SDI and HD-SDI interfaces

Composite 4fSC digital interfaces are obsolete. For details about them, consult the first edition of this book.

This chapter describes digital interfaces for uncompressed and compressed SD and HD. Tables 38.1 and 38.2 summarize video signal levels.

ITU-R Rec. BT.601-5, Studio encoding parameters of digital television for standard 4:3 and widescreen 16:9 aspect ratios.

BT.601 originated with 8-bit components, anticipating 10 bits.

Recall from page 124 that in 4:2:2 BT.601, CB and CR are cosited – each is centered on the same location as Y’j, where j is even; chroma samples are absent when j is odd.

Component digital SD interface (BT.601)

ITU-R Rec. BT.601, adopted in 1984, specifies abstract coding parameters (including 4:2:2 chroma subsampling) for 480i29.97 and 576i25 SD. Luma is sampled at 13.5 MHz; CB and CR colour difference components are horizontally subsampled by a factor of 2:1 with respect to luma – that is, sampled at 6.75 MHz each. Samples are multiplexed in the sequence {CB, Y0’, CR, Y1’}. Sync information and optional ancillary data is multiplexed; 10-bit words at 27 MW/s are then serialized for a total bit rate of 270 Mb/s. The external interface is called the serial digital interface (SDI); it uses coaxial cable and BNC connectors.

Sampling at 13.5 MHz produces a whole number of samples per total line (STL) in 480i systems (with 858 STL) and 576i systems (with 864 STL). Both 480i and 576i have 720 active luma samples per line (SAL). In uncompressed, 8-bit BT.601 video, the active samples consume about 20 MB/s.

The notation 4:2:2 originated as a reference to the chroma subsampling scheme that I outlined on

page 124. During the 1980s, 4:2:2 denoted a specific SD component digital video interface standard incorporating 4:2:2 chroma subsampling. In the 1990s, the 4:2:2 chroma subsampling format was adopted for HD; as a result, the notation 4:2:2 came to be independent of image size.

The notations BT.601 and BT.656 have fallen into disuse in studio video. However, desktop video hardware designers often use “Rec. 601” or “BT.601” to denote a parallel interface having separate wires for vertical and horizontal sync signalling, and “Rec. 656” or “BT.656” to denote a parallel interface wherein vertical and horizontal sync are represented by embedded TRS codes.

Figure 38.1 at the top of the facing page shows the luma (or R’, G’, or B’) waveform of a single scan line of 480i component video. The time axis shows sample counts at the BT.601 rate of 13.5 MHz; divide the sample number by 13.5 to derive time in microseconds. Amplitude is shown in millivolts (according to EBU Tech. N10 levels), and in 8-bit BT.601 digital interface code values.

Figure 38.2 The BT.656 component digital interface uses EAV to signal the start of each horizontal blanking 0V interval, and SAV to signal

the start of active video. Between EAV and SAV, ancillary data (HANC) can be carried. In a nonpicture line, the region between SAV and EAV can carry ancillary data (VANC). Digitized ancillary signals may be carried in lines other than those that convey either VANC or analog sync.

HANC

VANC Digitized ancillary signals

SMPTE 259M, 10-Bit 4:2:2 Component and 4fSC Composite Digital Signals – Serial Digital Interface.

The 23.976 Hz, 29.97 Hz, and 59.94 Hz frame rates are associated with a sampling rate of:

74.25 ≈74.176 Mpx/s

1.001

The corresponding HD-SDI serial interface bit rate is:

1.485 ≈1.483Gb/s

1.001

Serial digital interface (SDI)

Serial digital interface (SDI) refers to a family of interfaces standardized by SMPTE. The BT.601 or 4fSC data stream is serialized, then subjected to a scrambling technique. SMPTE ST 259 standardizes several interfaces, denoted by letters A through D as follows:

•Composite 4fSC NTSC video, about 143 Mb/s

•Composite 4fSC PAL video, about 177 Mb/s

•BT.601 4:2:2 component video, 270 Mb/s (This interface is standardized in BT.656.)

•BT.601 4:2:2 component video sampled at 18 MHz to achieve 16:9 aspect ratio, 360 Mb/s

All but scheme C are now obsolete.

SDI is standardized for electrical transmission through coaxial cable, and for transmission through optical fiber. The SDI electrical interface uses ECL levels, 75 Ω impedance, BNC connectors, and coaxial cable. Electrical and mechanical parameters are specified in SMPTE standards and in BT.656; see SDI coding on page 439. Fiber-optic interfaces for digital SD, specified in SMPTE 297M, are straightforward adaptations of the serial versions of BT.656.

Component digital HD-SDI

The basic coding parameters of HD systems are standardized in BT.709. Various scanning systems are detailed in several SMPTE standards referenced in Table 15.2, on page 145.

Component SD, composite 4fSC NTSC, and composite 4fSC PAL all have different sample rates and different serial interface bit rates. In HD, a uniform sample rate of 74.25 MHz is adopted (modified by the ratio 1000⁄1001 in applications where compatibility with 59.94 Hz frame rate is required). A serial interface bit rate of 20 times the sampling rate is used. Variations of the same standard accommodate mainstream 1080i30, 1080p24, and 720p60 scanning; 1080p30; and the

obsolete 1035i30 system. The integer picture rates 24, 30, and 60 can be modified by the fraction 1000⁄1001, giving rates of 23.976 Hz, 29.97 Hz, and 59.94 Hz.

The SDI interface at 270 Mb/s has been adapted to HD by scaling the bit rate by a factor of 5.5, yielding a fixed bit rate of 1.485 Gb/s. The sampling rate and serial bit rate for 23.976 Hz, 29.97 Hz, and 59.94 Hz