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YC transmission revisit for UHDTV production image format

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

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YC transmission revisit for UHDTV production image format

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  1. YC transmission revisitfor UHDTV production image format Masayuki SUGAWARA NHK HPA Tech Retreat

  2. Background • ITU-R WP6C • UHDTV image format standardization • Remained issue: Colour equations (YC transmission) • Special Rapporteur group HPA Tech Retreat

  3. 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 HPA Tech Retreat

  4. 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 YH GH YH GL GL YL Encode Decode YH BH BL BL C1L Up sample Down sample RH YH RL RL C2L HPA Tech Retreat

  5. 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” HPA Tech Retreat

  6. Why low frequency of luminance is correctly reproduced? Conventional（non-constant luminance） Encode Luminance transmitted partially Y（Luma） True luminance B-Y Blue Blue R-Y Decode Completely restored by using colour-difference signals HPA Tech Retreat

  7. Why constant luminance issue occurs? Conventional（non-constant luminance） Encode Y（Luma） Bandwidth reduction True luminance B-Y Blue R-Y Decode Modulation drop in high frequency HPA Tech Retreat

  8. Mechanism Non-constant Encode Source Input Yn Gi rR+gG+bB Gs Cbn Bi Bs B-Y LPF•SS Crn RI Rs R-Y LPF•SS Camera True luminance: Decode Gno Yn 1/g(Y-rR-bB) Bno Cbn CB+Y US•IPF Rno Crn CR+Y US•IPF Display Luminance and colour-difference equations are linear combinations of gamma-corrected (input) RGB signals. ･Low frequency of input signals is maintained. ･High frequency becomes achromatic. ･True luminance high frequency is NOT maintained. HPA Tech Retreat

  9. Introduction of constant luminance Non-constant constant Encode Y’（Luminance) Y’（Luma） True luminance Blue Decode Everything is OK? HPA Tech Retreat

  10. Mechanism Constant Encode Yc rR+gG+bB Source Input Gi Gs LPF•SS Cbc Bi Bs B-Y LPF•SS Crc RI Rs R-Y LPF•SS Camera Decode Yc HPF Gco 1/g(Y-rR-bB) US•IPF LPF•SS G+YH Bco Cb+Y Cbc US•IPF B+YH Rco Cr+Y Crc US•IPF R+YH Display Luminance and colour-difference equations are non-linear combinations of source RGB signals. ･Low frequency of source signals is maintained. ･High frequency becomes achromatic. ･True luminance high frequency is maintained. HPA Tech Retreat

  11. Gamma-corrected-referred / Linear-referred system YC systems transmit low frequency RGB and high frequency “luminance” Gamma-corrected referred system Linear referred system HPA Tech Retreat

  12. Impact of linear-referred system • Operational practice • Hardware • Picture quality improvement HPA Tech Retreat

  13. Impact: Signal handling Non-constant luminance system Linear world RGB Gamma correction Gamma-corrected world Signal process R’G’B’ Result Next stage Identical in terms of lower RGB component Encode Signal process Y’C’ Result HPA Tech Retreat

  14. Impact: Signal handling Constant luminance system Signal process RGB RGB Result Linear world Identical in terms of lower RGB component Signal process RGB Result Gamma correction Gamma correction Encode Gamma Decode Encode Gamma-corrected world R’G’B’ Next stage Y’C’ HPA Tech Retreat

  15. Picture quality improvement • Subjective observation • 4:2:0 sampling • Test materials • can not distinguish constant from non-constant. HPA Tech Retreat

  16. Picture quality improvement: example HPA Tech Retreat

  17. 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 HPA Tech Retreat

  18. 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. HPA Tech Retreat

  19. Thank you! sugawara.m-fq@nhk.or.jp HPA Tech Retreat

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