Embed Size (px)
DESCRIPTION
YC transmission revisit for UHDTV production image format. Masayuki SUGAWARA NHK. Background. ITU-R WP6C UHDTV image format standardization Remained issue: Colour equations (YC transmission) Special Rapporteur group. Why needed?. Compatibility with B&W television - PowerPoint PPT Presentation
Citation preview
YC transmission revisitfor UHDTV production image format
Masayuki SUGAWARA
NHK
12/02/17 1HPA Tech Retreat
Background
• ITU-R WP6C• UHDTV image format standardization• Remained issue: Colour equations (YC
transmission)• Special Rapporteur group
12/02/17 2HPA Tech Retreat
Why needed?
• Compatibility with B&W television• Robustness to analog error e.g. dc adjustment• Robustness to noise being added in
transmission path• Bandwidth reduction
12/02/17 3HPA Tech Retreat
Bandwidth reduction• Human visual system characteristics
– Less sensitive to high-frequency chroma than to luminance
• Transmitted and maintained information– Low frequency of RGB and High frequency of luminance
GL
GH
YL
YH
C1L
C2L
GL
YH
BL
RL
YH
YH
BL
BH
RL
RH
Encode Decode
12/02/17 4
Downsample
Upsample
HPA Tech Retreat
Combination with non-linear transfer function
• Why non-linear transfer function (gamma correction) is needed?– Pre-correction of CRT device characteristics– Data rate (bandwidth) reduction– And more?
• Well-known constant luminance issue– “the detail of high frequency of luminance signal
at highly colour-saturated part is lost”
12/02/17 5HPA Tech Retreat
Why low frequency of luminance is correctly reproduced?
True luminance
Y （ Luma ）
B-Y
R-YBlue Blue
Completely restored by using colour-difference signals
12/02/17 6
Encode
Decode
Luminance transmitted partially
Conventional （ non-constant luminance ）
HPA Tech Retreat
Why constant luminance issue occurs?
B-Y
R-YBlue
Modulation drop in high frequency
Bandwidth reductionY （ Luma ）
12/02/17 7
Encode
Decode
True luminance
Conventional （ non-constant luminance ）
HPA Tech Retreat
MechanismYn
Non-constant
LPF•
SS
rR+g
G+b
B
B-Y
R-Y
LPF•
SS
Gno
Encode
Decode
Cbn
Crn
Gs
Bs
Rs
Gi
Bi
RI
Yn
Cbn
Crn
Bno
Rno
US•
IPF
US•
IPF
CB+Y
CR+Y
1/g(
Y-rR
-bB
)
Camera
Display
･ Low frequency of input signals is maintained.･ High frequency becomes achromatic.･ True luminance high frequency is NOT maintained.
Luminance and colour-difference equations are linear combinations of gamma-corrected (input) RGB signals.
12/02/17 8
True luminance:
Source Input
HPA Tech Retreat
Introduction of constant luminance
Blue
Non-constant
Y’ （ Luma ）
constant
Y’ （ Luminance)
Everything is OK?
12/02/17 9
Encode
Decode
True luminance
HPA Tech Retreat
Yc
LPF•
SS
rR+g
G+b
B
B-Y
R-Y
LPF•
SS
Gco
Encode
Decode
Cbc
Crc
Gs
Bs
Rs
Gi
Bi
RI
Yc
Cbc
Bco
Rco
Cb+Y
1/g(
Y-rR
-bB
)
Camera
LPF•
SS
CrcU
S•IP
F
Cr+Y
Display
LPF•
SS
HPF
US•
IPF
US•
IPF
G+Y
H
B+Y H
R+
Y H
MechanismConstant
12/02/17 10
･ Low frequency of source signals is maintained.･ High frequency becomes achromatic.･ True luminance high frequency is maintained.
Luminance and colour-difference equations are non-linear combinations of source RGB signals.
Source Input
HPA Tech Retreat
Gamma-corrected-referred / Linear-referred system
1112/02/17
RGB Non-constant Constant
Source G
B
R
Input G
B
R
Encoded Y -
Cb -
Cr -
Signal process Gamma-corrected domain Linear domain
Decoded G
B
R
YC systems transmit low frequency RGB and high frequency “luminance”
Gamma-corrected referred system
Linear referred systemHPA Tech Retreat
Impact of linear-referred system
• Operational practice• Hardware• Picture quality improvement
12/02/17 12HPA Tech Retreat
Impact: Signal handling
RGB
R’G’B’
Linear world
Gam
ma-corrected w
orld
Y’C’
Result
Result
Gamma correction
Encode
Signal process
Signal process
Identical in terms of lower RGB component
Next stage
Non-constant luminance system
12/02/17 13HPA Tech Retreat
Impact: Signal handling
RGB
R’G’B’
Linear world
Gam
ma-corrected w
orld
Y’C’
Signal process
Signal process
Result
Result
Identical in terms of lower RGB component
Gamma correction
RGB
RGB
Gamma DecodeEncodeGamma correction Encode
Next stage
Constant luminance system
12/02/17 14HPA Tech Retreat
Picture quality improvement
• Subjective observation– 4:2:0 sampling– Test materials
– can not distinguish constant from non-constant.
12/02/17 15HPA Tech Retreat
Picture quality improvement: example
12/02/17 16HPA Tech Retreat
Impact of true constant luminance system
• Operational practice– Signal handling in linear domain at every stage– Non-linear conversion for monitoring is required
• Hardware– Increase for linear-nonlinear/nonlinear-linear
conversions in encoder and decoder
• Picture quality improvement– Almost no subjective difference in natural pictures
12/02/17 17HPA Tech Retreat
ConclusionFormat selection for UHDTV program production• Live production
– Reduced bandwidth is desirable.– Real time, therefore “look” is imposed at production.YC gamma-corrected-referred (non-constant) format
is preferable.
• Non-live production– Less desire for bandwidth reduction.– More desire for complicated signal manipulation such
as colour grading using colour science.– Full bandwidth for RGB signals are desirable for signal
processing.RGB format is preferable.
12/02/17 18HPA Tech Retreat
