Digital Sound Design: W3: Synthesis

Question 1

DSD: How do you perform additive synthesis? explain with example.

Answer 1

E.g. replicate a cello sound:

1) Analyse a real chello. Look at the timbre: the fundamental and the overtones. See where they are an what amplitude they have.
2) For every fundamental & overtone create a sine wave at that frequency (a sine wave does not have any overtones) with the same amplitude.
3) when you add enough overtones, it will start to sound like a cello.

Question 2

DSD: What is subtractive synthesis?

Answer 2

You take a complex waveform and filter some frequencies out, to create interesting sounds.

Question 3

DSD: What are the 4 primary types of oscillators.

Answer 3

• Sine wave: just a fundamental frequency with no partials. (does not occur in nature)
• Triangle waveform.
• Pulse wave (most common one is a square wave).
• Saw tooth wave

Question 4

DSD: what is a signal modifier?

Answer 4

Used for processing, things like filters, envelope generators, amplifiers,…

Question 5

DSD: What is a triangle wave form. Give an example. (start at 20hertz fundamental

Answer 5

Fundamental frequency + odd numbered partials (1,3, 5, ..) that have an amplitude relation 1/n^2 (n=partial number). This means we have very little amplitude in the partials.

1 * 20 hertz (ampl=100db)
3 * 20 hertz (ampl=(1/3^2) of the amplitude, thus 1/9 of the amplitude => 11 db)
5* 20 hertz (ampl=100db/25)

Question 6

DSD: What are pulse wave forms?

Answer 6

Most common one is the square wave. fundamental frequency + odd numbered partials in an amplitude relationship of 1/n (n=partial number). This means the amplitudes are bigger then the triangle. Other pulse waves are created by varying the ‘duty cycle’ (compression and rarefaction of a soundwave). This is the relationship of the compression to the rarefaction (sound wave goes up, then down). A standard sound has a compression (and rarefaction) half the size of a waveform cycle. A square wave is like this standard sound. By varying the pulse width we can change the ratio of compression to the full cycle. e.g. 4 to 1 ratio, where compression is only 1 fourth the size of the duty cycle. By doing this we cancel out some of the overtones and thus changing the timbre.

Question 7

DSD: Explain soundwave compression and rarefaction.

Answer 7

Because of the longitudinal motion of the air particles when producing a sound, there are regions in the air where the air particles are compressed together and other regions where the air particles are spread apart. These regions are known as compressions and rarefactions respectively. The compressions are regions of high air pressure while the rarefactions are regions of low air pressure. A soundwave is depicted as a line going up and down. up = compression, down = rarefaction.

Question 8

DSD: What is a saw tooth waveform?

Answer 8

A fundamental frequency + all partials with an amplitude relationship of 1/n (n = partial number).

Question 9

DSD: What are the four types of filters?

Answer 9

1) Low pass filter
2) High pass filter
3) Band pass filter
4) notch/ band reject filter

Question 10

DSD: Explain resonance control in a filter?

Answer 10

This will amplify the frequencies in the cutoff range.

Question 11

DSD: What is a time variant envelope generator?

Answer 11

Sets an envelope on amplitude of the sound

• Attack: Time from silence to maximum amplitude.
• Decay: time going from maximum to secondary (sustain) level
• Sustain: Secondary amplitude level
• Release: time it takes to get back to silence.

Question 12

DSD: How do you simulate percussive or string instruments in sound synthesis?

Answer 12

By modifying the amplitude envelope.

Question 13

DSD: What are the two types of signals an oscillator can generate?

Answer 13

A carrier signal (horizontal line in diagram) and a controller signal (vertical line) (e.g. an lfo).

Question 14

DSD: What is a LFO? What accoustic technique does this simulate (sine lfo).

Answer 14

A controller signal oscillator inaudible for the human ear. Hence low frequency oscillator between 0 and 20 hertz.
This simulates a tremolo (rapid alteration of volume).

Question 15

DSD: what happens when we increase an lfo to 20 Hertz

Answer 15

Our brain can no longer hear the alterations of volume and starts to pick it up as a new pitch. It starts to produce side-bands

Question 16

DSD: What is amplitude modulation?

Answer 16

Modulate the amplitude with an lfo in the hearing range so we create sidebands and thus hear other pitches.

Question 17

DSD: what are sidebands?

Answer 17

Other frequencies in sound that are created by some sort of modulation.

Question 18

DSD: what happens when you plug in an lfo into an oscillator? What accoustic technique does this represent when you use a sine wavee small amplitude on the lfo? Large amplitude?

Answer 18

You modulate the frequency of the oscillator.
Small amplitude represents vibrato.
Large amplitude represents glissando (an alarm system).

Question 19

DSD: What is an audio oscillator?

Answer 19

An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave. An audio oscillator produces frequencies in the audio range, about 16 Hz to 20 kHz.

Question 20

DSD: what accoustic technique do you get when you plug in a square wave lfo into an oscillator and set the amplitude so that it switches between octaves?

Answer 20

A trill

Question 21

DSD: How do create a timbral modification on a carrier signal (oscillator)?

Answer 21

You plug in an lfo into a filter so that it modifies the timbre.

Question 22

DSD: What is complex amplitude modulation?

Answer 22

Plug in an oscillator in the auditory range into an amplifier to control the amplitude levels. This creates sidebands.

Question 23

DSD: What happens if you plug in an 100hertz controller oscillator into an amplifier that has a 500 hertz carrier signal (both sine waves)? What happens if you set your controller to 500 hertz.

Answer 23

The sound that comes out consists of the 500 hertz signal plus a 600 hertz (500 + 100) and a 400 hertz (500-100) signal (two sidebands)
If you set your carrier and controller to the same frequency, you get an octave higher and a zero frequency (nothing) as sidebands. This is called an octave harmonizer.

Question 24

DSD: What is an octave harmonizer?

Answer 24

Complex amplitude modulation where the carrier signal and the controller signal have the same frequency.

Question 25

DSD: What is a ring modulator?

Answer 25

Similar to complex amplitude modulation. It will attenuate the carrier signal. So that only the sidebands will get through.

Question 26

DSD: What is complex frequency modulation? What is the formula for the number of sidebands and their frequencies for two sine waves?

Answer 26

If you modulate an oscillator with another oscillator in the auditory range so you get sidebands.
Calculate the modulation index: peak frequency deviation (=amplitude of the controller signal is in relation to this peak frequency)/ modulation frequency = number of sidebands below or above => times 2 is the total number of sidebands. the frequency of these bands = 1 * modulation frequency; 2 * modulation frequency;…. -4 * modulation frequency

Question 27

DSD: What happens to the formula for sidebands ands their frequencies if you use frequency modulation from a sine wave on a sawtooth?

Answer 27

You have to calculate the modulation index for every overtone as they will also have new sidebands.

Question 28

DSD: What happens to the formula for sidebands ands their frequencies if you use frequency modulation from a saw tooth wave on a saw tooth?

Answer 28

You have to calculate the modulation index for every fundamental and every overtone of both sawtooths (n^2)

Question 29

DSD: What happens when you create alot of frequencies/sidebands through some sort of modulation?

Answer 29

You get noise and the carier signal gets drowned out. you also get close to percussion instruments like cymbals.

Question 30

DSD: To what natural phenomenon can you compare granular synthesis?

Answer 30

The sound of rain. One raindrop you can hear separately, a thousand give a whole other sound.

Question 31

DSD: What is granular synthesis?

Answer 31

Granular synthesis is a basic sound synthesis method that operates on the microsound time scale.
It is based on the same principle as sampling. However, the samples are not played back conventionally, but are instead split into small pieces of around 1 to 50 ms. These small pieces are called grains. Multiple grains may be layered on top of each other, and may play at different speeds, phases, volume, and frequency, among other parameters.

Question 32

DSD: What is digital modeling?

Answer 32

In sound synthesis, physical modelling synthesis refers to methods in which the waveform of the sound to be generated is computed by using a mathematical model, being a set of equations and algorithms to simulate a physical source of sound, usually a musical instrument. Such a model consists of (possibly simplified) laws of physics that govern the sound production, and will typically have several parameters, some of which are constants that describe the physical materials and dimensions of the instrument, while others are time-dependent functions that describe the player’s interaction with it, such as plucking a string, or covering toneholes.