CA149 Midterm 2

Signal Processing

The task of manipulating sound's component elements

Signal Processing can be broken down into 4 basic types:

1. Frequency alterations → transposition

2. Timbre alterations → equalization and filtering

3. Time-based alterations → delays, reverbs, chorus, phasing, flanging, granular stretching

4. Dynamic (amplitude alteration) → compression expansion, gating

Pitch Shifting (Scaling)

Changes the pitch of a selection with/without preserving the duration of the sound file

What is Pitch Shifting useful for?

• To match the pitches of two pre-recorded clips for mixing when the clips cannot be reperformed or resampled

• To create effects such as increasing the range of an instrument

• Used in sound design

Pitch Bending

Changing the pitch of a note; equivalent to pitch control on turntables

Example: pushing a guitar string upwards to change pitch

What are examples of pitch correction?

• AutoTune

• Melodyne

What is Vibrato?

• True vibrato is a regularly occurring (generally) subtle change of pitch

• Common in most string instruments, singing, wind and brass instruments

• Not possible with fixed pitch instruments (e.g., the piano or mallet percussion like marimba or glockenspiel)

• At audio rates = frequency modulation (fm)

Filtering passes

• Low pass (high-frequency attenuation)

• High pass (low-frequency attenuation)

• Band pass (attenuation above and below a central portion)

• Band reject (attenuation of a central portion)

What is a Wah-Wah?

• Effect was popularized by many rock/blues guitarists favourite toy → wah-wah pedal

• Wah-wah pedal → consists of a band-pass filter which attenuates low and high frequencies in varying amounts

Vocoders characteristics

• Examines speech by measuring how its spectral characteristics change over time. Results in a series of signals representing these modified frequencies at any particular time as the user speaks

• Signal is split into a number of frequency bands (the larger this number, the more accurate the analysis) and the level of signal present at each frequency band gives the instantaneous representation of the spectral energy content

• Typically analyze an incoming signal by splitting the signal into a number of tuned frequency bands or ranges

• Modulator and carrier signal are sent through a series of these tuned bandpass filters

• Frequency components of the modulating signal are mapped onto the carrier signal as discrete amplitude changes in each of the frequency bands

What does feeding the delayed signal back into the delay device do?

Increases the effect and/or modifies it dramatically

Time Stretching

Changing the duration of a sound file without altering the pitch

When is Time Stretching useful?

• Trying to adjust a loop at a certain tempo

• Trying to adjust a sound effect to the length of some moving images (e.g., door closing)

-Slow speech down for foreign audiences

• Gives a sense of velocity to sound effects (e.g., car passing)

Normalizing

The volume adjustment that brings the peak amplitude at 0 dB (max amplitude)

When to use Gain?

• More gain can lead to saturation

• Sometimes wanted in analogue recording

• Avoid in digital recording

Limiter

• Like a compressor with a very high compression ratio

• Designed so that the audio level does not exceed a set limit

• Useful for protecting equipment (e.g., speakers, radio transmitters, etc)

Expander

Increases dynamic range

Gate/noise gate

• Electronic device or software that is used to control the volume of an audio signal

• In simplest form → a noise gate allows a signal to pass through only when it is above a set threshold (e.g., when the gate is 'open')

Gating

• A method used to lessen the prominence of noise that already exists within the signal

• Signal input to the noise gate falls below a preset threshold, the gate closes by reducing the output gain to zero at a rate defined by an attack control

• When the signal level exceeds the threshold, the gate opens by restoring full gain at a rate defined by a release control

• Reduces noise in a signal by passing the signal only when it is at a significantly high level

• When the signal falls below this level, the noise floor may become audible but the closing of the gate blocks it

Applications of Gating

• Noise gate often used in radio interviews

• Rhythmic gating used in music

• Percussive ducking

• Tremolo

Tremolo

• A cyclical variation of the amplitude, generally at sub-audio (less than 20 Hz) speeds

• At audio rates = amplitude modulation (am)

Amplitude modulation

Refers to cases where the volume varies through time (e.g., volume automation)

Early experiments in Technology

• Dominant technology being explored by inventors, engineers, and instrument builders was electricity

• Interests crossed, and composers looked to the new technology for new sounds and concepts

• Used new and modified instruments to create music that was previously impossible (Theremin - 1920)

• Varèse: "obtaining different vibrations of sound, just as the painter achieves different gradations and intensities of colour."

• Varèse came up with the theory that music is organized sound.

When did Varèse argue the music could only advance through the use of electrical technology?

As early as 1916

Pierre Schaeffer

• Coined the term 'musique concrète' to describe a music made 'concretely' by working directly with sounds, as against music made 'abstractly' by working with symbols for sounds (as in a musical score)

• First time in music history, the composer was able to work directly in his medium without the need for interpreters (performers)

Musique concrète

Style of avant-garde music that relies on recorded sounds, including natural environmental sounds and other noises that are not inherently musical, to create music

Speed change

Definition: Playing the tape back at a different speed from that at which it was recorded

• Would alter the character of a sound, raising or lowering its pitch and deepening its timbre

Schaeffer explored several transformational processes:

• Editing out portions of the sound (rearranging sounds or changing rhythm)

• Varying the playback speed (changing pitch and tempo)

• Playing the sounds backward

• Combining different sounds

In 1951, Schaeffer organized the Groupe de Recherche de Musique Concrète, the focus of which was working with ____

Tape recorders

Any natural sound could be treated as a ____

Sound object

The Sound Object

• All sounds were equally suitable as a basis for musical material