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8/9/2019 Wittek Stereo Surround (1)
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about Stereo and Surround
Considerations on Stereo andSurround recording, reproductionand perception
Helmut Wittek
SCHOEPS GmbH, Karlsruhe, Germany
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Contents
Contents:
- An overview of psychoacoustics and appliedmicrophone techniques
- Both theory and practical demonstration, bycalculating and listening
Topics
Source localisation / stereophonic localisation
Theories of microphone design Microphone setups and their properties
The microphone and its properties
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Source localisation
Where is the source?
At which distance?
In which room, what is the room like? What does the
source radiate?
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2 radiating sources
Only one perceived phantom source
Existing theory:Summing localisation = synthesis of theloudspeaker signals that leads to the original
source signal (Blumlein)
Stereophonic localisation
L R
Phantom
source
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Real source, 15on the right
Phantom Source,
perceived ca. 15on the right,L=7dB, t=0 ms
Phantom Source,
perceived ca. 15on the right,L=3.5dB, t=0.2ms
Phantom Source,perceived ca. 15on the right,
L=0dB, t=0.4 ms
Summing localisation
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Summing localisation
Real sourcePerceived Direction + 15
Phantom sourcePerceived Direction + 15Interchannel Level Difference
L = 7 dB 0,13 ms
0,13 ms
Interaural Cross Correlation( = Interaural Time Delay ITDvs. frequency)
-1 ms +1 ms0
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Summing localisation
Virtual sourcePerceived Direction + 15
Phantom sourcePerceived Direction + 15Interchannel Time Delay
t = 0,4 ms 0,4 ms
0,13 ms
Interaural Cross Correlation( = Interaural Time Delay ITDvs. frequency)
-1 ms +1 ms0
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Other hypothesis
Problems of the summing localisation theory: Works only below ca.1500 Hz
Works only for level panning
Works only in the sweet spot
It cannot explain the perceived sound colour
Other hypothesis for stereophonic
localisation/perception:Stereo signals can be perceivedseparately
Binaural decolouration (Salomons, Brggen)
Association model (Theile, 1980)
After the separate localisation, the fusion of thecoherent signals takes place no physicalsuperposition, no comb filtering
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Consequences for the sound engineer
1. Do we aim at a physical synthesis instereo? (as doAmbisonics, Blumlein)
2. Can only coincident microphones create
spatial sound?
3. Are time differences allowed?(see e.g. Lipshitz: Are the purists wrong?,JAES)
4. Can we use microphones at ear spacing?(like ORTF, SCHOEPS sphere microphone)
5. Is it wise to allow crosstalk in surroundmicrophones? (see Lee/Rumsey, AES)
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Source localisation
Where is the source?
At which distance?
In which room, what is the room like?
t0 ms 30 ms
Direction Distance,
Depth,Spaciousness
Spatial impression
Theile
Envelopment
Reverberance
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Source localisation
Discrete Signals:
Correlated at both ears
From discrete directions
Diffuse Signals:
Decorrelated at both ears
From all directions
t0 ms 30 ms
Direction Distance,
Depth,Spaciousness
Spatial impression
Theile
Envelopment
Reverberance
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s
Directions and points in time
010
2030 ms
time
Direct Sound
Early Reflections
Reverb
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5.1 Reflection pattern
L R
010
2030
mstime
Direct Sound
Early Reflections
Reverb
1. Envelopment is hardly possible in 2-ch stereo
2. No real depth is possible in 2-ch stereo3. Reflection density too high! has to be reduced
4. Reverb should be reproduced as diffuse as
possible decorrelated!
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Diffuse field correlation
Reproduction of thereverb tail:
- Should be diffuse,should be perceived
from everywhere- The more correlation,
the narrower theimage of the reverb tail
The diffuse soundshould be reproduceddecorrelated. The
decisive measure iscalled:
Diffuse field correlation
(DFC)
L R
L R
L R
DFC = 0
DFC = 0.5
DFC = 1
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Perceptual consequence of correlated diffusefield reproduction Demo
DFC of coincident microphone setups:
Diffuse field correlation
0
Blumlein,
90,Figure-8
XY,
180,Cardioids
XY,
120,Super-
cardioids
XY,
90,Cardioids
Setup
0.50.230.75DFC
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Correlation
Correlation vs. microphone angle
Supe
rcardioids
Co
rrelationcoe
fficient(DFC
)
XY
Omnis
Offset angle ()
Blumlein
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a/b and ORTF correlation
DFC of spaced microphones Calculation
Shuffling techniques could correct for that
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Directional Imaging
Level and timedifferences govern theperceived phantomsource direction
L = f (L) ;
t = f (t) ;
= L
+ t
;
The necessaryinterchanneldifferences are rather
similar to thoseknown from naturalhearing
7,5 % / dB
100%
50%
75%
25%Phantom
sourceshift
0 3 9 126
Interchannel level difference L [dB]
15
A
DC
B
100%
50%
75%
25%Phantoms
ourceshift
0 0,2 0,8 1,00,60,4Interchannel time differencet [ms]
13 % / 0,1ms
B
D
E
A
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Comparison of stereo setups
Recording angle Recording angle 75%
Image Assistant on www.hauptmikrofon.de
Comparison of different stereo setups sharing thesame recording angle Demo Showroom
www.schoeps.de/showroom/
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2-ch Microphone Design
Directional Image:can be calculated, differences betweenstereo techniques regarding the image
Diffuse Field Correlation DFC:should be minimal: not possible with smalla/b or normal cardioid XY
Further points of interest: Reality is not always ideal! Type and properties of single capsules
(pressure/pressure gradient transducer)
Direct/Reverb ratio controls reflection level Loudness balance
Size of the setup
Sensitivity to wind noise
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XY
UMS 20 UMS 20
M100C
CMXY
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MS
CMIT MS
CMC 64 + CCM 8
RCY
SGMSC
(CCM4 + CCM8)
MS-BLM
WSR MS
UMS 20
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ORTF and AB Stereo
STCg
MSTC 64 U, STC
M100C
UMS 20
KFM 6
MAB1000
ORTF
Outdoor Set
Decca3 * CCM 2H
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5.1 Reflection pattern
RS
L R
LS
C
010
2030
mstime
Direct Sound
Early Reflections
Reverb
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L R
C
LS RS
Early reflections
Reverb
Imaging area
5.1 Spatial reproduction
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Surround Microphone Design
Directional Image: Image Assistant
Crosstalk: no 3 correlated signals!
L h
C
R
L, R: Super-CardioidC: Cardioidb: 40 .... 100 cm
h: 8 cm
OCT
b
MK 41 MK 41V
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Stability in 5.1
Localisation curve on off-centre position
INA 3OCT 70 Quasi-ORTF
Einfallswinkel (Grad)
Sitzposition LINKS: Lokalisationskurven experimentell
(Mittelwert + 95% Vertrauensbereich)
R
Auslenkung
(Grad)
-60 0 60
-30
30
0
30-30
-15
15
30
15
0
-30
-15
Perceivedsourcedirection
Sound source direction
Quasi-ORTFINA 3OCT 70
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OCT Surround
Lh
C
R
OCT Surround
b
b + 20 cm
40 cm
LS RS
Early reflections
Reverb
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OCT Surround Setup
OCT Surround(using MAB1000 and CCM microphones)
40 cm
Front + 20cm
40 - 100 cm
8 cm
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OCT + Hamasaki
Lh
C
R
OCT + Hamasaki square
b
> 2 m
LS RS
Early reflections
Reverb
L R
> 2 m
+ -
+ -
+-
+-
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IRT cross
IRT cross for Surround atmos: 20-25cm
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Surround non-coincident
IRT cross
OCT Surround
Decca3 * CCM 2H
KFM 360
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Double MS: The M/S principle
R-LS
RLM
=
+=
)S-M(*
2
1R
)SM(*2
1L
=
+=
M
S
LR
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The Double M/S idea
Front M/S pair
Rear M/S pair
CombinedDouble M/S triplet
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The Double M/S decoding
Double M/S enables 2 different decodingmethods:a) each pair is decoded separately
b) decoding utilizes the third microphone as well
L R
L R
a)
b)
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Crosstalk
Crosstalk in Double M/S
Avoid crosstalk by optimal decoding
4ch 5ch
T l f D di
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Tools for Decoding
Decoding variants: 2 M/S Matrices
Hardware (MDMS U)
Software (VST PlugIn)
Try by yourself, its free!
www.schoeps.de/dmsplugin.htm
Fil di
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Film recording
Double M/S with shotgun
S d i id t
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Surround - coincident
CMIT-Double M/S
Double M/S
Higher Order Techniq es
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Higher Order Techniques
Which higher order technique can be used inpractice?
for reproduction:
Wavefield synthesis (WFS) and Higher OrderAmbisonics (HOA) can recreate sound fields
Stereo can (only) generate the sameperception in the sweet spot
for recording:
Higher order microphone techniques areeither noisy and/or can not yet deliver a fullysatisfying and stable timbral quality, but
proposals will come for special applications
WFS or Stereo?
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WFS or Stereo?
Wittek, Rumsey, Theile, Journal of the AES, Vol.55/9, 2007:
Colouration of WFS and Stereo:
Is stereo just a poor 2-ch wavefield synthesis?
WFS or Stereo?
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WFS or Stereo?
Pe
rceivedColouration
Real - WFS - Stereo
Wittek, Rumsey, Theile, Journal of the AES, Vol.55/9, 2007:
Colouration of WFS and Stereo
What does the microphone need?
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What does the microphone need?
The microphone is the basis for all. Well-known parameters govern the quality:
Good frequency response
Smooth polar pattern
Low noise floor
Good reliability
And the sound
Thank you.