unit 5 - surround and 3d sound systems

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Unit 5

Surround & 3D

Sound Systems

Digital Audio Processing (20023)Sound and Image in Telecommunication Engineering

Course 2015/2016

Sergio Bleda PérezDepartment of Physics, Engineering Systems and Signal Theory

1



Introduction

2



Introduction

! In this unit we are going to review the different soundsystems available to produce surround sound

! But prior to see these systems we must review the

concepts of spatial hearing

3



Spatial Perception of Sound

4



Spatial Perception

! If you haven’t noticed yet: we have 2 ears

! Why two?

"

Because our sound localization mechanism needs two

different signals"

Using only one, localization is very very difficult

! But not always impossible

! To explain the sound localization mechanism, Lord

Rayleight proposed the Duplex theory in its ‘Theory ofSound’ in 1877.

5



Duplex Theory

! The sounds perceived by both ears are similar bothnot identical

" Comparing both sounds the brain is able to locate the sound

source

! If not the exact position, at least the direction of arrival (DOA)

!

The hearing sense localizes sound sources using

fundamentally two different parameters:

" Inter-Aural Time Difference (ITD)

"

Inter-Aural Intensity Difference (IID)

! Also known as Inter-Aural Level Difference (ILD)

! These parameters allow a left/right localization

"

They allow Azimuth localization (lateralization)6



ITD

! It measures the temporal difference between thearrival of the sound to the left and right ears

7

In this case, right signalarrives before left signal



ITD

! It’s useful for low frequency signals

" The phase shift (delay) between both ears is easy to calculate

with low frequency signals

!

What is the low frequency limit?"

1.5 kHz

" This frequency has a wavelength similar to the head size

! For higher frequencies, hearing uses the ITD of the

envelopes (not the waveform)" But it’s much less relevant than ITD for lower frequencies

8



IID

! It measures the difference of signal level/intensitybetween both ears

" The head produces a shadow zone

"

Consequently both ears doesn’t receive the same amount of

signal

! Unless the sound source is in front of (or at the back of) the head

9

Left ear is inshadow zone



IID

! It is useful to perform localization of high frequencysources

" Head produces sound shadows above 1.5 kHz

"

Below this limit, diffraction reduces considerably the

shadowing, avoiding effective localization

10

No shadowdue to diffraction



Duplex Theory

! Hearing uses both ITD and IID

! They are not used alternatively, they are used at the

same time

"

For low frequency content ITD" For high frequency content IID

! But this theory has limits, It can’t distinguish between:

" In front of / behind sources

"

Above / Below sources

11



Duplex Theory

! Example of ambiguity:

12

Both cases produce:ITD = 0



Cone of Confusion

! If we trace a imaginary straight line between both ears,we obtain the: interaural axis

" Using only the duplex theory we are going to obtain revolution

symmetry

13

Every sound sourcelocated in the cone

produce the sameITD & IID

Ambiguity



Improving localization

! To eradicate the ambiguity we need to add somedifferent aspects to the localization mechanism

! We need to include the effects of:

"

Head" Shoulders

"

Pinna

14

All of them produce reflections & diffractionover the sound




! Head, shoulders & pinna effects:

15

Soundsource

Soundsource

Head, shoulders & pinnaproduce ‘reverb like’ reflections




! If the listener or the sound source moves, all changes

" Reflections (and diffraction) are completely different

!

But, how all of these affect localization?

!

When we are young, the brain learns that a soundarriving from a given direction has a distinctive ‘reverb

like’ pattern

" These ‘reverb like’ patterns are known as: cues (marcas)

!

So, a sound including a given cue is detected ascoming from a given direction

16




! The use of the cues improves considerably the spatiallocalization mechanism

! But, there are still some ambiguities that are not

solved" Example: still in front of / behind sources

! To eliminate the remaining ambiguities, humans tendto move involuntarily the head

"

When you are looking for something that makes noise,instinctively you start moving the head

17



Other Additional Factors

! There are some other factors that affect thelocalization

! Additional factors:

"

Kind of sound" Length of the sound

! Long sounds are easier to find

" Onset

! The instant were the sound begins

" Spectral content of the source

! Wider spectral contents are easier to find

! On the other hand, tones are extremely difficult to find

18



Location of Several Sound Sources

! When there is more than one sound source, the soundof all the sources is overlapped

" That is to say, they are mixed together

! If the sound sources are incoherent or partially

incoherent! " Hearing treats each one alone

"

So, hearing detects one different location for each source

! If the sound sources are coherent

"

They are joined together

" So, hearing detects only one location

" This unique location is known as Phantom Image

19



Phantom Image

! When dealing with coherent sources, the phantomimage appears

" There is only one unique location

! But, which location?

" The location is a mean of the locations of the different sources

" But the mean is weighted by the levels of each source

! This is known as: Sum of locations

! Moreover, phantom Image depends on the spectral

content of the signal" In high frequencies the pinna interferes the location

" So, location of high frequencies tends to be diffuse

! HF Sound sources seem wider20



Location with Reverb

! All of the localization mechanism said previously are infree field conditions

" There is no room involved

! But in real world there is always some kind of room

"

Or at least the floor

" So, there will always be some reverb

! What signal arrives to the ear?

"

Direct sound + Several reflections

21



Location with Reverb

! What signal arrives to the ear?

" Direct sound + Several reflections

! In this case the hearing can locate the sound without

problem " But, the perceived sound is colored due to comb filtering

effects

! But, direct sound & reflections are coherent signals!

"

Theory says that there should appear a phantom imageproviding an incorrect location

"

But hearing has other useful tool to avoid this: Haas effect

22



Haas Effect

! The Haas effect is known under two other names:

" Law of the first wavefront

" Precedence effect

! The Haas effect does the following:

" Once the first wavefront (usually the direct sound) arrives

"

It inhibits the location mechanism during the following 2 to 50

ms

" This way the location of the reflections is avoided, effectively

blocking the formation of the (wrong) phantom image

!

Haas Effect is influenced by the head, shoulders andpinna reflections

"

It works better in the horizontal plane

23



Distance Perception

! What does hearing notice when a sound is near of far?

" It depends on reverberation

!

In free field conditions (without reverberation) :

" We appreciate a drop of 6 dB

! Each time we double the distance

"

Low frequencies are less attenuated than high frequencies

! Another aspect is the wavefront curvature:

"

Near sources produce a spherical wavefront

"

Far sources produce a planar wavefront

! But this is not noticed by hearing, at least not if we do not move

24



Distance Perception

! In diffuse field conditions (with reverb.)

" We notice a change in the direct sound/reverb proportion

!

Near sources have high content of direct sound

" And relatively low content or reverb

!

Far sources have more reverberation

" The reverb proportion is increased with distance

25



Movement Perception

! When the sound source and/or the listener are inmovement we obtain the Doppler effect

! When they are getting closer / away:

"

Apparent frequency is raised / lowered respectively

26

s

r

a f vc

c f !!

"

#$$

%

&

+

=

f a : apparent frequencyf s : real frequencyv r : relative speedc : speed of sound



Distance + Movement

! There is one additional effect due to distance andmovement both at the same time: Motion Parallax

! Motion Parallax:

"

Near moving sources produce large sound level differences

" Far moving sources have a constant level

" E.g: a flying mosquito at night (when you are trying to sleep)

! When is far you didn’t notice it (it has a constant low level)

! When is near ! it seems like a plane (it produces large soundlevel differences)

! And, of course: doppler

27



Distance + Movement

! This effect is not limited to sound!

! Example: A car running fast in a motorway

" High Speed: traffic barrier (near)

"

Low Speed: mountains (far)

28



Spatial Sound Systems

29



Spatial Sound Systems

! There are 3 different families:

" Stereophonic Systems

" Binaural Systems

"

Sound Field Reconstruction Systems

! It may be systems that are a mix between families

30



Monoaural Sound

! The first sound system was the monoaural, in whichthere is only one channel

! With this system there is no possibility to move the

sound out of the position of the speaker

!

All the sound sources are located in the position of the

loudspeaker

!

This is our starting point

31



Stereophonic Systems

32



Stereo

! The stereo systems uses 2 channels: L & R

" It produces a substantial enhancement in sound quality

!

It allows the (apparent) movement of the sound source

" The movement is only based on IID (it does not use ITD)

" Producing phantom sources

!

The use of IID for movement has a severe

consequence:

" We are using a parameter that uses the head as reference

33

In any stereophonic system, the reference point is

ALWAYS THE HEAD



Stereo

! Phantom sources are located between the arc thatunites both speakers

34

o! =

p!

Maximum aperture

must be 60º

Otherwise phantom

gets unstable

SWEET SPOTUnique place in which

phantom source is perceived OK



Stereo Panning

! Since we are using the IID the movement of thesource is controlled with a panning

" Sending more or less signal to each channel

! There are several options to calculate the proportion ofthe signals:

"

Linear law

" Sine law

" Tangent law

35



Stereo Panning

!

Linear law:

" This is the easiest law, but it is not advisable

" Since it does not maintain the apparent level of the source

during the movement

! Hearing is not linear, is logarithmic

36

L R

100 %

0 %

Phantom Source

Gain appliedto each channel

GR

GL

g L =1! g R



Stereo Panning

!

Sine law:

" This more elaborated law maintains a constant level when the

source moves

37

R L

R L

o

p

g g

g g

+

!

=

"

#

sin

sin

Maximumaperture

Phantomposition



Stereo Panning

!

Tangent law:

" This law maintains a constant energy level when the source

moves

38

Maximumaperture

Phantomposition

R L

R L

o

p

g g

g g

+

!

=

"

#

tan

tan

This is the mostaccurate panning law



Stereo Panning

!

Both previous laws only say the proportion betweengains

! But we need another equation to be able to calculate

the gains:" (Two equations for two unknowns)

39

1=+ R L

GG 122=+

R L GG

For Free Field For Diffuse Field



Stereo Panning

!

To take into account: mixing consoles and otherequipment has a panning control

! Panning control uses tangent law

"

But the maximum aperture used is 90º

!

In this case, tangent law with difusse field can be

simplified to the following equations:

40

)cos()sin(

P L

P R

G

G

!

!

=

=

This is why tangent law is alsoknown as Sine-Cosine law



Quadraphonic

!

Since the apparition of the stereo was a revolution,manufacturer of Hi-Fi equipment decided to go further

! They designed the quadraphonic systems

"

Instead of 2 channels they use 4 channels

!

It was a complete disaster

" In 1970 this was very difficult to implement

" Each manufacturer produced it’s own standard

"

With vinyl discs we needed 2 discs being reproduced at thesame time

" The use was simple with eight-track cartridges

41



Quadraphonic

!

As you can see, speakers do not maintain the rule:" The maximum aperture must be 60º

42



Phantom with several speakers

!

How do we locate phantom sources with more than 2speakers?

" The answer is the same for every stereo based system

! Phantom sources are always created using only 2speakers

"

Speakers are always used by pairs for creating phantom

sources, only two are used at a time

"

But remember to maintain always the maximum aperture of

60 degrees

43



Dolby Stereo

!

The Dolby Stereo system was designed for cinema

! There were two different domestic versions:

" Dolby Surround

" Dolby Surround ProLogic

!

It uses 4 different channels: L, R, C, S

" All of them were mixed in only two channels

" C (center) is used to maintain the dialogues stable

"

S (surround) is used for ambient sound

44



Dolby Stereo

!

Maximum aperture is 60º

45

30º-30º

0º

Phantom images

are only possible

with L & R speakers



Dolby Stereo

!

Example: Dolby Stereo in Cinema" As you can see, surround channel uses several speakers

" But everyone emits the same sound

46



Dolby Stereo

!

Example: Dolby Surround ProLogic at Home

47

The Dolby Surround version does not have the center channel



3/2 System

!

Also known as 5.0 system" Or 5.1 If it includes the subwoofer channel (LFE)

!

It uses 5 channels:

" L, R, C, LS & RS

! All the speakers must reside over a circle

! Since it is a stereo based system, it’s location power is

the same as stereo" It only allows a better ambient definition (surround)

48



3/2 System

!

Diagram:

49

100-120º100-120º

Surround

channelsare not fixed

They are not

used forlocation ofsources



3/2 System

!

Using more speakers does not enhance localization

50

6.1 7.1



10.2 System

!

This system has a slogan:" Twice as good as 5.1

" It’s designer is the engineer creator of THX

! As the name implies, it uses 10 channels plus 2 LFE

!

The interesting part is that is uses:

" 2 channels for elevation at 45º

! They allow the reproduction of sounds above the stage

"

2 Channels at 55º! They enhance localization

! And allow lateral reflections: increasing the spatial sensation

51



10.2 System

!

LH & RH: Left/Right height

! LW & RW: Left/right wide

52



22.2 System

!

This is a System proposed by the NHK" The national television of Japan

! It consist of 22 channels + 2 LFE

!

Channels are divided in layers, allowing elevation

effects

53



22.2 System

!

The placement of the speakers is as follows:

54



VBAP

!

Vector Based Amplitude Panning

! As the name implies, it uses an amplitude panning to

move the source around

! But now the movement can be in 3D too

"

Well, 3D with severe restrictions

! To allow elevation it needs an additional speaker

"

So now we are going to make pannings with 3 speakers"

Applying a generalized version of the tangent law

55



VBAP

!

We are going to maintain the listener as referencepoint

56

[ ]!!!

"

#

$$$

%

&

=

321

321

321

k k k

mmm

nnn

k mn

l l l

l l l

l l l

p p p g

Phantomcoordinates

Speakerscoordinates

Gain(1x3) We can use more than 3 speakers

But always making triangles



Binaural Systems

57



Binaural Systems

!

Stereo systems has severe limitations when dealingwith precise spatial location

" They are only using IID

! We could choose to use ITD instead (or both at a time)

"

Delaying the sound from each channel we can achieve thesame results than with IID

" But it’s more complex and it enhances nothing

! So, instead of using only IID (or ITD) we can make

another approach" Use everything: ITD + IID + Reflections & Diffraction on head,

shoulders and pinna

" These are Binaural systems58



Binaural Systems

!

So, binaural systems try to:" Reconstruct exactly the same signals that are produced by

real sound sources over the ears

! But, at first sight this is very, very complex

! How do we include head, shoulders & pinna

reflections & diffraction?

" With the HRTF

59



HRTF

!

HRTF: Head Related Transfer Function

! A HRTF is a filter

" This filter introduces all the needed effects

" Reflections & diffraction due to head, shoulders & pinna

! Indeed, we need a large set of filters

" Two filters (HRTFs) for each direction of arrival

! One for the left ear & one for the right ear

!

How do we compute/measure the HRTF filters?

" With a bit or work

60



HRTF Measurement

!

We need to use 2 small microphones" Located inside the ears

!

An then, with the head still, we are going to producesound from every possible direction

" With a given resolution (e.g. 5 or 10 degrees)

61



HRTF Measurement

!

To speed up the process we can use a semicirculararray of speakers

62

Anechoic chambergets rid of

room reflections



HRTF Measurement

!

From each speaker at a time we send an impulse" Or any other signal useful for obtaining an impulse response

63

HRIR

Impulse

HRIR: Head Related Impulse Response



HRTF Measurement

!

If we represent all the HRIR for the horizontal planewe obtain the following map:

64

This exampleis for the

right ear



HRTF Measurement

!

We have measured an HRIR then, what is a HRTF?" A HRTF is the Fourier transform of the HRIR

! The previous chart in frequency becomes:

65



HRTF

!

One big problem that comes with the HRTF is theuniqueness

!

People tend to have different heads, shoulders and

ears!

"

So, each person will have a different set of HRTFs

!

If we are going to use them!

" We will have to measure the HRTFs for each person

!

There are sets of ‘general’ HRTF that work more orless with everyone

" But you will achieve much more accuracy with your own

HRTFs 66



Binaural Systems

!

Once we have measured the HRTFs the binauralsystems are very easy to implement

" You only have to filter the sound with the corresponding pair

of HRTFs of the desired direction

" And apply a delay and gain proportional to the distance of the

phantom source! Since HRTF always measured at a given (short) distance

from the head

67

HRTFL

HRTFR

L

R

Source



Binaural Systems

!

One severe drawback of binaural systems is that weneed to use headphones for reproduction

" We are computing the sound ‘in the ears’ once it has passed

the pinna!

" If we use speakers we are going to include the pinna twice

! If you do this, everything is ruined

68



Binaural Systems

!

More problems due to headphones:" It appears the ‘Inside the Head’ effect: in several positions of

the sources, the sound seems to come from inside the head,

not the outside

! People like speakers over headphones for something

"

We are bond to the head: If the listener moves the head,the scene moves with him/her

! To break the bond there are two options:

"

Obvious: Do not move the head!

"

Complex: Include a tracking mechanism. Following themovement of the head and compensating for it

(changing the HRTFs)

69



Transaural Systems

!

The use of headphones is a severe drawback in thevast majority of scenarios"

Worst scenario ever: Cinema

" Best scenario: videogames

!

The transaural systems appear as an alternative to binauralsystems that tries to overcome the headphone problem

!

Using a bit of signal processing we can change the

headphones for speakers

!

Let’s see what we will hear using speakers!

70



Transaural Systems

!

If we change headphones for speakers, we need todeal with crosstalk

" We need left speaker to left ear, and right speaker to right ear

"

But we also have crosstalk

! We need to cancel the crosstalk using signal

processing 71

Crosstalk:dashed lines



Transaural Systems

!

How do we do this?" Emitting two signals that cancel each other crosstalk when

received by ears

"

x: signals emitted by speakers

" y: signals received by ears

" Hab: HRTF between speaker-ear pair

! a # speaker

! b # ear

72

x1 x2

y1 y2

H11

H12 H21

H22



Transaural Systems

!

We need to solve the following system of equations:

73

x1 x2

y1 y2

H11

H12 H21

H22

y1

y2

!

"

#

#

$

%

&

&=

H 11

H 12

H 21 H 22

!

"

#

#

$

%

&

&

x1

x2

!

"

#

#

$

%

&

&

This is the desired sound(already known)

This is the soundthat we must send



Transaural Systems

!

Block diagram:" So, at the end we apply the following block diagram

74



Transaural Systems

!

Now, the question is: does this really work?" The answer is: Yes & No

!

Transaural systems work only in controlled scenarios

" Room reverberation ruins our effort

! It is very difficult to include & cancel reverberation issues

"

The head of the listener must be completely still

! A small movement changes reflections drastically and avoiding

an effective crosstalk cancellation

! In this case, we should perform a tracking of the head, andcompensate for it

" Although during the time of reaction crosstalk is clearlynoticeable

! In an anechoic chamber, it works great75



Sound Field

Reconstruction Systems

76



Sound Field Reconstruction

!

This last family of sound systems use a differentapproach to solve the spatial sound problem

!

Stereophonic & binaural systems are focused on the

hearing sense

"

They try to deceive the localization mechanism with more orless skill

" But these approaches have a common problem: they are

head dependent

!

The new family will try a completely different and novelapproach

"

They are going to try to synthesize the sound field in a whole

area, not only in the ears 77




!

Let’s explain it:" Instead of deceiving the localization mechanism!

" We are going to build a sound field identical to the one that

the real sound source would produce

" The sound field reconstruction is not limited to the sound in

the ears only, we will reconstruct it in the whole room

! This way, in a cinema with a lot of people, each onewill perceive the scene in a different way (as happens

in the real world)

"

Instead of making only one scene for all people at the sametime

78




!

Example:

79

Virtual Sound Source

Listener 1

Listener 2

Listener 3(moving)

All listeners perceive thesame source position

But each one with adifferent DOA

DOA depends onthe current position

of the listener

Now we are:Head Independent!




!

To achieve our goal there are two different systems

! They are very different at first sight but, at the end,

they are based on the same physics

" Both perform the same task but from different point of views

!

Sound Reconstruction Systems:

" Ambisonics

" Wave-Field Synthesis

80



Ambisonics

!

Ambisonics is a sound system that started as amicrophone technique used to store spatial sound

! It stores the original sound filed of a given (andunique) point in a finite number of channels

" It measures the sound field in a given point

!

Then in reproduction it extrapolates the signal that

must be sent to each speaker

"

When the signal emitted by each speaker arrives to theoriginal (measured) point it recreates the same sound field

that was stored for this point

81



Ambisonics B-Format

!

The most simple version is the Ambisonics B-Format

! It stores 4 different channels:

"

W: pressure

" X: velocity in x axis

" Y: velocity in y axis

"

Z: velocity in z axis

!

This is achieved with 4 different microphones:

" One omnidirectional (W) and three bidirectional (X, Y, Z), onefor each axis

82*Image source: wikipedia



Ambisonics B-Format

!

Having stored only the pressure (W) we can decide inwhich direction we want to be the source:

" Giving to angles: Azimuth(") & Elevation(#)

!

The signals to be sent to the speakers will be:

"

We only need to compute the D matrix" D: decoding matrix

! It depends on the speaker locations

83



Ambisonics

!

Example of use:

! Ambisonics does not store the sound to be emitted by

each speaker in a different channel" It computes the signal to be sent to each speaker during

reproduction time

"

Using the decoding matrix84



Ambisonics

!

Advantages: " The format does not define the position of the speakers

" We can locate the speakers as we want

! Disadvantages:

"

Speakers can’t be located anywhere! Decoding matrix depends on speaker positions

! To be able to compute the decoding matrix we need speakers at

regular positions (shapes: circle, sphere, semi-sphere, !)

"

It only works in the sweet spot

! The center of the regular shape array of speakers

! As in stereo systems!

85



Ambisonics

!

But! we said that this system was a Sound FieldReconstruction System

" Not something similar to a stereo system

!

Yes, but this is only possible with: HOA"

Higher Order Ambisonics

86




!

We have seen the most basic Ambisonics system" B-Format

" It uses only 4 channels to encode the sound

!

With HOA we are going to store more than 4 channels

" Increasing the number of stored channels, we achieve a

greater sweet spot

" Sweet spot becomes a Sweet Area

! More channels# wider area

! The channels now represent Spherical Harmonics

"

But the complexity of the calculus needed to compute the

decoding matrix is increased notably

87




!

Representation of the Spherical Harmonics up to 3rd order:

88*Image source: wikipedia




!

The equations of the spherical harmonics up to 4th order are:

89

4th order HOA needs 25 channels




!

Example of a 3D 3rd order ambisonics reproductionsystem, CHESS (Guillaume Pottard):

90




!

The worst drawback of ambisonics is the need of aregular shape array of speakers

! In 2D the common regular shape is the circle

" Think about a cinema with the speakers forming a circle

around the seats!

" It’s not operative

91



Wave-Field Synthesis

!

WFS is the second system that falls inside the SoundField Reconstruction Systems

! At first sight is very different from Ambisonics

" But in fact, they are almost the same

! It’s starting point is the Huygens Principle

" So! let’s review it

92



Huygens Principle

!

“Every point of the wavefront can be seen as a newsource of spherical waves”

93




!

WFS uses the Huygens principle to reconstruct thesound filed

! If the wave front is created combining the spherical

waves of the secondary sources! " Let’s replace secondary sources by speakers

" This is known as ‘acoustic curtain’ principle94

Originalsource

Secondarysources



Acoustic Curtain Concept

!

Step 1 – Put an speaker over each secondary source! Step 2 – Form a linear array, instead of curve

! Step 3 – Get rid of the original source

95




!

Stereo VS WFS comparison:

96




!

Available types of sound sources:

97

Stereo: OnlyPoint Sources

Focusedsource




!

Now, the thought part! " Which sound must be emitted by each speaker?

" We must calculate it!

! Kirchhoff-Helmholtz integral:

"

It is used the describe (with Huygens principle) the sound field

that is inside a given surface (S)

"

It says: The acoustic field inside a given volume can berecreated using secondary sources distributed around the

surface that encloses the volume

98




!

Kirchhoff-Helmholtz integral:

99

ListenerSoundSource

Secondarysources




!

Simplifying:" Let’s suppose that the volume is an infinite cube, the surface

is now only a plane that divides inside from outside

" Now let’s reduce even more, from 3D we change to 2D, the

surface is now a straight line

" And last, we discretize the surface in N points

100




!

So, at the end, each speaker must emit:" A delayed and attenuated version of the original sound

" Plus a high pass filter:

! This filter appears when reducing from 3D to 2D

" All speakers form a linear array

101

Pressure

Delay due

to distance

Distanceattenuation

HP Filter



WFS Limitations

!

Linear array:" Since we are using linear arrays of speakers, the sound

emitted has cylindrical divergence instead of spherical

! The sound decays only 3 dB each time we double the distance

102



WFS Limitations

!

Diffraction:" The array of speakers is finite, it has limits

" In both extremes it will appear diffraction

103



WFS Limitations

!

Spatial Aliasing:" Speakers must be located at some distance between them (at

least their size)

! In the Huygens principle, each secondary source is infinitely

small and there are infinite sources

! High Frequencies are not well reconstructed

104

Pure tonewith and

withoutaliasing



WFS Limitations

!

Loudspeakers directivity:" In theory, secondary sources are omnidirectional

" In practice, speakers are not omnidirectional at all the

frequencies

" But this is not so important, in low frequencies they work ok

! In high frequencies, spatial aliasing is worse than this

105



WFS E l



WFS Examples

!

Prototype at the IRCAM (France)" It uses planar DML speakers

107

WFS E l



WFS Examples

!

Prototype at UPV (Valencia)" Uses 96 channels

108

WFS E l



WFS Examples

!

Prototype at the UA

109

A bi i VS WFS



Ambisonics VS WFS

!

Ambisonics:" It allows elevation (only for small orders of HOA)

! Higher orders are too expensive with computing power needs

"

It needs a regular array of speakers

" To obtain a big ‘Sweet Area’ it needs a very high order

"

At the end, with the maximum order = WFS

!

WFS:

" At the moment, without elevation

! But equations allow it, the problem is the money (& computer power)

"

It allows any array configuration! Using combinations of linear arrays

" There is no sweet spot

! But the center is a preferential location110



Sound in Movie Theatres

111

S d i Ci



Sound in Cinema

!

The sound in Cinema has had two great milestones:" The apparition of sound

" The stereo system

! The use of more than two channels enhances the

spatial perception"

But the difference of perception between mono & stereo is far

better

!

In the following slides we are going to review briefly

the different attempts made in cinema sound" And we are going to learn how to store sound in a film

112

M l S d



Monoaural Sound

!

At the beginning of the cinema, the sound and theimage were stored separately

! During the reproduction they used a gramophone

" Varying the speed to maintain the synchronization

113

M l S d



Monoaural Sound

!

Later, when the sound was already treated as aelectric current (over 1926)

" The sound started to be stored jointly with the image

!

The sound was stored as an opaque band over a

transparent section of the film (optic format)

114

Soundband

M lti h l S d



Multichannel Sound

!

The first multichannel Sound Track was “One hundredmen and a girl” in 1937

" It was recorded in 9 channels

"

But at the end all channels were mixed together in the

monoaural optic band

!

From this moment and on, several attempts were

made to enhance the sound in cinema

"

But, the vast majority were a failure

"

The technology was very expensive for the moment

115

Fantaso nd (1940)



Fantasound (1940)

!

In 1940, it was performed the first projection of a truemultichannel film: ‘Fantasia’

" 9 different sound channels

"

Mixed in 4 different optic bands

" Using 3 speakers behind the screen and 65 small speakers

around the theatre

" It was amazing but too much expensive, only used in Fantasia

116

Cinerama (1952)



Cinerama (1952)

!

It used 3 joined projectors to produce a panoramic format!

And 7 audio channels stored in a magnetic multitrack player

!

As with fantasound, it was so expensive that there were only a

few compatible cinemas and short list of produced films

117

Cinemascope (1953)



Cinemascope (1953)

!

Born in 1953" The system used an anamorphic lens to deform the image

and convert a 4:3 film in a 16:9 format

" The lens must be used both at recording & reproduction

" It used 4 audio channels stored in 4 magnetic tracks over the

film

118Magnetic audio tracks

Lens

Dolby (1970)



Dolby (1970)

!

In 1970 Ray Dolby introduced the Dolby-A noisereduction system

" Stanley Kubrick’s ‘A Clockwork Orange’ was the first film to

use it (1971)

!

In 1974 is introduced the Dolby Stereo system

119

The 4 channelsare mixed in

2 optic bands

Later itbecame an

ISO standard

Sensurround (1974)



Sensurround (1974)

!

This (defunct) system was an extension of the 4channel system

" And the precursor of the subwoofer channel

! It added 4 very big speakers behind the screen

"

2 at each side

"

Each one driven with a 1 kW amplifier

" Emitting only infrasounds (bellow 20 Hz)! All of them controlled with 1 additional track

!

It was presented with the film:" Earthquake

120

Digital Sound (1986 )



Digital Sound (1986 - )

!

Digital Sound in cinemas was introduced in 1986

! There are several standards:

"

Dolby Stereo Digital

! Dolby Digital or Dolby SR-D

"

DTS! Digital Theater System

"

SDDS

! Sony Dynamic Digital Sound

!

From the previous unit we already know how toprocess the sound

" But, the question is! How is stored the digital sound in the

film? 121

Digital Sound



Digital Sound

!

Digital sound is stored in optic format!

122

SDDS

DolbyDigital

DTS

Dolby

Stereo(analog)

Digital Sound



Digital Sound

!

As you have seen in the previous slide, the sound isstored near the holes of the film

" It seems like the QR codes used for smartphones!

! Dolby Digital is stored in a very bad place

"

The projector trailing mechanism deteriorates the storedsignal each time is reproduced

" After two weeks presenting the film (or even less), the sound

system will change to the analog sound track due to the errors

encountered when decoding dolby digital

! And DTS!

" It’s so simple! only a small amount of dashes

123

DTS



DTS

!

DTS is a system very similar to Dolby Digital" It is supposed to produce better quality (with more bitrate)

" But you must be well trained to notice it

! The major difference is the storing mechanism

"

It does not store the sound in the film"

It stores the sound in CDs (or DVD / Bluray)

" In the film it only stores a Time-Code to synchronize the

player

unit 5 - surround and 3d sound systems

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