“Rec. 709”的意思、由来-开放百科全书

ITU-R Recommendation BT.709, also known by the abbreviations Rec. 709, BT.709, and ITU709, standardizes the format of high-definition television, having 16:9 (widescreen) aspect ratio. The first edition of the standard was approved in 1990.

Technical details

Pixel count

Rec. 709 refers to HDTV systems having roughly two million luma samples per frame. Rec. 709 has two parts:

Part 2 codifies current and prospective 1080i and 1080p systems with square sampling. In an attempt to unify 1080-line HDTV standards, part 2 defines a common image format (CIF) with picture parameters independent of the picture rate.

Part 1 codifies what are now referred to as 1035i30 and 1152i25 HDTV systems. The 1035i30 system is now obsolete, having been superseded by 1080i and 1080p square-sampled ("square-pixel") systems. The 1152i25 system was used for experimental equipment in Europe and was never commercially deployed.

Frame rate

Rec. 709 specifies the following picture rates: 60 Hz, 50 Hz, 30 Hz, 25 Hz and 24 Hz. "Fractional" rates having the above values divided by 1.001 are also permitted.

Initial acquisition is possible in either progressive or interlaced form. Video captured as progressive can be transported with either progressive transport or progressive segmented frame (PsF)

transport. Video captured as interlaced can be transported with interlace transport. In cases where a progressive captured image is transported as a segmented frame, segment/field frequency must be twice the frame rate.

In practice, the above requirements result in the following frame rates ("fractional" rates are specified in commonly used "decimal" form): 25i, 25PsF, 25p, 50p for 50 Hz systems; 23.976p, 23.976PsF, 24p, 24PsF, 29.97i, 29.97p, 29.97PsF, 30PsF, 30p, 59.94p, 60p for 60 Hz systems.

Digital representation

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 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. Values 0 and 255 are reserved as timing references, and may not contain color data. 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.^[1]

Primary chromaticities

Note that red and blue are the same as the EBU Tech 3213 primaries while green is halfway between EBU Tech 3213 and SMPTE C (two types of Rec.601). 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%.^[3]

Standards Conversion

When converting between the various HD and SD formats, it would be correct to compensate for the differences in the primaries (e.g. between the Rec. 709, EBU Tech 3213, and SMPTE C primaries). In practice, this conversion is rarely performed because the difference is negligible in real-world scenes, except the ones with large patches of very saturated colors.^[4]{{rs|date=November 2017}}

Luma coefficients

HDTV according to Rec. 709 forms luma (Y’) using R’G’B’ coefficients 0.2126, 0.7152, and 0.0722. This means that unlike Rec. 601, the coefficients match the primaries and white points, so luma corresponds more closely to luminance. Some experts feel that the advantages of correct matrix coefficients do not justify the change from Rec. 601 coefficients.^[5]

Transfer characteristics

Rec. 709 specifies the OETF (opto electrical transfer function) of HDTV encoding in reference to the camera, known as camera gamma (sometimes indicated as "scene-referred"^[6] 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

range:^[7]

The power function of the majority of the gamma 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, display gamma is discussed in EBU Tech 3320 and specified in ITU-R BT.1886 as a gamma of 2.4. 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. ^[8]

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 a gamma of 2.2. ^[9]

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).^[10]^[11]^[12]