an FM synthesizer?
With reference to sounds creation, briefly explain the following;
a) Sample and Hold
b) Ring Modulator
With regard to FM synthesis, briefly explain the following;
a) Sidebands
b) Modulation Ratio
c) Modulation Index
With reference to physical modelling synthesis, briefly explain;
a) driver
b) Resonator
c) Modifier
In an FM algorithim, what differences in sound would occur from running 2 operators in series rather than parallel.
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Mon Sep 26, 2005 5:45 pm |
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AndyBarber
Joined: 09 Sep 2005
Posts: 18
Location: North Wales |
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Posted: Tue Sep 27, 2005 3:45 am Post subject: Week 5 - Synthesis 2
Andy Barber
1.
a) Which method of additive synthesis had widespread commercial success in the 1980s?
In the 1980s especially with the release of the Yamaha DX7, synths had become more affordable to the mass market. It was the first digital synth implementing FM synthesis.
b) Describe the basic principles of this method of synthesis.
Initial signal is created using a number of oscillators/operators producing sine waves
Linking the oscillators in different ways produces many creative possibilities
Using additive, the oscillator outputs are summed together.
FM is created when one oscillator’s output or an LFO is used to modulate the frequency of another creating a series of harmonics in the signal.
c) How this was implemented in the synthesisers that were commercially available.
These modern synth are basically a digital implementation of an analogue synth structure
The FM method of synthesis became affordable due to the extensive use of integrating circuits and microprocessors
They had to wait for the next generation in digital chip technology, called large-scale integration, before chips had enough power to handle the process. It wasn't until 1983 that they released their first really popular product, the DX-7 synthesizer
2. What are the ADSR characteristics of the following instruments;
Amplitude envelopes describe how volume of a sound changes over time
Midi note velocity can be used to affect envelope levels
Non sustained sounds, will gradually fade away (organ sustained)
Piano
Maximum loudness almost at the instant the hammer makes contact with the string then fades away with time. Fast attack, long decay, zero sustain, medium release (when key held down there is no sustain)
Guitar
Fast attack, long decay, zero sustain, zero release (key not held down, only plucked)
Snare Drum
Fast attack, short decay, zero sustain, zero release (sharp quick sound)
Violin
Sound fades in and out with note sustained between bow movements
Medium attack, zero decay, maximum sustain, medium release

3. Describe the effect of the following modulations:
a) VCO going into a VCF and to a VCA. The VCO is being modulated by another
VCO producing a square wave with a frequency of 15 Hz.
The signal coming out of the VCF and VCA will be two frequencies with a bandwidth proportional to the amplitude of the modulating VCO. The rate of deviation between the two frequencies will be 15Hz
b) A VCO is going into a VCF and to a VCA. The VCF is being modulated by an ADSR.
The ADSR envelope will define the changing of the filters cut-off frequency over time. Depending on the filer used, certain frequencies, at certain times, will be removed/attenuated from the signal coming out of the VCO
c) A VCO is going into a VCF and to a VCA. The VCA is being modulated by another VCO producing a sine wave with a frequency of 10 Hz.
The signal coming out of the VCO will be the same going into the VCA. The amplitude of the signal coming out of the VCA will oscillate like a sine wave with a frequency of 10 Hz.
4. In early analogue subtractive synthesis, describe the difference between a voltage controlled oscillator and a conventional oscillator.
A conventional oscillator produces a signal with physically set (variable resistor) amplitude and frequency. A VCO’s frequency and amplitude can be varied (modulated) in ‘real time’ using a control voltage. The control voltage can come from a number of sources, other oscillators, Envelope generators and LFOs
5. In subtractive synthesis, describe the function of the resonance control in a low-pass filter.
A filters main function is to remove/attenuate harmonics from a waveform. A cut-off frequency (threshold value) is assigned which determines what range of frequencies are processed. A low pass filter removes all frequencies above the cut-off frequency. A resonance control determines how the frequencies at the threshold behave. This is achieved by using a feedback loop. Increasing the resonance exaggerates any signal near cut-off frequency. Resonance makes filter more audible and adds character
6. Describe three common ways an LFO might be used in analogue synthesis. Include references to other components of analogue synthesisers. Do not use abbreviations in your answer.
An LFO source can be used to modulate many destinations within a synth. LFO frequency is usually below 20Hz
Assigning an LFO to the frequency of an oscillator creates a periodic change in frequency, known as ‘vibrato’. The depth and rate of vibrato is determined by that of the LFO. Vibrato can also be created by using mod wheel, pitch bend, after touch, joystick, ribbon and key velocity to modulate a frequency. These are good for creating personalized vibrato’s
An LFO output can be assigned to modulate amplitude, either an oscillator’s or VCA’s. Known as ‘tremolo’. Depth and character can be varied in real time
Assigning an LFO to a filter’s (VCF‘s) cut-off frequency causes it to sweep through a band of frequencies. The effect called ‘trill’
7. Give four terms used in FM (Frequency Modulation) synthesis and not in analogue synthesis.
Operator
Frequency deviation
Modulation index
Algorithm
8. Describe the waveform source in a wavetable synthesiser.
A waveform is stored as digital audio segments. The segments are stored in the synths memory as a table of waveforms. Notes of a particular sound/timbre are played back at different pitches by playing back the waveform at different speeds.
Usually three segments to a wavetable sound. First segment represents the attack of the sound. Second segment represents the sustain (looped until key released) and plays when the key is held down. After the key is released the third segment plays the release of the note.
The wavetable waveform can then be modulated like normal digital signals
9. What technical problem would you expect to encounter when using an old analogue synthesizer for a lengthy live performance, even if the synthesizer is working to its original specifications?
FM synthesis using analogue oscillators is not generally feasible due to their inherent pitch instability. They have a fundamental problem that they drift out of tune due to temperature fluctuations. When the synth is used for log periods of time it will heat up requiring periodic retuning
10. What would be the effect of modulating the VCF of an analogue synthesizer with the LFO?
VCF’s cut-off frequency is altered by a DC control voltage rather than a variable resistor
There are two variables for the filter, depth control (how much LFO effects filter) and LFO waveform (frequency and shape). Modulation allows the filter to sweep through a frequency range periodically, attenuating/removing frequencies/harmonics along the way. An unlimited combination of tonal effects can be created
11. On the ADSR of an analogue synthesizer, how would you make a note decay to complete silence, no matter how long the key is pressed?
Setting the sustain to zero will mean the amplitude will reduce to zero at a rate specified by the decay level. Release should be set so note will also decay gradually when key is released
12. In an FM synthesizer, how would you quickly make a major (but probably random) change in the sound produced?
A change in the ‘amount’ of modulation applied to an oscillator can affect the frequency bandwidth dramatically. The pitch of the signal is attenuated as the sidebands created increase in amplitude
13. In an FM synthesizer, what would you adjust to change the high frequency content of a sound?
A filter with resonance can be used to exaggerate or attenuate frequencies. A low pass filter will remove high frequencies and a high pass filter will single them out. A band pass can do both, increasing resonance will exaggerate high frequencies if cut-off frequency is set in their range
14. Which would you expect to sound best on an FM synthesizer: a tinkly electric piano sound or an imitation of string instruments?
FM is not a particularly realistic method of creating sounds. It approximates musical sounds by reproducing how their strength varies with frequency. A limitation of this technique is that it can only match the sound of an instrument as it is measured at one point in time. Tinkly electric piano sounds have relatively simple waveforms with little variation over time
Stringed instruments are complex with evolving texture. FM can’t create nuances in its sounds as they evolve. Wavetable synthesis much better at imitating musical instruments as any sound can be made available by providing samples.
15. Describe how frequency modulation can be achieved with a traditional analogue synthesizer, or modern equivalent. How will the end result differ compared to an FM synthesizer?
FM is not exclusive to digital synths can also be applied to analogue oscillators
A VCO signal can be used as a modulation source. The amplitude of modulation is determined by a VCA which is controlled by a CV source. When gain of VCA is zero there will be no modulation. A slight increase in VCA gain will generate gentle vibrato effect. Increase in gain will make carrier sweep over a wider range of frequencies.
Analogue systems are limited to producing side bands within their finite bandwidth
Compared to digital systems an analogue synth implementing FM will be inconsistent and difficult to calibrate.

16. With reference to sounds creation, briefly explain the following;
a) Sample and Hold
A sample-and-hold circuit samples a signal at a specified time interval. By periodically capturing the frequency of a dynamic waveform the analogue signal can be converted into a digital one. The module captures the signal’s voltage level and holds it until the next sample is taken. A sequence of voltages is obtain with a stepped output.
Most commonly used as a modulator
Can have more than one source triggering it

b) Ring Modulator
Modulation where two bipolar signals are multiplied together, modulator and carrier signals dissapear and replaced by tones at the sum and difference of their frequencies
With an ideal ring modulator:
x and y input signals = output signal contains (x+y) and (x-y) but not x or y
If ‘y’ signal is a low frequency sine wave, result is amplitude modulation or tremolo
If ‘x’ and ‘y’ have same signal, result is linear doubling of inputs frequencies (signal an octave higher with distorted partials)

17. With regard to FM synthesis, briefly explain the following;
Mathematical interpretation of FM synthesis, basic setup:
- output of modulator is offset by a constant, fc, (carrier frequency)
- combined signal used to control frequency of carrier
- if ‘amplitude’ of modulator is zero then there is no modulation (carrier output will be fc)
- if modulator’s amplitude is greater than zero then modulation occurs
- frequency of carrier output proportional to ‘amplitude’ of modulator
Amplitude of modulator is known as the frequency deviation ‘d’ (Hz)
‘d’ (Hz) - frequency deviation modulation amplitude
fm (Hz) - modulator frequency
fc (Hz) - offset carrier frequency
ac - Carrier amplitude
a) Sidebands
Spectrum of an FM sound is composed of the off set carrier frequency (fc) and a number of partials (sidebands) either side of it. Sidebands are spaced at a distance equal to the modulator frequency and are calculate with:
fc + (k x fm) and fc - (k x fm) k = integer number greater than zero
Partials’ amplitudes are determined by the frequency deviation. When d=0 signals power is entirely in fc. Increasing d produces sidebands at expense of power in fc. Greater values of d mean more partials are created
b) Modulation Ratio, Modulation Index
FM is a useful tool to control number of audible partials
d and fm drastically change form of the carrier wave
Ratio between d and fm is known as the modulation index ‘i’
i = d /fm
i controls the spectral richness of the sound. As ‘i’ increases from zero, number of audio partials increases and energy of offset fc is distributed among them
18. With reference to physical modelling synthesis, briefly explain;
Synthesis of sound using a set of equations and algorithms simulating physical source of sound
Example of wave guide modelling using a trumpet. Typically there are three main parts that influence the sound, DRIVER, RESONATOR, MODIFIER:
1.DRIVER (excitation)
Produces the sound in the first place. Instruments way of creating the initial sound . Input of synth must be able to approximate various ways in which instrument can be sounded by player. Mouthing techniques of trumpet player, blowing softer or louder effects timbre and pitch. Model accounts for constraints placed upon vibrating object (ie. Vibrating lips)
2.RESONATOR (wave control)
Changes initial vibration into what we recognise as the sound of the instrument. Trumpet player can control pitch by opening and closing valves. Ultimately varying the length of the tube/wave guide. Tube acts as filter and is main resonant body of instrument. Longer tubes amplify lower frequencies, shorter tubes amplify higher frequencies
3.MODIFIER (amplification)
A trumpets amplifier is the bell which acts as a transformer for the sound. Point at which sound wave reaches most resistance. Air outside has a large acoustic impedance and some of the sound is reflected back into the instrument
19. In an FM algorithim, what differences in sound would occur from running 2 operators in series rather than parallel.
In parallel the signal from each operator can be heard at the same time. Connected in series the two signals modify each other creating a signal with the superposition of the two sounds
http://arts.ucsc.edu/ems/music/equipment/synthesizers/Synthesizing/usesynth.html
http://www.brokengadget.co.uk/artfrm.htm
http://en.wikipedia.org/wiki/ADSR_envelope
http://arts.ucsc.edu/EMS/Music/tech_background/TE-17/teces_17.html
http://www.geocities.com/SunsetStrip/Underground/2288/2fmsynth.htm#Ops
http://en.wikipedia.org/wiki/Frequency_modulation_synthesis
http://www.harmony-central.com/Synth/Articles/Physical_Modeling/
http://www.synergymwave.com/Articles/PDF/Frequency-Synthesizers.pdf
http://www.stanford.edu/dept/news/pr/94/940607Arc4222.html
http://www.samplecraze.com/tutorial.php?xTutorialID=7
http://jpmullan.com/stan/index_glossary.php
http://www.sfu.ca/~gotfrit/courses/247_2001/w5_synthesis_02.html |
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Thu Sep 29, 2005 9:41 am |
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tobyh1000
Joined: 10 Jul 2005
Posts: 13
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Week 5 - Synthesis 2
a)Which method of additive synthesis had widespread commercial success in the 1980s?
Frequency modulation
b) Describe the basic principles of this method of synthesis.
Two oscillators are fed into each other – the first signal from the first operator is modulated by the second operator. More operators can be added to alter the sound after that to create varying and a wide range of sounds
c) How this was implemented in the synthesisers that were commercially available.
This was implemented in synthesizers when six operator able synths became available like the Yamaha DX7.
What are the ADSR characteristics of the following instruments;
Piano
Attack – Fast
Decay – Slow
Sustain – Slow
Release - Fast
Guitar
Attack – Fast
Decay – Slow
Sustain – Medium
Release – Fast
Snare Drum
Attack – Fast
Decay – Instant
Sustain – None
Release – None
Violin
Attack – Slow
Decay – None
Sustain – High
Release – Fast
Describe the effect of the following modulations:
a) A VCO is going into a VCF and to a VCA. The VCO is being modulated by another VCO producing a square wave with a frequency of 15 Hz.
b) A VCO is going into a VCF and to a VCA. The VCF is being modulated by an ADSR.
c) A VCO is going into a VCF and to a VCA. The VCA is being modulated by another VCO producing a sine wave with a frequency of 10 Hz.
In early analogue subtractive synthesis, describe the difference between a voltage controlled oscillator and a conventional oscillator.
A conventional oscillator makes no use of voltage, so it doesn’t rely on it to control the frequency values, so the frequency is fixed at one value. Where voltage controlled oscillators can vary the frequency through different voltages.
In subtractive synthesis, describe the function of the resonance control in a low-pass filter.
When resonance is increased the amount of overtones close to the cut off frequency are also increased.
Describe three common ways an LFO might be used in analogue synthesis. Include references to other components of analogue synthesisers. Do not use abbreviations in your answer.
The low frequency oscillator can modulate the frequency of a voltage controlled oscillator. It can also modulate the output level of the envelope generator and the voltage controlled amplifier which produces the effect of vibrato.
Give four terms used in FM (Frequency Modulation) synthesis and not in analogue synthesis.
Operators, carriers, algorithms and modulation index.
Describe the waveform source in a wavetable synthesiser.
The source of the waveform in a wavetable synthesizer is the instruments played which were recorded and stored on memory as digital samples that can be recalled.
What technical problem would you expect to encounter when using an old analogue synthesizer for a lengthy live performance, even if the synthesizer is working to its original specifications?
The technical problem you would expect would be overheating, which can alter the state of the pitch level stability.
What would be the effect of modulating the VCF of an analogue synthesizer with the LFO?
The effect of modulating the VCF with a LFO would be distortion to the sound and also create a ‘wah wah’ effect.
On the ADSR of an analogue synthesizer, how would you make a note decay to complete silence, no matter how long the key is pressed?
To make a note decay to complete silence, no matter how long the key is pressed, you must not use sustain and release, but only attack and decay.
In an FM synthesizer, how would you quickly make a major (but probably random) change in the sound produced?
To make a major change in the sound you could alter the algorithm in the synthesizer.
In an FM synthesizer, what would you adjust to change the high frequency content of a sound?
Increase the modulation index.
Which would you expect to sound best on an FM synthesizer: a tinkly electric piano sound or an imitation of string instruments?
I would expect a tinkly electric piano to sound best on a FM synthesizer.
Describe how frequency modulation can be achieved with a traditional analogue synthesizer, or modern equivalent. How will the end result differ compared to an FM synthesizer?
Frequency modulation can be achieved with a traditional analogue synthesizer by plugging a carrier into an operator, then that operator into more if needed. The end result will not be polyphonic and may have an unstable frequency.
With reference to sounds creation, briefly explain the following;
a) Sample and Hold
Basically is a circuit with 2 inputs and 1 output, where in audio – a signal can be fed into input 1 and then stored inside the circuit, sent only to the output after a trigger from input 2 is sent.
b) Ring Modulator
Ring modulators multiply two audio signals together to produce one audio signal, therefore being able to produce very disconcordant sounds.
With regard to FM synthesis, briefly explain the following;
a) Sidebands
Sidebands are the frequencies above and below of the carrier frequency after frequency modulation has occurred.
b) Modulation Ratio
Modulation ratio defines the sideband frequencies amount and placement around the frequency modulation carrier frequency.
c) Modulation Index
The higher the modulation index the louder the sideband frequencies are after frequency modulation.
With reference to physical modeling synthesis, briefly explain;
a) Driver
The driver creates the virtual sound vibrations – the sound that is produced first of all, like the breath through a mouthpiece on a trumpet.
b) Resonator
The resonator defines how much the sound is perceived through its own harmonics, like the horn on the trumpet shapes the sound.
c) Modifier
The modifier changes the sound made by the driver into the sound of the instrument.
In an FM algorithm, what differences in sound would occur from running 2 operators in series rather than parallel.
In series the first operator signal is changed when modulated by the second operator, as it would lead directly into it. With parallel, the first and second operators sound is played together. |
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Fri Sep 30, 2005 11:50 am |
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Polarman
Joined: 24 Jun 2005
Posts: 55
Location: Barbados |
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a) Which method of additive synthesis had widespread commercial success in the 1980s?
FM synthesis had a widespread commercial success in the 1980s. The most famous synth was the Yamaha DX7. It had a big success with its bright and complex timbers but it was regarded hard to programme.
b) Describe the basic principles of this method of synthesis.
FM synthesis is in one way the opposite of analogue synthesis. Instead of removing harmonics in FM synthesis we are adding harmonic overtones. The simplest way to explain this is to think of an LFO connected to the amplitude of an oscillator. The amplitude of the output signal varies with the frequency on the LFO. If set to 1 HZ on the LFO the amplitude of the signal out of the oscillator increases the first half second and fades the other half. If you use another sinus wave to modulate another sinus wave the modulating waves will distort the other sinus wave and introduce extra harmonics. You can say it's a way of mixing two sounds together to create a third or putting two "pure" sound waves together in such a way that what you hear is not the original waves but the mix - or modulation.
This is same technology as FM radio and it works for both analogue and digital oscillators and the technique became very famous with the release of Yamaha DX7.
c) How this was implemented in the synthesisers that were commercially available.
The big breakthrough came with Yamahas DX7. It had a big success with its bright and complex timbers. It was regarded hard to programme. The DX7 worked with six operators to generate it sounds and had a fixed amount of algorithms.
What are the ADSR characteristics of the following instruments?
Piano
Attack - fast
Decay – short
Sustain – quite high
Release - fast
Guitar
Attack - fast
Decay – short
Sustain – quite high
Release – short
Snare Drum
Attack - fast
Decay – short
Sustain - 0
Release - 0
Violin
Attack - medium
Decay – medium
Sustain – quite high
Release – medium
Describe the effect of the following modulations:
a) A VCO is going into a VCF and to a VCA. The VCO is being modulated by another VCO producing a square wave with a frequency of 15 Hz.
The frequency of the signal will vary between to states with 15 Hz according to the square wave this will produce a "trill" (a sudden varying of pitch).
b) A VCO is going into a VCF and to a VCA. The VCF is being modulated by an ADSR.
The VCFs envelope will follow the ADSRs envelope.
c) A VCO is going into a VCF and to a VCA. The VCA is being modulated by another VCO producing a sine wave with a frequency of 10 Hz.
The amplitude of the VCA will modulated with 10 Hz this will produce a "tremolo" (smooth varying up and down of amplitude).
In early analogue subtractive synthesis, describe the difference between a voltage controlled oscillator and a conventional oscillator.
A voltage controlled oscillator can vary its frequency according to the input voltage while conventional oscillators delivers only one waveform and a specific frequency.
In subtractive synthesis, describe the function of the resonance control in a low-pass filter.
A low-pass filter is made to cut-off frequencies above the cut-off level. The resonance control boosts the amplitude of the frequencies around that cut off point. As the resonance is increased, the boosted band becomes narrower. Resonance creates all sorts of wah-wah effects.
Describe three common ways an LFO might be used in analogue synthesis. Include references to other components of analogue synthesisers. Do not use abbreviations in your answer.
Low Frequency Oscillator is an oscillator which works at a set slow rate (perhaps between 0.1Hz to 10 Hz) and its a very common Modulator on synthesizers. A Low Frequency Oscillator will have the following parameters:
Wave (Triangle, Square, Saw, Sine),
Rate (the speed of the oscillation)
Delay (a time delay before the oscillator starts).
Low Frequency Oscillator Modulates the Voltage Controlled Oscillator
Using an Low Frequency Oscillator to modulate the pitch of the Voltage Controlled Oscillator oscillator causes the oscillator's pitch to "wobble" with the Low Frequency Oscillator .
Using a Triangle or Sine on the Low Frequency Oscillator will produce "vibrato" (a smooth up and down varying of the pitch).
Using a Square on the Low Frequency Oscillator will produce a "trill" (a sudden varying of pitch).
Using a Saw on the Low Frequency Oscillator will produce something like an alarm siren.
Low Frequency Oscillator modulates Voltage Controlled Filters Cuf-Off
Using the Low Frequency Oscillator to modulate the Cut-Off of the filter causes the Cut-Off to vary with the Low Frequency Oscillator. Used in extremes, it will sound like a "Wah-wah" sound.
Low Frequency Oscillator modulates Voltage Controlled Amplifier
Using the Low Frequency Oscillator to modulate the Amplifier causes the loudness to vary with the Low Frequency Oscillator.
Using a Tri or Sine on the Low Frequency Oscillator will produce "tremolo" (smooth varying up and down of amplitude).
Using a Square on the Low Frequency Oscillator will produce a sharp "gated" sound (ie loud, quiet, loud, quiet etc).
Give four terms used in FM (Frequency Modulation) synthesis and not in analogue synthesis.
1. Operator
2. Algorithm
3. Carrier
4. Digital oscillator
Describe the waveform source in a wavetable synthesiser.
A wavetable synthesiser has a ROM containing data describing the waveforms. These waveforms may be digitally encoded versions of the basic sound sources of analogue synthesis, or they may be completely new types of waveforms or even samples.
What technical problem would you expect to encounter when using an old analogue synthesizer for a lengthy live performance, even if the synthesizer is working to its original specifications?
The pitch of the oscillator of the analogue synthesizers tended to drift. There were problems with air circulation inside the unit. Cooling wasn’t the same for all of the oscillator cards, so they would drift at different rates. To combat these tuning problems, technicians tried various strategies such as installing internal cooling fans for improved air circulation.
What would be the effect of modulating the VCF of an analogue synthesizer with the LFO?
Using the Low Frequency Oscillator to modulate the Cut-Off of the filter causes the Cut-Off to vary with the Low Frequency Oscillator. Used in extremes, it will sound like a "Wah-wah" sound.
On the ADSR of an analogue synthesizer, how would you make a note decay to complete silence, no matter how long the key is pressed?
Set sustain level to zero
In an FM synthesizer, how would you quickly make a major (but probably random) change in the sound produced?
A quick change to the sound on a FM synthesizer is to change the algorithm.
In an FM synthesizer, what would you adjust to change the high frequency content of a sound?
To change the high frequency content of a sound on a FM synthesizer is to adjust the parameters of one of the operators.
Which would you expect to sound best on an FM synthesizer: a tinkly electric piano sound or an imitation of string instruments?
I would expect a tinkly electric piano. The sound of a FM synthesizer is very clean. FM synthesizers are not normally good at or used for imitating real instruments.
Describe how frequency modulation can be achieved with a traditional analogue synthesizer, or modern equivalent. How will the end result differ compared to an FM synthesizer?
One VCO can modulate another VCO. The problem with Voltage Controlled oscillators is that they are not stable enough they drift a little bit. You need a DCO to keep to oscillator stable.
With reference to sounds creation, briefly explain the following;
a) Sample and Hold
The Sample and Hold enables the system to generate tunes and riffs and create different modulations on control signals. It enables the immediate value (voltage) of a waveform to be captured, and continues to output that value until the next sample is taken. This voltage is then used to control for example a filter. A sample and hold circuit can produce pretty random sounding fluctuations in one or more aspects of a sound.
b) Ring Modulator
A ring modulator is a type of audio mixer that combines two audio signals, and outputting their sum and difference. The frequencies found in the original signals are not passed through to the output. For example, if two sine waves (single frequency waveforms containing no overtones) are inputted, one with a frequency of 1000 Hz, and the second at 400 Hz, the ring modulator will output two frequencies: 600 Hz and 1400 Hz. With more complex waveforms (which contain many more overtone frequencies) ring modulators produce a "metallic" result often used for special effects, in synth programming etc. One popular use has been to process vocals, which produces sci-fi sounding "robotic" voices.
With regard to FM synthesis, briefly explain the following;
a) Sidebands
The sidebands is the result of one signal or waveform being modulated by another (or others). When a signal is either frequency modulated (FM) by another signal, sum and difference frequencies are produced that appear with the signal. These are known as sidebands. The Upper Sidebands (USB), which are the result of adding the signals together, and Lower Sidebands (LSB), which are the result of subtracting them. They are a big part of what gives FM synthesizers their unique sound.
b) Modulation Ratio
Modulation ratio is the ratio between the carriers frequency and the modulators frequency. If an oscillator is set to zero frequency no sound is generated. Modulating a frequency of zero Hz means that the frequency will be swept around zero Hz.
c) Modulation Index
The modulation index is the amount of modulation or volume level we apply to the second oscillator. Normally the more modulation you apply the brighter the sound gets, if the modulation index becomes very high the result is noise. The change into noise goes quite fast and this can be used in many creative ways.
With reference to physical modelling synthesis, briefly explain;
a) driver
The driver (sometimes called exciter) is the initial energy burst required to produce the sound and it is different for all instruments. The driver for air blown through a reed on a flute is different from a hammer striking a piano string. The resonator determines the tonal characteristics of an instrument. For example the energy of a vibrating string on an acoustic guitar would be passed via the bridge to the instrument body through which the sound is modified by amplification and resonance.
The driver is constantly fluctuating. A bowed instrument such as a violin displays this clearly with the amount of expression that can be achieved simply by changing the velocity and pressure of the bow.
b) Resonator
The resonator determines the tonal characteristics of an instrument. For example the energy of a vibrating string on an acoustic guitar would be passed via the bridge to the instrument body through which the sound is modified by amplification and resonance.
As the structure of most musical instruments does not change the resonator is a fairly consistent factor that defines the instruments sound.
c) Modifier
A modifier is a filter and an amplifier that is applied the output of the source that changes the tonal characteristics of the sound.
In an FM algorithm, what differences in sound would occur from running 2 operators in series rather than parallel.
Running two operators in parallel both signals will be heard at the same time on the output and no modulation will occur. Running them in series will result in a modulated signal on the output, operator one modulating operator 2.
Sources:
http://www-users.york.ac.uk/~adh2/1st_yr/3/sld008.htm
http://www.computermusic.co.uk/tutorial/fm/fm2.asp
http://www.soundonsound.com/sos/apr00/articles/synthsecrets.htm
http://www.fm-alive.com/fm_explained.htm
http://www.computermusic.co.uk/tutorial/a_sforbeg/asynth2.asp
http://www.xgfactory.com/downloads/synthinfo.pdf
http://www.keyboardmag.com/story.asp?sectioncode=28&storycode=10652
http://www.clavia.se/nordmodular/Modularzone/SampleHold.html
http://www.sweetwater.com/insync/search.php
http://www.headjog.co.uk/report/
http://www.clavia.se/nordmodular/Modularzone/FMsynthesis.html |
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Sat Oct 01, 2005 11:36 am |
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