Mason R., Rumsey F. - An Assessment of the Spatial Performance of Virtual Home Theatre Algorithms by Subjective and Objective Methods.pdf - Akustyka - dnbdbstp

DepartmentofMusicandSoundRecording

TheInstituteofSoundRecordingpapers

UniversityofSurrey Year2000

AnAssessmentoftheSpatial

PerformanceofVirtualHomeTheatre

AlgorithmsbySubjectiveandObjective

Methods

RussellMason FrancisRumsey

ThispaperispostedatSurreyScholarshipOnline.

http://epubs.surrey.ac.uk/recording/4

An Assessment of the Spatial Performance of

Virtual Home Theatre Algorithms by Subjective and Objective Methods

5137 (L - 6)

Russell Mason and Francis Rumsey

Institute of Sound Recording, University of Surrey, Guildford, Surrey, UK

Presented at

the 108th Convention

2000 February 19-22

Paris, France -

AUDIO

This preprint has been reproduced from the author’s advance

manuscript, without editing, corrections or consideration by the

Review Board. The AES takes no responsibility for the

contents.

Additional preprints may be obtained by sending request and

remittance to the Audio Engineering Society, 60 East 42nd St.,

New York, New York 10165-2520, USA.

thereof, is not permitted without direct permission from the

Journal of the Audio Engineering Society.

AN AUDIO ENGINEERING

SOCIETY PREPRINT

An assessment of the spatial performance of virtual home theatre

algorithms by subjective and objective methods

Russell Mason and Francis Rumsey, Institute of Sound Recording,

School of Performing Arts, University of Surrey,

Guildford, Surrey, GU2 5XH, UK

r.mason@surrey.ac.uk

f.rumsey@surrey.ac.uk

Abstract:

A controlled subjective test was carried out to assess selected spatial qualities of three virtual

home theatre processors. The subjective results were used to evaluate a number of objective

measurements based on the interaural cross-correlation coefficient (IACC). A novel

implementation of the IACC was found which appears to correlate well with the subjective

data.

0 Introduction

This paper documents part of the work carried out under the Eureka 1653 MEDUSA

(Multichannel Enhancement of Domestic User Stereo Applications) project. The MEDUSA

project involves collaborative research between the following partners: the British

Broadcasting Corporation, the Institute of Sound Recording at the University of Surrey,

Nokia Research Centre, Genelec Oy, and Bang & Olufsen A/S.

The purpose of the project is to examine the variables of the domestic multichannel sound

system, with and without picture, to carry out the essential optimisation leading to consumer

end products. These products will combine the requirements of multichannel reproduction

together with the less complex modes of reproduction, such as stereo and mono. This involves

linked studies of programme production and perceptual elements, leading to a single

optimised approach to domestic reproduction.

A great deal of the research carried out within the MEDUSA project involves subjective

listening tests. These subjective experiments are both expensive and time consuming to carry

out. As an alternative to this, objective measures that correlate well with certain subjective

parameters would be more accurately repeatable and would save time and money [1].

Therefore, it would be useful if subjective assessments could be replaced or complemented by

objective measurement methods. Currently it may be an impossible task to replace subjective

assessments completely. However, there are measures that are established or under

development which may correlate well with some aspects of spatial perception.

Whilst a great deal of research has been completed into the aspect of localisation in

reproduction systems, ‘spatial impression’ has so far been left behind [2]. Perhaps one reason

for this is the comparative simplicity with which localisation can be evaluated in listening

tests. In contrast, spatial impression is a much more complicated multidimensional subjective

phenomenon. In this case, spatial impression is defined as the auditory perception of the

location, dimensions, and other physical parameters of a sound source and the acoustic

environment in which the source is located.

In some areas, spatial impression has been researched in detail. This includes the perception

of concert hall acoustics. Not all of the measurable or perceivable categories used in concert

hall acoustics are relevant to the reproduction of sound in small rooms, but there are definite

parallels. Beranek provides a good overview of this [3].

The research into concert hall acoustics also proposes a number of objective measurements

that help to predict how a listener will perceive the sound of a concert hall. Among these is

the interaural cross-correlation coefficient (IACC), a measurement which was first worked on

in the late 1960s. Work by Schroeder, Gottlob and Siebrasse found that IACC was one of a

number of physical measures that correlated well with listener preferences [4]. Ando

confirmed this and found that it was independent of reverberation time [5].

The IACC has seldom been tested using reproduction systems [6, 7, 8]. There are also

arguments that the IACC is inadequate due to its poor low frequency differentiation [9], and

that the IACC does not work for small rooms [10].

Therefore, an experiment was undertaken to test a reproduction system in a small room using

both subjective and objective measurements. The objective measurements were based on the

IACC and the results were examined to find correlations with subjective spatial attributes.

The reproduction chosen implemented various ‘virtual home theatre’ (VHT) systems as

described in [11]. This type of system aims to reproduce the spatial attributes of the original

multichannel material using only two loudspeakers. This is usually attempted by simulating

head-related transfer function (HRTF) cues with cross-talk cancelling. By using a system in

which some of the loudspeaker signals are already artificially spatialised, the challenge for the

objective measurement is possibly made more difficult.

1 Programme material

In order to quantify attributes of sound reproduction using subjective tests, it is necessary to

conduct controlled listening tests. Within these listening tests an experimental design needs to

limit extraneous variables to an absolute minimum. Because of this, the programme material

needs to contain a wide range of auditory cues, yet be limited enough not to confuse the

listener. Whilst this may in some cases limit the external validity of a test, it is sometimes

necessary in order to obtain sensitive results.

In this case, certain spatial attributes of various virtual home theatre algorithms were judged.

Based on the work of Berg [12], these attributes were limited to those of Apparent Source

Width (ASW), Listener Envelopment (LEV) – both defined in [13], and Depth (perceived

distance of the source from the listener). The simplest programme material available that

would sufficiently excite all three spatial attributes was a single source in a reverberant

environment. To produce a range of auditory cues, a number of acoustic environments and

sound sources were needed.

Therefore, programme material was recorded specifically for this experiment, consisting of a

number of sound sources in a number of acoustic environments. In order to separate the

variables of acoustic environment and sound source, the source was sounded and recorded in

each environment. If this had been done in the conventional manner of recording a

performance in each space, there would have been an additional variable. For a given musical

extract, even with the very best musicians, it would have been impossible to play exactly the

same twice or more. This would have added performance as a confounding variable in

judging the reproduction of the acoustic environments.

Replaying anechoically recorded excerpts through a loudspeaker in each of the acoustic

environments eliminated this variation. This method was necessarily a compromise as there

was no longer a real source sounding in each acoustic. The disadvantages of this approach

were due to the artificiality of this ‘virtual source’. This included the directionality of the

source and the physical coupling of the source to the air. Such factors as timbre, attack, decay,

and musicality should have been reproduced effectively by high quality reproduction. This

approach has been used successfully in previous experiments [14, 4].

1.1 Anechoic recordings

The most readily available source of anechoic recordings was the Bang and Olufsen CD that

contains anechoic recordings made for the Archimedes project. The recording of this is well

documented in [15].

In order to present a wide range of auditory cues, the programme material needs to contain a

range of sound sources. This should ideally include examples such as transients, sustains,

both wide-band and narrow-band (tuned) signals, a wide range of frequencies, and a human

voice. There should also be sufficient gaps in the extracts so it is possible to hear the effect of

the acoustics.

The extracts used from the B&O CD were Cello (sustained, tuned, low frequency) and

Trumpet (mixture of transient attacks and sustains, tuned, mid-high frequencies). Two

additional extracts were recorded in the free-field room at BBC Research and Development in

Kingswood Warren, UK. These were snare drum (transient, wide frequency range, separated

hits) and a male speaking voice (a mixture of noise and modulated tonal sounds - a popular

test item).

The recordings were made in mono with a Brüel and Kjær 4006 omnidirectional microphone

connected via a custom pre-amp and phantom power supply to a Tascam DA-30 DAT

recorder using the internal converters. The aim of the recording was to produce a result which

when replayed sounded as natural as possible. In order to do this, the recording was

monitored on a single large loudspeaker and compared with the natural sound from the

source.

It has been found that it is easier and more efficient to judge audio signals that are stationary

and possibly repetitive [2, 16]. Because of this, the snare drum and trumpet excerpts were

made up of a short loop of a bar or so. This loop was repeated for 60 seconds to match the

duration of the other extracts.

The relative reproduction level of each of the sound sources is also important in recreating it

as accurately as possible. Using a Brüel and Kjær SPL meter with a Brüel and Kjær 4145 1-

inch capsule, A-weighted SPL measurements with a fast time constant were made of an

example of each sound source represented. From this, the relative level of each source was

calculated and referenced to a calibration signal of pink noise at 85 dBA at 1 metre from the

loudspeaker. A DAT was compiled of the excerpts adjusted to the correct level. As a final

check, the DAT was replayed at its reference level next to the corresponding source

reproducing a similar phrase.

1.2 Choice of reproduction loudspeaker

After informal listening, it was apparent that the choice of loudspeaker for reproducing the

anechoic recordings was important as it had a significant effect on the perceived result. The

ideal situation would be to reproduce each sound source through a loudspeaker that matches

the source most closely in terms of size, shape, directionality and frequency response.

Mason R., Rumsey F. - An Assessment of the Spatial Performance of Virtual Home Theatre Algorithms by Subjective and Objective Methods.pdf

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: