2D to 3D ambience upmixing based on perceptual band allocation

3D multichannel audio systems employ additional elevated loudspeakers in order to provide listeners with a vertical dimension to their auditory experience. Listening tests were conducted to evaluate the feasibility of a novel vertical upmixing technique called “perceptual band allocation (PBA),” which is based on a psychoacoustic principle of vertical sound localization, the “pitch height” effect. The practical feasibility of the method was investigated using 4-channel ambience signals recorded in a reverberant concert hall using the Hamasaki-Square microphone technique. Results showed that the PBA-upmixed 3D stimuli were significantly stronger than or similar to 9-channel 3D stimuli in 3D listener-envelopment (LEV), depending on the sound source and the crossover frequency of PBA. They also significantly produced greater 3D LEV than the 7-channel 3D stimuli. For the preference tests, the PBA stimuli were significantly preferred over the original 9-channel stimuli

The Effect of Interchannel Time Difference on Localisation in Vertical Stereophony

Listening tests were conducted in order to analyse the localisation of band-limited stimuli in vertical stereophony. The test stimuli were seven octave bands of pink noise, with centre frequencies ranging from 125–8000Hz, as well as broadband pink noise. Stimuli were presented from vertically arranged loudspeakers either monophonically or as vertical phantom images, created with the upper loudspeaker delayed with respect to the lower by 0, 0.5, 1, 5 and 10ms (i.e. interchannel time difference). The experimental data obtained showed that localisation under the aforementioned conditions is generally governed by the so-called “pitch-height” effect, with the high frequency stimuli generally being localised significantly higher than the low frequency stimuli for all conditions. The effect of interchannel time difference was found to be significant on localisation judgments for both the 1000-4000Hz octave bands and the broadband pink noise; it is suggested that this was related to the effects of comb filtering. Additionally, no evidence could be found to support the existence of the precedence effect in vertical stereophony

Investigation on the Phantom Image Elevation Effect

Listening tests have been carried out in order to evaluate the phantom image elevation effect depending on horizontal stereophonic base angle. Seven ecologically valid sound sources as well as four noise sources were tested. Subjects judged the perceived image positions of phantom centre image created with seven loudspeaker base angles. Results generally showed that perceived images were elevated from front to above as the loudspeaker base angle increased up to around 180°. This tendency depended on the spectral characteristics of sound source. The perceived results are explained from both physical and cognitive points of view

Effect of Vertical Microphone Layer Spacing for a 3D Microphone Array

Subjective listening tests were conducted to investigate how the spacing between main (lower) and height (upper) microphone layers in a 3D main microphone array affects perceived spatial impression and overall preference. Four different layer spacings of 0m, 0.5m, 1m, and 1.5m were compared for the sound sources of trumpet, acoustic guitar, percussion quartet, and string quartet using a nine-channel loudspeaker setup. It was generally found that there was no significant difference between any of the spaced layer configurations, whereas the 0m layer had slightly higher ratings than the more spaced layers in both spatial impression and preference. Acoustical properties of the original microphone channel signals as well as those of the reproduced signals, which were binaurally recorded, were analyzed in order to find possible physical causes for the perceived results. It is suggested that the perceived results were mainly associated with vertical interchannel crosstalk in the signals of each height layer and the magnitude and pattern of spectral change at the listener’s ear caused by each layer

Evaluation of the Phantom Image Effect for Phantom Images

This paper introduces the author’s recent research on the elevation effect perceived with horizontal phantom images. Early research in stereophony suggests that a phantom centre image produced by two loudspeakers symmetrically placed from the listener position would be perceived in an elevated position, with its elevation angle increases as the loudspeaker base angle increases. In particular, an image presented from loudspeakers placed around the listener’s sides would be perceived overhead. With 3D audio formats employing height and overhead channels in mind, the aforementioned elevation effect is considered to be useful for creating a virtual overhead loudspeaker image, especially for sound effects using just ear-level loudspeakers (e.g. in downmix scenarios). Another important psychoacoustic principle relevant to 3D audio formats is the so- called ‘pitch-height’ effect, which suggests that the higher the frequency of a sound is the higher its image will be perceived. However, past research in this topic only considered loudspeakers placed in the median plane. From the above background, several subjective experiments have been conducted on the elevation of horizontally oriented phantom image. This paper first presents a vertical localisation test conducted with frontal stereo loudspeakers using octave-band noise stimuli. The results not only confirm the elevation effect for broadband noise, but also show the existence of an elevation effect for middle frequency bands. The second experiment introduced in this paper verifies the existence of the virtual overhead perception depending on loudspeaker base angle but also shows the effect heavily depends on the type of sound source

Towards the development of a universal listening test interface generator in Max

This engineering brief describes HULTI-GEN (Huddersfield Universal Listening Test Interface Generator), a Cycling ‘74 Max-based tool. HULTI-GEN is a user-customisable environment, which takes user-defined parameters (e.g. the number of trials, stimuli and scale settings) and automatically constructs an interface for comparing auditory stimuli, whilst also randomising the stimuli and trial order. To assist the user, templates based on ITU-R recommended methods have been included. As the recommended methods are often adjusted for different test requirements, HULTI-GEN also supports flexible editing of these presets. Furthermore, some existing techniques have been summarised within this brief, including their restrictions and how they might be altered through using HULTI-GEN. A finalised version of HULTI-GEN is to be made freely available online at: http://www.hud.ac.uk/research/researchcentres/mtprg/projects/apl

Psychoacoustic Considerations in Surround Sound with Height

This paper presents recent research findings in the psychoacoustics of 3D multichannel sound recording and rendering. The addition of height channels in new reproduction formats such as Auro-3D, Dolby Atmos and 22.2, etc. enhances the perceived spatial impression in reproduction. To achieve optimal acoustic recording and signal processing for such formats, it is first important to understand the fundamental principles of how we perceive sounds reproduced from vertically oriented stereophonic loudspeakers. Recent studies by the authors in this field provide insights into how such principles can be applied for practical 3D recording and upmixing. Topics that are discussed in this paper include the interchannel level and time difference relationships in terms of vertically induced interchannel crosstalk, the effectiveness of the precedence effect in the vertical plane, the aspect of tonal coloration resulting from vertical stereophonic reproduction, the effect of vertical microphone spacing on envelopment, the effect of interchannel decorrelation, and the use of spectral cues for extending vertical image spread

Sound Source and Loudspeaker Base Angle Dependency of the Phantom Image Elevation Effect

Early studies found that, when identical signals were presented from two loudspeakers equidistant from the listener, the resulting phantom image was elevated in the median plane and the degree of the elevation increased with the loudspeaker base angle. However, sound sources used in such studies were either unknown or limited to noise signals. In order to investigate the dependencies of the elevation effect on sound source and loudspeaker base angle in details, the present study conducted listening tests using eleven natural sources and four noise sources with different spectral and temporal characteristics for seven loudspeaker base angles between 0° and 360°. The elevation effect was found to be significantly dependent on the sound source and base angle. Results generally suggest that the effect is stronger for sources with transient nature and a flat frequency spectrum than for continuous and low-frequency-dominant sources. Theoretical reasons for the effect are also discussed based on head-related transfer function measurements. It is proposed that the perceived degree of elevation would be determined by a relative cue related to the spectral energy distribution at high frequencies, but by an absolute cue associated with the acoustic crosstalk and torso reflections at low frequencies

Towards the development of intelligent microphone array designer

This paper introduces a novel stereophonic localisation prediction algorithm and the concept of new interactive software that can assist recording engineers in configuring a microphone array to achieve desired stereophonic localisation characteristics in acoustic recording

Perceptual Band Allocation (PBA) for the Rendering of Vertical Image Spread with a Vertical 2D Loudspeaker Array

A series of subjective experiments were conducted to investigate a novel vertical image rendering method named “Perceptual Band Allocation (PBA),” using octave bands of pink noise with a vertical 2D reproduction setup with main and height loudspeaker pairs. The perceived height of each octave band was first measured for the main and height loudspeakers individually. Results suggested a significant difference between monophonic and stereophonic images in the perceived relationship between frequency and height. Six different test conditions have been created aiming for various degrees of vertical image spread, in such a way that each frequency band was mapped to either the main or height loudspeaker layer based on the results from the localization experiment. Multiple comparison tests were conducted to grade the perceived magnitude of vertical image spread. It was generally found that various degrees of vertical image spread could be rendered using different PBA schemes, but the perceived results did not fully match predicted results based on the localization results. Differences between the main and height loudspeaker layers in the spectral weightings of ear-input signal at certain frequencies was identified as one of the factors that influenced this result