DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

SMPTE 240M, 1125-Line High-
Definition Production Systems –
Signal Parameters.
In 1993, CCIR was renamed
ITU-R.
ITU-R Rec. BT.1361, Worldwide
unified colorimetry and related
characteristics of future television
and imaging systems.
SMPTE 240M transfer function
SMPTE Standard 240M for 1125/60, 1035i30 HDTV
was adopted two years before Rec. 709 was adopted.
Virtually all HDTV equipment deployed between 1988
and 1998 uses the SMPTE 240M parameters. SMPTE
and ATSC standards for HDTV now specify Rec. 709
parameters; however, the standards previously accommodated
an “interim implementation” having the identical
transfer function to SMPTE 240M. SMPTE 240M’s
transfer function is this:
⎧⎪
4. 0L; 0 ≤ L<
0. 0228
V'240
= ⎨ 045 .
⎩⎪ 1. 1115L −0. 1115; 0. 0228 ≤ L≤1
Eq 23.5
To recover scene tristimulus values, use this relation:
⎧V'240
⎪ ; 0 ≤ V'240
< 0. 0913
⎪ 40 .
L = ⎨
1
⎪⎛
V'240
+ 0. 1115⎞
045 .
⎪⎜
⎟ ; 0. 0913 ≤V'240 ≤1
⎩⎝
1. 1115 ⎠
Eq 23.6
The difference between the SMPTE 240M and Rec. 709
transfer functions is negligible for real images. It is
a shame that international agreement could not have
been reached on the SMPTE 240M parameters that
were widely implemented in 1990, when the CCIR
discussions were taking place. The transfer function of
Rec. 709 is closely representative of current studio
practice, and should be used for all but very unusual
conditions.
Rec. 1361 transfer function
Rec. 1361 is intended to enable future HDTV systems
to achieve wider color gamut than Rec. 709, through
use of tristimulus signals having negative values and
values greater than unity. The Rec. 1361 transfer function
is identical to Rec. 709’s for RGB tristimulus values
within Rec. 709’s range, that is, between 0 and 1. Tristimulus
values from -1 ⁄4 to zero are subject to a transfer
function that is Rec. 709’s function mirrored, and scaled
by a factor of 1 ⁄4, on both axes. Tristimulus values from
unity to + 4 ⁄3 are subject to a straightforward extension
of the Rec. 709 curve.
CHAPTER 23 GAMMA 265