Noise reducing audio filters
Lukas Everding
Outline
● Analogue signal filtering
○ dual ended noise reduction
○ single ended noise reduction
● Digital signal filtering
○ signal conversion and quantisation noise
○ dither
○ Fourier analysis
Basics
Signal-to-noise ratio (SNR)
● Mean signal power/Mean noise power
● Measured in dB
● SNR = 10 log( MSP/MNP) dB
Analogue signals
Dual ended noise reduction systems (Companding)
● Ray Dolby used this in 1966 for magnetic tape recorders
(Dolby A and Dolby B filter)
● signal strength of weak signals is amplified during recording
time (compressing of dynamical range)
● the process is reversed for playback (expanding of DR)
● SNR enhanced by ~10dB
● improved version amplifies in dependence on the frequency
example: xylophone (from: https://www.hsu-hh.de/ant/)
Analogue signals
Preemphasis & Deemphasis
● amplifying now dependent on frequency
● higher frequencies are boosted
Picture from: https://secure.wikimedia.org/wikipedia/de/wiki/Preemphasis
Analogue signals
Single ended noise reduction systems (dynamic NR)
● not suited for recording, produces artifacts and reduces
original amplitude information
● used in telephony
● weak signals are split
○ Signal 1 is phase shifted by 180°
○ Signal 1 is high pass filtered with cut-off 4kHz
● recombining erases noise above 4kHz
● SNR approx. 10 dB enhanced
● very fast and cheap
Digital signals
Conversion of analogue signals into digital data
● SNR ~ k²
(for N bits ~ 22N)
● ~6.02*N dB
(CD 16bit = 96.3 dB)
● quantisation noise
=> oversampling
=> non-linear
quantisation
=> noise shaping
Picture from: https://en.wikipedia.
org/wiki/Quantization_%28signal_processing%29
Digital signals
Dithering
● quantisation error causes non-neglectable distortions for
small amplitudes and high frequencies
● Solution: add small amount of random noise
● white noise can be filtered more easily by the human ear
than harmonic distortions
(a very good article with examples on this can be found
under: http://www.audiocourses.com/article1627.html )
Picture from: https://secure.wikimedia.org/wikipedia/en/wiki/Dither
Digital signals
Fourier analysis & synthesis
● Signal is decomposed in Fourier space
● frequencies that reach a given threshold are back
transformed
● can be done in almost real time by today's microchips
Digital signals
Fourier analysis & synthesis
Pictures from: https://secure.wikimedia.org/wikipedia/de/wiki/Rauschunterdrückungsverfahren
Sources
Thomas Görne: Tontechnik. 1. Auflage, Carl Hanser Verlag, Leipzig,
2006
Friedrich, Hans Jörg: Tontechnik für Mediengestalter: Töne hören,
Technik verstehen, Medien gestalten. Springer-Verlag Berlin
Heidelberg: 2008. (Format: PDF)
http://www.audiotools.com/noise.html
http://www.audiocourses.com/
http://web.audacityteam.org/
http://clas.mq.edu.au/speech/perception/
http://de.wikimedia.org/wiki/Kategorie:Rauschen
http://en.wikipedia.org/wiki/Category:Noise
Thank you for your attention!
Questions?