Rec. 709

Recommendation ITU-R BT.709-6 defines a common image format (CIF) where picture characteristics are independent of the frame rate. The image is 1920x1080 pixels, for a total pixel count of 2,073,600.[2]

Previous versions of BT.709 included legacy systems such as 1035i30 and 1152i25 HDTV systems. These are now obsolete, and replaced by the system defined in the 2015 ITU BT.709-6.

BT.709 offers over a variety of frame rates and scanning schemes, which along with separating the picture size from frame rate has provided the flexibility for BT.709 to become the worldwide standard for HDTV. This allows manufacturers to create a single television set or display for all markets world-wide.

BT.709-6 specifies the following frame rates, where P indicates a progressively scanned frame, PsF indicates progressive segmented frames, and I indicates interlaced:

24/P, 24/PsF, 23.976/P, 23.976/PsF, which match the frame rate used for theatrical motion pictures. The fractional rates for compatibility with the "pull-down" rates used with NTSC.

50/P, 25/P, 25/PsF, 50/I (25fps), for regions that formerly used 50 Hz systems such as PAL or SECAM. There are no fractional rates as PAL and SECAM did not have the pull-down issue of NTSC.

60/P, 59.94/P, 30/P, 30/PsF, 29.97/P, 29.97/PsF, 60/I (30 fps), 59.94/I (29.97 fps), for regions that formerly used 60 Hz systems such as NTSC. Here again, the fractional rates are for compatibility with legacy NTSC pull-down rates.

Per BT.709, cameras may capture in either progressive or interlaced form. Video captured as progressive can be recorded, broadcast, or streamed as progressive or as progressive segmented frame (PsF). Video captured using an interlaced mode must be distributed as interlace unless a de-interlace process is applied in post production.

In cases where a progressive captured image is distributed in segmented frame mode, segment/field frequency must be twice the frame rate. Thus 30/PsF has the same field rate as 60/I.

Note that red and blue and y_G are the same as the EBU Tech 3213 (PAL) primaries while the x_G is halfway between EBU Tech 3213's x_G and SMPTE C's x_W (PAL and NTSC are two types of BT.470). In coverage of the CIE 1931 color space the Rec. 709 color space (and the derivative sRGB color space) is almost identical to Rec. 601 and covers 35.9%.[4] It also covers 33.24% of CIE 1976 u’v’[5][6] and 33.5% of CIE 1931 xy.[6] White point is D₆₅ as specified in 2° standard observer.

Rec. 709 specifies only the OECF/OETF (opto-electrical transfer function) of HDTV encoding in reference to the camera, known as camera gamma (sometimes indicated as "scene-referred"[7] gamma). The Rec. 709 transfer function from the linear signal (luminance) to the nonlinear (voltage) is linear in the bottom part and then transfers to a power function for the rest of the ${\displaystyle [0..1]}$ range:[8]

${\displaystyle V={\begin{cases}4.500L&L<0.018\\1.099L^{0.45}-0.099&L\geq 0.018\end{cases}}}$

Here 1.099 number (called α) has the value 1 + 5.5 * β = 1.099296826809442... and β has the value 0.018053968510807..., while 0.099 is 1.099 - 1.[9] Those values are coming from these simultaneous equations[10] that are required to connect the two curve segments smoothly:

${\displaystyle {\begin{cases}4.5\beta =\alpha \beta ^{0.45}-\alpha +1\\4.5=0.45\alpha \beta ^{-0.55}\end{cases}}}$

The conversion to linear is as follows.

${\displaystyle L={\begin{cases}{\dfrac {V}{4.5}}&V<0.081\\\left({\dfrac {V+0.099}{1.099}}\right)^{\frac {1}{0.45}}&V\geq 0.081\end{cases}}}$

The power function of the majority of the TRC (tone response curve) is 0.45, but because it is offset by the linear section the resulting equivalent gamma is more approximate to 0.50-0.53 (the inverse of which is approximately gamma 1.9-2.0 to convert back to linear).

While Rec. 709 does not specify the display referred gamma (EOCF/EOTF), display gamma is discussed in EBU Tech 3320 and specified in ITU-R BT.1886 as an equivalent gamma of 2.4, that is deviating from it in black region depending on how deep the black is.[11] This is a higher gamma than the 2.0 the math shown above would indicate, because the television system has been deliberately designed with an end-to-end system gamma of about 1.2, to provide compensation for the ‘dim surround’ effect. Therefore, the monitor gamma is not the inverse of the camera gamma.[12]

It is worth noting that Rec. 709 and sRGB share the same primary chromaticities and white point chromaticity; however, sRGB is explicitly output (display) referred with an equivalent gamma of 2.2 (the actual function is also piecewise).[13]

In typical production practice the encoding function of image sources is adjusted so that the final picture has the desired aesthetic look, as viewed on a reference monitor with a gamma of 2.4 (per ITU-R BT.1886) in a dim reference viewing environment (per ITU-R BT.2035).[14][15][16] Nevertheless the BT.1886 does have a problem in HDR and thus BT.2390 defines the Hermite spline (EETF) that adds a tapering factor (1 – E₂)⁴ to BT.1886.

Rec. 709 defines an R’G’B’ encoding and a Y’C_BC_R encoding, each with either 8 bits or 10 bits per sample in each color channel. In the 8-bit encoding, the R’, B’, G’, and Y’ channels have a nominal range of [16..235], and the C_B and C_R channels have a nominal range of [16..240] with 128 as the neutral value. So in limited range R’G’B’, reference black is (16, 16, 16) and reference white is (235, 235, 235), and in Y’C_BC_R, reference black is (16, 128, 128), and reference white is (235, 128, 128). Values outside the nominal ranges are allowed, but typically they would be clamped for broadcast or for display (except for Superwhite and xvYCC). Values 0 and 255 are reserved as timing references, and may not contain color data (for 8 bits, for 10 bits more values are reserved and for 12 bits even more, no values are reserved in files or RGB mode). Rec. 709's 10-bit encoding uses nominal values four times those of the 8-bit encoding. Rec. 709's nominal ranges are the same as those defined in ITU Rec. 601.[17]

The vast legacy library of standard definition programs and content presents further challenges. NTSC, PAL, and SECAM are all interlaced formats in a 4:3 aspect ratio, and at a relatively low resolution. Scaling them up to HD resolution with a 16:9 aspect ratio presents a number of challenges.

First is the potential for distracting motion artifacts due to interlaced video content. The solution is to either up-convert only to an interlaced BT.709 format at the same field rate, and scale the fields independently, or process them to de-interlace and remove the inter-field motion, creating progressive frames.

Second is the issue of aspect ratios. Cropping the top and or bottom of the standard definition frame may or may not work, depending on if the composition allows it and if there are graphics or titles that would be cut off.

In addition, the NTSC color primaries of red, green, and blue are different than those of BT.709. The red and blue primaries for PAL and SECAM are the same as BT.709, with only a minor change in the green primary. Converting NTSC properly means using a LUT (lookup table) to convert the colors to the new colorspace.

When encoding Y’C_BC_R video, BT.709 creates gamma-encoded luma (Y’) using R’G’B’ coefficients 0.2126, 0.7152, and 0.0722 (together they add to 1). BT.709-1 used slightly different 0.2125, 0.7154, 0.0721 (changed to standard one in BT.709-2). Although worldwide agreement on a single R’G’B’ system was achieved with Rec. 709, adoption of different luma coefficients for Y’C_BC_R requires the use of different luma-chroma decoding for standard definition and high definition.[18]

These problems can be handled with video processing software which can be slow, or hardware solutions[19] which allow for realtime conversion, and often with quality improvements.

A more ideal solution is to go back to original film elements for projects that originated on film. Due to the legacy issues of international distribution, many television programs that shot on film used a traditional negative cutting process, and then had a single film master that could be telecined for different formats. These projects can re-telecine their cut negative masters to a BT.709 master at a reasonable cost, and gain the benefit of the full resolution of film.

On the other hand for projects that originated on film, but completed their online master using video online methods would need to re-telecine the individual needed film takes and then re-assemble, a significantly greater amount of labor and machine time is required in this case, versus a telecine for a conformed negative. In this case, to enjoy the benefits of the film original would entail much higher costs to conform the film originals to a new HD master.