3D sound reproduction with “OPSODIS” and its commercial applications Takashi Takeuchi, PhD Chief Technical Officer OPSODIS Limited Institute of Sound and Vibration Research, University of Southampton Kajima Technical Research Institute

University of Southampton University of Southampton History 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 Student University of Southampton Acoustic Consultant (Kajima) Staff ISVR University of Southampton 3D sound Business MSc PhD (Invention) (OPSODIS) Auralisation OPSODIS Joint R&D Joint R&D Stereo Dipole OPSODIS (UK Ltd.)

Optimal Source Distribution What is OPSODIS™? Optimal Source Distribution Principle - 3D sound reproduction - Binaural - Loudspeaker reproduction - 2 channel system (3ch option) - Stereo & Surround - reproduction compatible What is special? - Lossless crosstalk cancellation! - Signal processing by loudspeaker - Good & natural sound quality - Robust control - Elevation (height) reproduction - Multiple listeners Picture above; acoustic image created with Stereophony. panning between two loudspeakers could generate a segment of sound images. could feel depth Picture below; acoustic image created with OPSODIS. true 3D sound image nothing to do with loudspeaker direction or distance. with high quality sound.

Spatial hearing and binaural signal Sound source signal s Characteristics of space (physics) （relative position，response of space） Spatial hearing (human) （source position，movement，room size，reverberation） stereo surround A2 binaural A1 Binaural signals d any number of sources … Physics 　source signal　　　　propagation　　　　two different signal at ears Perception 　Separates signal and spatial information　　　　binaural signals

Inverse filtering (crosstalk cancellation) Loudspeaker reproduction interweaves loudspeaker position In order to reproduce spatial information arbitrarily, independent transmission to each ear is necessary. Minimum of two independent (real) sound source necessary C22 Inverse filtering H C12 binaural Signals d Synthesised binaural signals C21 w C11

Conventional cross-talk cancellation Independent control at both ears Cancelling each other Right ear t Left ear t Loudspeaker output t R t Right ear L t t Left ear

Conventional cross-talk cancellation Independent control at both ears Right ear t Left ear t Loudspeaker output t R t Right ear L t t Left ear

Conventional cross-talk cancellation Independent control at both ears Massive destructive interference Right ear t Left ear t Loudspeaker output t R t Right ear L t t Repetitive delay = Large output (Frequency dependent) Left ear

Crosstalk cancellation (frequency) phase amplitude Destructive interference at both ears. Inefficient, quality loss, sensitive to errors.

Frequency dependency Conventional frequency frequency Left ear from Left SP from Right SP reproduced frequency frequency Left ear Right ear

Why have 3D systems not performed well so far? Frequency response to listener 6dB (two point sources) (_____ ) 　　　　　 0dB In-phase signal level (_ _ _ ) Out-of-phase signal -40dB 20Hz Frequency (linear) 20kHz Sound emitted by loudspeakers do not reach listener’s ears at certain frequencies. Large output required to compensate it.

Loss of performance through cross-talk cancellation process Typical frequency response of the inverse filters for 3D reproduction. 0dB Out-of-phase signal level In-phase signal -40dB 20Hz Frequency 20kHz

Loss of performance through cross-talk cancellation process 0dB Original Performance Dynamic range loss = Low Quality ≈ 5 bit Synthesized sound -30dB Distortion Noise level Exact matching 20Hz Frequency 20kHz Cross-talk cancellation degrades performance severely.

Analogy to picture quality Reduce; Constant resolution Magnify; Pixel size Quality Loss

Loudspeaker span 180° 10° (Stereo Dipole) ≈ - 3 bit frequency (log) Original Performance 0 dB ≈ - 3 bit ≈ - 7 bit -16dB Smaller loss More complexity Less complexity -43dB Larger loss Synthesized sound 20Hz 20kHz 20Hz 20kHz frequency (log) frequency

Optimal Source Distribution Conventional Transducer position vs Performance (excess amplification) poor 20kHz OPSODIS Conventional Stereo Dipole 2kHz frequency 200Hz high frequency low OPSODIS 20Hz good 0° 60° 120° 180° transducer position

Conventional OPSODIS (Stereo Dipole) The worst conditioned frequency dictates performance for all frequencies!! (Stereo Dipole) OPSODIS

OPSODIS loudspeaker Frequency response to listener with OPSODIS. No frequency dependency on amplitude response and phase response OPSODIS 0dB level -40dB frequency

OPSODIS principle Frequency response of the OPSODIS inverse filters. No frequency dependency on amplitude response and phase response 0dB robust Simple process No loss resulting from signal manipulations High quality 3D reproduction Natural & pure sound quality Elevation reproduction level -40dB frequency

Crosstalk cancellation of OPSODIS Independent control at both ears Right ear phase amplitude Left ear Cancellation R Right ear ½ amplitude, -90º phase Addition ½ amplitude L Left ear Loudspeaker output

Crosstalk cancellation of OPSODIS Independent control at both ears Right ear phase amplitude Left ear R Right ear ½ amplitude, -90º phase ½ amplitude L Left ear Loudspeaker output

Crosstalk cancellation of OPSODIS Independent control at both ears Right ear phase amplitude Left ear Destructive interference R Right ear ½ amplitude, -90º phase Constructive interference ½ amplitude L Left ear Loudspeaker output Efficient, Lossless, Robust to errors.

Reproduction quality Example: 16bit hardware Stereo Dipole frequency Reproduced dynamic range ≈ +3 dB Reproduced dynamic range ≈ -43 dB 16 bit 16 bit OPSODIS 16 bit 16 bit ≈9 bit L channel R channel reproduced

Electro-acoustic transducer for OPSODIS Flat panel loudspeakers plate high frequency low shaker driving plate narrow width wide Acoustic waveguide large stiffness small This principle requires a pair of monopole transducers whose position from which sound is radiated varies continuously as frequency varies. This might be realised by exciting a plate at each position individually. Or, exciting a triangular shaped plate whose width and stiffness varies along its length. Alternatively, changing the width of a slot along an acoustic waveguide might do the job. slot driver high frequency low narrow slot wide high frequency low Variation in excited frequency and radiation impedance

Discrete system Width of n enables discretisation frequency(Hz) It is also possible to realise a practical system even when a continuous variable span transducer pair is not available. By allowing n to have small width nu, it is possible to discretise into a number of transducer pairs with a limited frequency range. This principle is extremely useful and practical since a single transducer which can cover the whole audible frequency range is not practically available anyway. Discretisation also expands the operational frequency range by lowering fl. Please note that this is still a simple 2-channel sound system where only two independent signals are necessary to synthesise full 3D spatial sound environment. transducer span Width of n enables discretisation 2 channel, multi-way system Use of conventional transducers Wide frequency range U-shaped valley Small performance loss

Effects of room reflection (sound radiation) Small radiation by OPSODIS Not affected by reflection - dB 0 dB - dB 0 dB 0 dB Reference to radiation by mono loudspeaker Very large sound radiation other than receiver directions Needs anechoic room conventional OPSODIS

Radiation pattern of Conventional 0dB -∞dB Radiation pattern is different depending on the frequency. Single listener． Very small sweet area.

Radiation pattern of OPSODIS B A B A 0.6m 0.6m A: 1 or 3 listener(s) B: 2 listeners Radiation pattern is constant regardless the frequency; Large sweet area No spectral change; elevation works everywhere. Multiple listener listening with a little performance loss. The interval of L-R cross-talk cancellation is about 0.3m; Alternative seating positions.

Reproduced sound level (lateral position） Left ear signal Right ear signal Lateral position　（m） 3 seater or 2 seater

Aspects of the “OPSODIS” No dynamic range loss Quality (Lossless) Good S/N No distortion Robust to errors in plants and filters Robust to individual difference in HRTFs Generic inverse filters Flat amplitude response of inverse filters Elevation (height etc) works at any location No colouration at any location Small sound radiation other than receiver directions Robust to room reflections Consistent radiation over frequency Multiple listener listening With this assumption, the expression for the inverse filter matrix H becomes very simple like this, in fact, g is a real positive number close to 1, so the binaural synthesis over loudspeakers is nearly achieved by simple addition with 90° phase change. ||H|| is 1 over square root of 2 for all frequencies, so there are no dynamic range loss, which means good S/N and no distortion. The condition number is 1 which is smallest possible for all frequencies, so the system is robust to errors in plant and inverse filter matrix at all frequencies. Inverse filters have flat frequency response so there are no colouration at any location in the listening space, even outside the sweet spot. As illustrated here, the sound radiation by the OSD system is always smaller than by a single monopole transducer producing the same sound level at the ears. So it is also robust against reflections of a listening room.

3 channel OPSODIS

Inverse filter ~ 2 channel OSD frequency high low 0dB Out-of-phase component level In-phase component 2ch OSD -40dB Frequency/azimuth

Inverse filter ~ 3 channel OSD frequency high low high low low 0dB Out-of-phase component level In-phase component 3ch OSD -40dB Frequency/azimuth

Extension to 3 channel 2ch OSD 3ch OSD 2ch OSD 3ch OSD Out-of-phase component 2ch OSD 3ch OSD 2ch 2ch OSD In-phase component 3ch 3ch OSD Singular values of the inverse filter matrix H

3 channel OSD 2ch OSD peak 3ch OSD 2ch OSD 3ch OSD valley excess amplification peak (in phase) = valley (out of phase) 3ch OSD peak 20kHz peak (out of phase) = valley (in phase) 2kHz frequency 2ch OSD 200Hz 3ch OSD 20Hz valley 0° 60° 120° 180° transducer azimuth

Crosstalk cancellation of 3ch OPSODIS Independent control at both ears Right ear phase amplitude Left ear Cancellation ¼ amplitude, -90º phase R Right ear C ½ amplitude Addition Left ear L ¼ amplitude, +90º phase Loudspeaker output

Crosstalk cancellation of 3ch OPSODIS Independent control at both ears Right ear phase amplitude Left ear ¼ amplitude, -90º phase R Right ear C ½ amplitude Left ear L ¼ amplitude, +90º phase Loudspeaker output

Crosstalk cancellation of 3ch OPSODIS Independent control at both ears Right ear phase amplitude Left ear Destructive interference ¼ amplitude, -90º phase R Right ear C ½ amplitude Constructive interference Left ear L ¼ amplitude, +90º phase Loudspeaker output Efficient, Lossless, Quality, Robust to errors.

Crosstalk cancellation of 3ch OPSODIS Independent control at both ears Right ear phase amplitude Left ear Destructive interference 1/3 amplitude, -120º phase R Right ear C 1/3 amplitude Constructive interference Left ear L 1/3 amplitude, +120º phase Loudspeaker output Efficient, Lossless, Quality, Robust to errors.

Summary Combination of signal processing and the new type of loudspeaker based on a new principle enables ideal 3D sound delivered to ears. OPSODIS Enabling high definition 3D sound reproduction with 2 channel (or 3ch) loudspeakers Natural sound quality without distortion Natural sound at any location Effective in any room Compatible with surround reproduction