Audio signal processing - ETH Zisistaff/courses/ak2/... · Audio signal processing e ects equalizer compressor, limiter noise gates anger chorus phaser leslie pitch-shift - time scaling

Audio signalprocessing

effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie

pitch-shift - time scaling

time reversal

signal restoration

clicks

tape speed variations

hum

noise

examples

back

Acoustics II:audio signal processing

Kurt Heutschi2013-01-18


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

effects


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

effects: introduction

I effects: alteration of the original sound such as:I adjustment of the frequency responseI reduction of the audibility of unwanted signal

componentsI ....I creation of new sounds


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

equalizer


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

equalizer: function

I function:I manipulation of the amplitude response of a sound

or a transmission system

I applications:I flatten a non-ideal frequency response of a

loudspeakerI loudness filteringI attenuation of the low frequency end in PA

systems for speechI equalization of headphonesI ....


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

equalizer: realizations

I parametric equalizer:I several bandpass/notch filters with variable

I center frequencyI bandwidthI amplification/attenuation

I universal applicability, however not that intuitiveto reproduce a predefined amplitude response


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

equalizer: realizations

I graphical equalizer:I series of adjacent bandpass filters of constant

relative bandwidthI third-octave filters (typ. 30 bands)I octave filters (typ. 10 bands)

I adjustable amplification/attenuation in each band(typ. +/- 15 dB)

I setting with linear controllers → graphicalrepresentation of the amplitude response


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

compressor/limiter


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

compressor/limiter: function

I function:I reduction of the signal dynamics

I applications for compressors:I increase of loudness with given maximal signal

amplitudeI music productionI commercialsI radio stations (listeners are often in noisy

environment)

I applications for limiters:I avoid clipping in digital recordings


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

compressor/limiter: principle of operation

I VCA: Voltage Controlled Amplifier

I steering voltage: smoothed input signal

I amplification is lowered above an adjustablethreshold


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

compressor/limiter: characteristics

I example (threshold = 0 dB)

I input level → amplification


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

compressor/limiter: characteristics

I example (threshold = 0 dB)

I input level → output level


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

compressor/limiter: time constantsI VCA is steered by the level of the audio signal

I determination of the level needs an average process

I choice of the time constant is criticalI implementation of two constants: positive for signal

increase and negative for signal decreaseI positive slope time constant too small →

distortion of start transients of musical instrumentsI negative slope time constant too small →

pumping: modulation of the signal by dominantcomponents

I positive/negative slope time constant too large →signal dynamics is not properly reduced due todelayed reaction

I typical values:I positive slope: order of millisecondsI negative slope: 10 to 3000 milliseconds


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

compressor/limiter: example

I original sampleI → original

I compression starting 25 dB below full-scale →approx. 10 dB higher signal power at identical peakvalue

I → with compression


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

compressor/limiter: finalizer

I loudness warI producers push perceived loudness to maximumI increasing concern regarding audio qualityI loss in dynamic rangeI unofficial dynamic range data base:

http://www.dr.loudness-war.info/


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

compressor/limiter: finalizer: loudness war

→ Metallica: Death Magnetic: My Acopalypse, 2008


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

noise gates


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

noise gates: function

I function:I attenuation of low level signals

I applications:I suppression of noise during pauses of the signal


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

noise gates

I parameters: level and step


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

flanger


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

flanger: principle

I summation of signal and a delayed copy

I slow variation (0.1 Hz) of the delay (couple ofmilliseconds)


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

flanger: comb filter

I → comb filter for delay τ :I maxima: f = 0, 1

τ ,2τ ,

3τ

I minima: f = 12τ ,

32τ ,

52τ


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back


I time dependency of the delay variation functionI e.g. sinusoidal → uneven variation of the

maxima/minima frequenciesI in general → non-sinusoidal time function


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back


I example of a non-sinusoidal time function of thedelay:


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

flanger: exampleI original sample

I → original

I Flanger minDelay: 0.5ms, maxDelay: 5ms, variation:sinusoidal with 0.1Hz

I → with Flanger

spectrogram:


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

chorus


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

chorus: principle

I parallel arrangement of independent flangers

I delay times in the order of 10. . .25 ms

I listening impression of multiplication of aninstrument or voice


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

chorus: example

I original sampleI → original voice

I ChorusI → with Chorus


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

phaser


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

phaser: principle

I summation of the signal and a phase shifted copy

I slow variation of the phase shift

I spectral - non-harmonic - amplifications andattenuations


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

leslie


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

leslie: principle

I loudspeaker cabinet with rotating chassis

I effects:I Doppler effect → frequency shiftingI chassis directivity → amplitude modulationI room reflections → varying directional impression


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

leslie: example of realization


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

leslie: sound examples

I Hammond organ with LeslieI → sequence I +I → sequence III → John Lee Hooker: I Cover the Waterfront


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back



effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

pitch-shift - time scaling: principle

I aim:I pitch variation without scaling of the time axisI time axis scaling without pitch variation

I principle:I audio signal is written in bufferI up-shifting:

I clock of writing data < clock of reading dataI samples have to synthesized → ”identification of

loops”I down-shifting:

I clock of writing data > clock of reading dataI ”removal of samples”


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

pitch-shift - time scaling: sound examples

I original sampleI → original

I 20 % fasterI → 80 % of original length

I original sampleI → Original

I half as fastI → doubling of length

I twice as fastI → length halved


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

time reversal


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

time reversal: audio samples

I speech:I → originalI → inverted time axis

I music:I → originalI → inverted time axis


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

restoration of distortedaudio signals


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

restoration of distorted audio signals

I usually applied during transfer of old sound storagemedia to digital formats

I vinyl discs → CDI compact cassettes → CD


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back


I typical artifacts of old recordings:I clicksI tape speed fluctuationsI hum (mains frequency)I noiseI distortions


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back


I aims:I recovery of the original soundI in some cases make improvements with respect to

original soundI e.g. extension of a limited frequency response

I in future: model-based solutionsI detection of sound objectsI re-synthesis of the signal in perfect audio quality


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

removal of clicks


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back


I removal of clicks:I typical for signals stemming from vinyl discsI strategy:

I detection of the clicksI delete and replace the section by estimated signal

(interpolator)


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

removal of tape speedvariations


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

removal of tape speed variations

I removal of tape speed variations (→ pitchvariations):

I typical for signals stemming from tape recordingsI strategy:

I short-time spectral analysis with pitch detectionI with a priori knowledge about the original pitch

→ identification of the time axis distortionI apply inverse time axis distortion


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

removal of hum


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

removal of hum

I removal of hum (and possible harmonics):I strategy:

I narrow-band notch filterI problem:

I → notch filter impulse response: pulse at τ = 0and at τ = 10ms

I → addition of a sort of echo → possible audibleartifacts


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

removal of broad-band noise


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back


I removal of broad band noise:I typical for all early sound storage mediaI principle:

I establish filter bank with time variableamplification in each band

I if signal is present → reference amplificationI dominating noise → very small amplification


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back


I strategy:I assumption of constant power spectral density of

the noise componentI estimation of the noise spectrum in signal pauses

(”minimum during the whole piece”)I short-time spectral analysis of the signal →

evaluation of S/NI determine amplification of each spectral line from

S/NI delicate choice of the time constant for the

amplification adaptation


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

examples


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

restoration of distorted audio signals:

example

De-Noise


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back


example

De-Click


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back


example

Remove Hum


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back


example

De-Clip


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back


example

original: originalde-clicking: step 1de-hissing: step 2fadings: step 3


effects

equalizer

compressor, limiter

noise gates

flanger

chorus

phaser

leslie


time reversal

signal restoration

clicks


hum

noise

examples

back

eth-acoustics-2

Documents

Audio signal processing - ETH Zisistaff/courses/ak2/... · Audio signal processing e ects equalizer compressor, limiter noise gates anger chorus phaser leslie pitch-shift - time scaling