Acoustics II: time reversal signal restoration clicks audio - - PowerPoint PPT Presentation
Audio signal processing effects equalizer compressor, limiter noise gates flanger chorus phaser leslie pitch-shift - time scaling Acoustics II: time reversal signal restoration clicks audio signal processing tape speed variations
Audio signal processing 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
Kurt Heutschi 2013-01-18
Audio signal processing 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
Audio signal processing 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
◮ effects: alteration of the original sound such as:
◮ adjustment of the frequency response ◮ reduction of the audibility of unwanted signal
components
◮ .... ◮ creation of new sounds
Audio signal processing 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
Audio signal processing 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
◮ function:
◮ manipulation of the amplitude response of a sound
◮ applications:
◮ flatten a non-ideal frequency response of a
loudspeaker
◮ loudness filtering ◮ attenuation of the low frequency end in PA
systems for speech
◮ equalization of headphones ◮ ....
Audio signal processing 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
◮ parametric equalizer:
◮ several bandpass/notch filters with variable ◮ center frequency ◮ bandwidth ◮ amplification/attenuation ◮ universal applicability, however not that intuitive
to reproduce a predefined amplitude response
Audio signal processing 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
◮ graphical equalizer:
◮ series of adjacent bandpass filters of constant
relative bandwidth
◮ third-octave filters (typ. 30 bands) ◮ octave filters (typ. 10 bands) ◮ adjustable amplification/attenuation in each band
(typ. +/- 15 dB)
◮ setting with linear controllers → graphical
representation of the amplitude response
Audio signal processing 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
Audio signal processing 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
◮ function:
◮ reduction of the signal dynamics
◮ applications for compressors:
◮ increase of loudness with given maximal signal
amplitude
◮ music production ◮ commercials ◮ radio stations (listeners are often in noisy
environment)
◮ applications for limiters:
◮ avoid clipping in digital recordings
Audio signal processing 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
◮ VCA: Voltage Controlled Amplifier ◮ steering voltage: smoothed input signal ◮ amplification is lowered above an adjustable
threshold
Audio signal processing 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
◮ example (threshold = 0 dB) ◮ input level → amplification
Audio signal processing 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
◮ example (threshold = 0 dB) ◮ input level → output level
Audio signal processing 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
◮ VCA is steered by the level of the audio signal ◮ determination of the level needs an average process ◮ choice of the time constant is critical ◮ implementation of two constants: positive for signal
increase and negative for signal decrease
◮ positive slope time constant too small →
distortion of start transients of musical instruments
◮ negative slope time constant too small →
pumping: modulation of the signal by dominant components
◮ positive/negative slope time constant too large →
signal dynamics is not properly reduced due to delayed reaction
◮ typical values:
◮ positive slope: order of milliseconds ◮ negative slope: 10 to 3000 milliseconds
Audio signal processing 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
◮ original sample
◮ → original
◮ compression starting 25 dB below full-scale →
value
◮ → with compression
Audio signal processing 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
◮ loudness war
◮ producers push perceived loudness to maximum ◮ increasing concern regarding audio quality ◮ loss in dynamic range ◮ unofficial dynamic range data base:
http://www.dr.loudness-war.info/
Audio signal processing 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
→ Metallica: Death Magnetic: My Acopalypse, 2008
Audio signal processing 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
Audio signal processing 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
◮ function:
◮ attenuation of low level signals
◮ applications:
◮ suppression of noise during pauses of the signal
Audio signal processing 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
◮ parameters: level and step
Audio signal processing 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
Audio signal processing 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
◮ summation of signal and a delayed copy ◮ slow variation (0.1 Hz) of the delay (couple of
milliseconds)
Audio signal processing 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
◮ → comb filter for delay τ:
◮ maxima: f = 0, 1
τ , 2 τ , 3 τ
◮ minima: f =
1 2τ , 3 2τ , 5 2τ
Audio signal processing 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
◮ time dependency of the delay variation function
◮ e.g. sinusoidal → uneven variation of the
maxima/minima frequencies
◮ in general → non-sinusoidal time function
Audio signal processing 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
◮ example of a non-sinusoidal time function of the
delay:
Audio signal processing 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
◮ original sample
◮ → original
◮ Flanger minDelay: 0.5ms, maxDelay: 5ms, variation:
sinusoidal with 0.1Hz
◮ → with Flanger
spectrogram:
Audio signal processing 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
Audio signal processing 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
◮ parallel arrangement of independent flangers ◮ delay times in the order of 10. . .25 ms ◮ listening impression of multiplication of an
instrument or voice
Audio signal processing 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
◮ original sample
◮ → original voice
◮ Chorus
◮ → with Chorus
Audio signal processing 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
Audio signal processing 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
◮ summation of the signal and a phase shifted copy ◮ slow variation of the phase shift ◮ spectral - non-harmonic - amplifications and
attenuations
Audio signal processing 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
Audio signal processing 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
◮ loudspeaker cabinet with rotating chassis ◮ effects:
◮ Doppler effect → frequency shifting ◮ chassis directivity → amplitude modulation ◮ room reflections → varying directional impression
Audio signal processing 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
Audio signal processing 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
◮ Hammond organ with Leslie
◮ → sequence I + ◮ → sequence II ◮ → John Lee Hooker: I Cover the Waterfront
Audio signal processing 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
Audio signal processing 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
◮ aim:
◮ pitch variation without scaling of the time axis ◮ time axis scaling without pitch variation
◮ principle:
◮ audio signal is written in buffer ◮ up-shifting: ◮ clock of writing data < clock of reading data ◮ samples have to synthesized → ”
identification of loops”
◮ down-shifting: ◮ clock of writing data > clock of reading data ◮ ”
removal of samples”
Audio signal processing 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
◮ original sample
◮ → original
◮ 20 % faster
◮ → 80 % of original length
◮ original sample
◮ → Original
◮ half as fast
◮ → doubling of length
◮ twice as fast
◮ → length halved
Audio signal processing 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
Audio signal processing 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
◮ speech:
◮ → original ◮ → inverted time axis
◮ music:
◮ → original ◮ → inverted time axis
Audio signal processing 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
Audio signal processing 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
◮ usually applied during transfer of old sound storage
media to digital formats
◮ vinyl discs → CD ◮ compact cassettes → CD
Audio signal processing 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
◮ typical artifacts of old recordings:
◮ clicks ◮ tape speed fluctuations ◮ hum (mains frequency) ◮ noise ◮ distortions
Audio signal processing 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
◮ aims:
◮ recovery of the original sound ◮ in some cases make improvements with respect to
◮ e.g. extension of a limited frequency response ◮ in future: model-based solutions ◮ detection of sound objects ◮ re-synthesis of the signal in perfect audio quality
Audio signal processing 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
Audio signal processing 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
◮ removal of clicks:
◮ typical for signals stemming from vinyl discs ◮ strategy: ◮ detection of the clicks ◮ delete and replace the section by estimated signal
(interpolator)
Audio signal processing 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
Audio signal processing 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
◮ removal of tape speed variations (→ pitch
variations):
◮ typical for signals stemming from tape recordings ◮ strategy: ◮ short-time spectral analysis with pitch detection ◮ with a priori knowledge about the original pitch
→ identification of the time axis distortion
◮ apply inverse time axis distortion
Audio signal processing 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
Audio signal processing 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
◮ removal of hum (and possible harmonics):
◮ strategy: ◮ narrow-band notch filter ◮ problem: ◮ → notch filter impulse response: pulse at τ = 0
and at τ = 10ms
◮ → addition of a sort of echo → possible audible
artifacts
Audio signal processing 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
Audio signal processing 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
◮ removal of broad band noise:
◮ typical for all early sound storage media ◮ principle: ◮ establish filter bank with time variable
amplification in each band
◮ if signal is present → reference amplification ◮ dominating noise → very small amplification
Audio signal processing 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
◮ strategy:
◮ assumption of constant power spectral density of
the noise component
◮ estimation of the noise spectrum in signal pauses
(” minimum during the whole piece” )
◮ short-time spectral analysis of the signal →
evaluation of S/N
◮ determine amplification of each spectral line from
S/N
◮ delicate choice of the time constant for the
amplification adaptation
Audio signal processing 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
Audio signal processing 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
De-Noise
Audio signal processing 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
De-Click
Audio signal processing 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
Remove Hum
Audio signal processing 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
De-Clip
Audio signal processing 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
de-clicking: step 1 de-hissing: step 2 fadings: step 3
Audio signal processing 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
eth-acoustics-2