Acoustic Quadraphony:

a new meta-format for cinema surround audio

Dr. Diego GonzalezDr. Davide Bonsi

What should be the future of sound for Digital Cinema?

• Cinema sound has gone through many quality jumps along its history, from mono to current multi-channel surround systems, to follow the evolution of image quality

• With the advent of Digital Cinema, a major quality jump regarding the audio experience is expected

• What requisites shall the new systems accomplish to satisfy the market requirements?

From the point of view of the audience, the new systems shall improve quality, overcoming current limitations such as:

• limitations of the optimal listening area (sweet spot)

• lack of naturalness in acoustic environment rendering

• lack of third spatial dimension (vertical component)

From the point of view of the industry, new systems should feature:

• cost effectiveness of baseline service, backwards compatibility with existing systems and sufficient flexibility for forward compatibility with possible future developments

• universality of the format, allowing for effective data encryption and compression

• improvement of theatrical playback for the quality of the audio experience

• relevant metadata support

Acoustic Quadraphony is a very promising approach to tackle many of these requirements!

A fundamental problem in developing a universal format is being able to provide a signal that contains all the information about the sound event, including the acoustic environment.

This challenge can be solved by means of a physical approach that consists of capturing the pressure and the three components of the air velocity related to the sound event. These four signals are the quadraphonic data.

Acoustic Quadraphony implementation

Contrarily to the measurement of pressure, developed in the 19th century, the direct measurement of air velocity has been possible only recently, thanks to the invention of intensimetric probes at Twente University

• Thanks to the linearity of the acoustic environment response, it is possible to generate quadraphonic data by a 4-dimensional convolution between quadraphonic metadata (4-impulse responses) and a target anechoic sound.

• This approach allows for the implementation of a complete audio workflow for Digital Cinema

Rendering procedure

The quadraphonic audio workflowProduction

Quadraphonic Recording

& Metadata Measurement

Post-production

Quad Post-prod.

Real&Virtual Environments

Anechoic

Exhibition

Quadraphonic Processing

Up-Downwards Conversion

5.1 and others

Library of Quad Impulse-responses

From Simulation

Virtual environments

P

Vx

Vy

Vz

gp

gx

gy

gz

4-Convolution(pressure signal)

Acoustic

Data

Quadraphonic

Metadata

Pseudo

Anechoic

Other Quad

Related Metadata

-relative position

(source-receiver)

-source movement

-absolute source

reference

-intensimetric

indices,etc.

From pre-recorded

environments

From Calculation

(Effects)

ExhibitionUp-downwards Compatibility

Monophonicpressure signal

Pseudo-anechoic

Quadraphonic audio Workflow

Centre channel(Dialogs)

Decoding

Stereo(Music)

5.1(Sound effects)

ProprietarySystem

Wave-fieldSynthesis

& otherprototype systems

Active control- one-dimensional- three-dimensional

AcousticAmbience

3rdDimension

From therecordinglocation

DTS® or Dolby®

encoding

Rendering of

extended

source

Post-processing

Quad

Signal

Direct recording

Real environments

4-Impulse Responses

Deconvolution(pressure signal)

Acoustic Quadraphony:the right approach to audio virtual reality

Direct measurement ofsound pressure and air particle velocity

Air velocity sensors are based on double hot-wire anemometry

The air flow produces a variation of the temperature profile near the wires, causing a change in their electrical resistance, generating a voltage difference that is proportional to the air velocity

The Microflown® USP probe combines three o r t h o g o n a l v e l o c i t y transducers and a pressure microphone in coincident configuration

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Direct recording

The recording of the dry sound of actors and musical sources can be made in anechoic chambers or in reflection-free environments.

As an alternative, direct quadraphonic recordings can be made live on location capturing both sound pressure and velocity.

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Video:Pseudo-anechoic recording of a choir

Quad IR measurement

FSSG-CNR has implemented a procedure for the measurement of pressure and velocity impulse responses, allowing for the complete character izat ion and the fa i th fu l reconstructions of the acoustic properties of the environments

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Video: Measurement of quadraphonic impulse

responses in San Giorgio Church - Venice

Intensimetric indices

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1Adimensional index

Centre Frequency (Hz)

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From the Quad IR it is possible to derive intensimetric indices that describe the energetic properties of a sound field, such as the sound energy flow directions, the energy decay curves and the reverberation time

Post-processing

Anechoic source

Quadraphonic convolution

Virtual acoustic environment

Quadraphonic acoustic sound

Transcoding to consumer format

(mono, stereo, surround)

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dvd authoring

Quad audio can be encoded in any file format with any number of channels and configuration: future proof compatibility is guaranteed!

Surround sound produced with Acoustic Quadraphony can be delivered in any standard format, including Dolby® and DTS® for DVD-Video and DVD-Audio.

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Playback

Quad audio is suitable to be reproduced by any standard and future playback system. Realistic surround sound can be reproduced in 5.1 home theatre systems and cinemas as well.

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Moreover, quad technology allows for the assessment of the playback quality.

Active playback of Quad sound

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FSSG-CNR developed a prototype system that allows for the faithful reconstruction of the original sound field (pressure & velocity) and its energetic properties based on the solution of the acoustic inverse problem.

Quad audio signals are sent to the speakers through a feed-forward control unit processing metadata previously measured in the listening room. This ensures the best reconstruction of the natural timbre and spatial characteristics of the recorded sound.

The process, tested and validated on one-dimensional fields, has recently been extended to s tandard surround configurations and other 3D systems with an arbitrary number of channels (Patent Pending)

Thank you for your attention!