Decorrelation algorithms with application to artificial reverberation
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Typ
Examensarbete för masterexamen
Program
Sound and vibration (MPSOV), MSc
Publicerad
2020
Författare
Favill, Guy
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Decorrelation is a process which decreases the cross correlation between signals
without destroying other qualities of those signals. In relation to audio, that means
that the cross correlation of decorrelated signals is reduced but the audio is perceived
as sounding the same. Decorrelation is known to reduce the effect of colouration,
phantom sources and precedence effect in a multi-channel audio situation, but can
also impart a greater spatial extent, diffuseness or headphone externalisation on
sound [1].
This thesis has two main parts. It first investigates and expands upon six different
known decorrelation algorithms. The performance of these algorithms is analysed
by various metrics, such as the extent to which they can decorrelate, impact upon
the frequency content of an input signal and the group delay introduced. Three
of the six decorrelation algorithms are selected as the best performing in these
regards: IIR allpass cascade decorrelation, Sub-band decorrelation and White Noise
decorrelation.
The second part of the thesis is to use these three chosen algorithms to decorrelate
the reverberant part of a Room Impulse Resonse (RIR) in order to simulate the
diffuse sound field of sound in a room on a multi-loudspeaker setup inside a nonreverberant
room. A listening test was performed, where participants were asked
to rate sounds with different cross correlation or different decorrelation methods in
terms of spaciousness and timbre.
The conclusion drawn as a result of this listening test is that for the RIR with a
longer reverberation time, a more decorrelated reverberant part of the audio does not
result in a more spacious perception, but for the less reverberant RIR, it does. The
Sub-band and White noise decorrelation algorithms produce the most perceptually
spacious sound.
The most decorrelated signals tested produce the largest difference in timbre when
compared to less decorrelated signals, but it is important to note that all levels of
decorrelation have a small to moderate difference in timbre when compared.
Further investigation into this topic would most usefully be focused on the cause of
an increase in low frequency content of audio for the least decorrelated audio when
compared to less decorrelated. This was noted in informal conversation with most of
the subjects that participated in the listening test, but not conclusively evidenced in
the results of that listening test. It is suspected that this low frequency “booming”
characteristic of the stimuli was percieved as spacious sounding, when it was not
necessarily an intended part of any decorrelation algorithm.
Beskrivning
Ämne/nyckelord
decorrelation, correlation, room impulse response, listening test