ITU-R BT 601

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ITU-R Recommendation BT.601 (old name: CCIR 601 ) is a standard defined by the CCIR (now ITU-R ) has been published and which specifies how digital interlaced - video signals to be encoded. It contains methods of digitizing analog television signals with a frame rate of 60  Hz and 525 lines or alternatively with a frame rate of 50 Hz and 625 lines.

Frequencies

The timing of the CCIR-601 signal largely corresponds to that of its analog predecessors, so that a conversion in one as well as in the other direction is possible without great effort. There are thus also time ranges for the synchronization signals and the blanking intervals of the analog signal. The pixel clock is 13.5 MHz regardless of the refresh rate.

In the 50 Hertz version, the duration of a line is set to 64 µs, just like with the analog television signal, which results in a nominal horizontal resolution of 864 pixels per line. The 12 µs for the blanking signal must be subtracted from the 64 µs. This leaves 52 µs for the visible line. The active area of ​​a line is 53 s µs (720 pixels) a little wider than the analog standard with 52 µs (corresponds to 702 pixels). The active area also rises in the image height: there are now 576 visible lines instead of the previous 574 whole and half each on the top right and bottom left. When digitizing an analog signal, there are therefore small black borders.

The standard says nothing about whether and how a digital signal can be converted back into an analog one.

Color coding

CCIR 601 further states that the color signal values ​​C b and C r are to be skipped from every other pixel. This color model is called YC b C r -4: 2: 2, corresponding to the ratio of the three values brightness , color deviation blue / yellow and color deviation red / turquoise . For two consecutive pixels, the two brightness values ​​Y 1 and Y 2 and the two color difference values ​​are stored in the order C b : Y 1 : C r : Y 2 . The brightness values ​​are to be quantized with at least 8 bits, the color deviation values ​​with at least 4 bits . In the studio area, all values ​​are quantized with the higher resolution of 10 bits.

Auxiliary signals

With CCIR 601 (more precisely: CCIR 656) the synchronization signals of the analog video signal are not transmitted in digitized form. Instead, the visible video area is marked by special digital synchronization signals, which signal the beginning ( SAV = Start of Active Video) and the end ( EAV = End of Active Video) of a video line. The digitized video information is transmitted between the SAV and EAV . The time ranges between EAV and SAV are either filled with meaningless content or, for example, digital audio data are embedded in them. Such devices are called embedders . In 8-bit systems, the specially reserved values ​​0x00 (0) and 0xFF (255) are used to encode the SAV and EAV signals in the form 0xFF, 0x00, 0x00, 0x YY . The value 0x YY is a bit field and defines, whether it is a line beginning or end of a line, a picture start or a visual or a straight or odd field ( Field treated).

The temporal maintenance of the horizontal and vertical blanking intervals in the digital data stream is necessary in order to guarantee the temporal relationship between the represented analog video signals and their digital mapping. Without maintaining these areas, the line frequency of the analog and digital video data would not match at the same frame rate, which would mean that a complete image would have to be buffered during the conversion.

Interfaces

The first version of CCIR 601 only defined a parallel interface, but later versions introduced the bit-serial family of serial digital video signals (better known as SDI ) that are now widely used. The 8-bit serial protocol was previously used in D-1 digital tape recording. It also quantizes the color difference values ​​with 8 bits and thus comes to a data rate of according to Y 1 : Y 2 : C b1 : C r1

13.5 MHz × 1/2 × (8 bit + 8 bit + 8 bit + 8 bit) = 216 Mbit / s

Modern standards use a coding table that adds one or two check bits. The 9-bit series version has a data rate of 243 Mbit / s. The 10-bit version used in the D5 digital tape recording has a data rate of 270 Mbit / s.

There is an 8-bit version in which only the actual image area is transmitted. This requires 165.9 Mbit / s.

Since according to the specification, the digital values ​​0 and 255 are used as synchronization values, the valid brightness and color value range is reduced. Therefore, the luminance (Y) 220 values ​​are available (16 ... 235) in order to achieve a sufficient buffer zone above and below the valid values. These areas are called the headroom and footroom, which are used to absorb any (invalid) overshoots without endangering synchronization. The chrominance (C b , C r ) is mapped to 225 valid values ​​(16 ... 240), whereby the value 128 defines the zero point and thus the colorless state of the two color signals. In this area, a pure black and white image or a gray level can now be transmitted.

application

The CCIR-601 video raster format has been reused in several later standards including MPEG . The FourCC identifier for a CCIR-601 data stream is V422 .

Remarks

  1. ↑ The EBU Technical Recommendation R92-1999 : Active picture area and picture centering in analogue and digital 625/50 television systems (PDF; 14 kB) makes it clear that the 52 µs (702 pixels) comes from BT.470. BT.601 on the other hand comes to 53 ⅓ µs (720 = 864 - 144 pixels).

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