06_CM0340_Synthesis

06_CM0340_Synthesis - 173 JJ II J I Back Close Some...

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Unformatted text preview: 173 JJ II J I Back Close Some Practical Multimedia Digital Audio Applications Having considered the background theory to digital audio processing, let’s consider some practical multimedia related examples: • Digital Audio Synthesis — making some sounds – Related : Digital Audio Effects — changing sounds via some standard effects ( See CM0268 ). • MIDI — synthesis and effect control and compression 174 JJ II J I Back Close Digital Audio Synthesis We have talked a lot about synthesising sounds. Several Approaches: • Subtractive synthesis • FM (Frequency Modulation) Synthesis • Additive synthesis • Sample-based synthesis • Wavetable synthesis • Granular Synthesis • Physical Modelling 175 JJ II J I Back Close Subtractive Synthesis Basic Idea : Subtractive synthesis is a method of subtracting overtones from a sound via sound synthesis, characterised by the application of an audio filter to an audio signal. First Example: Vocoder — talking robot (1939). Popularised with Moog Synthesisers 1960-1970s 176 JJ II J I Back Close Subtractive synthesis: Simple Example Simulating a bowed string • Take the output of a sawtooth generator • Use a low-pass filter to dampen its higher partials generates a more natural approximation of a bowed string instrument than using a sawtooth generator alone. 2 4 6 8 10 12 14 16 18 20 − 0.5 − 0.4 − 0.3 − 0.2 − 0.1 0.1 0.2 0.3 0.4 0.5 2 4 6 8 10 12 14 16 18 20 − 0.4 − 0.3 − 0.2 − 0.1 0.1 0.2 0.3 • subtract synth.m MATLAB Code Example Here. 177 JJ II J I Back Close Subtractive Synthesis: A Human Example We can regard the how humans make noises as subtractive synthesis: Oscillator — the vocal cords act as the sound source and Filter — the mouth and throat modify the sound. • Saying or singing ”ooh” and ”aah” (at the same pitch.) • Vocal chords are generating pretty much the same raw, rich in harmonic sound Difference between the two comes from the filtering which we apply with the mouth and throat. • Change of mouth shape varies the ”cutoff frequency” of the filter, so removing (subtracting) some of the harmonics. • The ”aah” sound has most of the original harmonics still present, • The ”ooh” sound has most of them removed (or to be more precise, reduced in amplitude.) 178 JJ II J I Back Close Subtractive Synthesis: Another Human Example A sweeping filter ”ooh”s to ”aah”s again • By gradually changing from ”ooh” to ”aah” and back again – simulate the ”sweeping filter” effect • Effect widely used in electronic music • Basis of the ”wahwah” guitar effect, so named for obvious reasons. • We will see how we produce this effect in MATLAB code shortly. 179 JJ II J I Back Close Subtractive Synthesis: One More Human Example Making Aeroplane Noise Make a ”ssh” sound — white noise • Now ”synthesise” a ”jet plane landing” sound • Should mostly by use mouth shape to filter the white noise into pink noise by removing the higher frequencies.pink noise by removing the higher frequencies....
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This note was uploaded on 01/26/2012 for the course CM 0340 taught by Professor Davidmarshall during the Fall '09 term at Cardiff University.

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06_CM0340_Synthesis - 173 JJ II J I Back Close Some...

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