Serial digital interface
Serial digital interface is a family of digital video interfaces first standardized by SMPTE in 1989. For example, ITU-R BT.656 and SMPTE 259M define digital video interfaces used for broadcast-grade video. A related standard, known as high-definition serial digital interface, is standardized in SMPTE 292M; this provides a nominal data rate of 1.485 Gbit/s.
Additional SDI standards have been introduced to support increasing video resolutions, frame rates, stereoscopic video, and color depth. Dual link HD-SDI consists of a pair of SMPTE 292M links, standardized by SMPTE 372M in 1998; this provides a nominal 2.970 Gbit/s interface used in applications that require greater fidelity and resolution than standard HDTV can provide. 3G-SDI consists of a single 2.970 Gbit/s serial link that allows replacing dual link HD-SDI. 6G-SDI and 12G-SDI standards were published on March 19, 2015.
These standards are used for transmission of uncompressed, unencrypted digital video signals within television facilities; they can also be used for packetized data. SDI is used to connect together different pieces of equipment such as recorders, monitors, PCs and vision mixers. Coaxial variants of the specification range in length but are typically less than. Fiber optic variants of the specification such as 297M allow for long-distance transmission limited only by maximum fiber length or repeaters.
SDI and HD-SDI are usually available only in professional video equipment because various licensing agreements restrict the use of unencrypted digital interfaces, such as SDI, prohibiting their use in consumer equipment. Several professional video and HD-video capable DSLR cameras and all uncompressed video capable consumer cameras use the HDMI interface, often called clean HDMI. There are various mod kits for existing DVD players and other devices such as splitters that ignore HDCP, which allow a user to add a serial digital interface to these devices.
Electrical interface
The various serial digital interface standards all use coaxial cables with BNC connectors, with a nominal impedance of 75 ohms.This is the same type of cable used in analog composite video setups, potentially allowing for easier "drop-in" equipment upgrades. The specified signal amplitude at the source is 800 mV peak-to-peak; far lower voltages may be measured at the receiver owing to attenuation. Using equalization at the receiver, it is possible to send 270 Mbit/s SDI over without use of repeaters, but shorter lengths are preferred. The HD bitrates have a shorter maximum run length, typically.
Uncompressed digital component signals are transmitted. Data is encoded in NRZI format, and a linear feedback shift register is used to scramble the data to reduce the likelihood that long strings of zeroes or ones will be present on the interface. The interface is self-synchronizing and self-clocking. Framing is done by detection of a special synchronization pattern, which appears on the serial digital signal to be a sequence of ten ones followed by twenty zeroes ; this bit pattern is not legal anywhere else within the data payload.
Standards
Bit rates
Several bit rates are used in serial digital video signal:- For standard-definition applications, as defined by SMPTE 259M, the possible bit rates are 270 Mbit/s, 360 Mbit/s, 143 Mbit/s, and 177 Mbit/s. 270 Mbit/s is by far the most commonly used; though the 360 Mbit/s interface is sometimes encountered. The 143 and 177 Mbit/s interfaces were intended for transmission of composite-encoded video digitally and are now considered obsolete.
- For enhanced definition applications, there are several 540 Mbit/s interfaces defined, as well as an interface standard for a dual-link 270 Mbit/s interface. These are rarely encountered.
- For HDTV applications, the serial digital interface is defined by SMPTE 292M. Two bit rates are defined, 1.485 Gbit/s, and 1.485/1.001 Gbit/s. The factor of 1/1.001 is provided to allow SMPTE 292M to support video formats with frame rates of 59.94 Hz, 29.97 Hz, and 23.98 Hz, in order to be compatible with existing NTSC systems. The 1.485 Gbit/s version of the standard supports other frame rates in widespread use, including 60 Hz, 50 Hz, 30 Hz, 25 Hz, and 24 Hz. It is common to collectively refer to both standards as using a nominal bit rate of 1.5 Gbit/s.
- For very high-definition applications, requiring greater resolution, frame rate, or color fidelity than the HD-SDI interface can provide, the SMPTE 372M standard defines the dual link interface. As the name suggests, this interface consists of two SMPTE 292M interconnects operating in parallel. In particular, the dual link interface supports 10-bit, 4:2:2, 1080P formats at frame rates of 60 Hz, 59.94 Hz, and 50 Hz, as well as 12-bit color depth, RGB encoding, and 4:4:4 colour sampling.
- A nominal 3 Gbit/s interface was standardized by SMPTE as 424M in 2006. Revised in 2012 as SMPTE ST 424:2012, it supports all of the features supported by the dual 1.485 Gbit/s interface but requires only one cable rather than two.
Other interfaces
An 8-bit parallel digital interface is defined by ITU-R Rec. 601; this is obsolete.
Data format
In SD and ED applications, the serial data format is defined to 10 bits wide, whereas in HD applications, it is 20 bits wide, divided into two parallel 10-bit datastreams. The SD datastream is arranged like this:whereas the HD datastreams are arranged like this:
; Y: '
; C: '
For all serial digital interfaces, the native color encoding is 4:2:2 YCbCr format. The luminance channel is encoded at full bandwidth, and the two chrominance channels are subsampled horizontally and encoded at half bandwidth. The Y, Cr, and Cb samples are co-sited, and the Y' sample is acquired at the time halfway between two adjacent Y samples.
In the above, Y refers to luminance samples, and C to chrominance samples. Cr and Cb further refer to the red and blue "color difference" channels; see Component video for more information. This section only discusses the native color encoding of SDI; other color encodings are possible by treating the interface as a generic 10-bit data channel. The use of other colorimetry encodings, and the conversion to and from RGB colorspace, is discussed [|below].
Video payload may use any 10-bit word in the range 4 to 1,019 inclusive; the values 0–3 and 1,020–1,023 are reserved and may not appear anywhere in the payload. These reserved words have two purposes; they are used both for [|Synchronization packets] and for [|Ancillary data] headers.
Synchronization packets
A synchronization packet occurs immediately before the first active sample on every line, and immediately after the last active sample. The synchronization packet consists of four 10-bit words, the first three words are always the same—0x3FF, 0, 0; the fourth consists of 3 flag bits, along with an error correcting code. As a result, there are 8 different synchronization packets possible.In the HD-SDI and dual link interfaces, synchronization packets must occur simultaneously in both the Y and C datastreams.. In SD-SDI and enhanced definition interfaces, there is only one datastream, and thus only one synchronization packet at a time. Other than the issue of how many packets appear, their format is the same in all versions of the serial-digital interface.
The flags bits found in the fourth word are known as H, F, and V. The H bit indicates the start of horizontal blank; and synchronization bits immediately preceding the horizontal blanking region must have H set to one. Such packets are commonly referred to as End of Active Video, or EAV packets. Likewise, the packet appearing immediately before the start of the active video has H set to 0; this is the Start of Active Video or SAV packet.
Likewise, the V bit is used to indicate the start of the vertical blanking region; an EAV packet with V=1 indicates the following line is part of the vertical interval, an EAV packet with V=0 indicates the following line is part of the active picture.
The F bit is used in interlaced and segmented-frame formats to indicate whether the line comes from the first or second field. In progressive scan formats, the F bit is always set to zero.
Line counter and CRC
In the high definition serial digital interface, additional check words are provided to increase the robustness of the interface. In these formats, the four samples immediately following the EAV packets contain a cyclic redundancy check field, and a line count indicator. The CRC field provides a CRC of the preceding line and can be used to detect bit errors in the interface. The line count field indicates the line number of the current line.The CRC and line counts are not provided in the SD and ED interfaces. Instead, a special ancillary data packet known as an EDH packet may be optionally used to provide a CRC check on the data.
Line and sample numbering
Each sample within a given datastream is assigned a unique line and sample number. In all formats, the first sample immediately following the SAV packet is assigned sample number 0; the next sample is sample 1; all the way up to the XYZ word in the following SAV packet. In SD interfaces, where there is only one datastream, the 0th sample is a Cb sample; the 1st sample a Y sample, the 2nd sample a Cr sample, and the third sample is the Y' sample; the pattern repeats from there. In HD interfaces, each datastream has its own sample numbering—so the 0th sample of the Y datastream is the Y sample, the next sample the Y' sample, etc. Likewise, the first sample in the C datastream is Cb, followed by Cr, followed by Cb again.Lines are numbered sequentially, starting from 1, up to the number of lines per frame of the indicated format. Determination of line 1 is somewhat arbitrary; however, it is unambiguously specified by the relevant standards. In 525-line systems, the first line of vertical blank is line 1, whereas in other interlaced systems, the first line after the F bit transitions to zero is line 1.
Note that lines are deemed to start at EAV, whereas sample zero is the sample following SAV. This produces the somewhat confusing result that the first sample in a given line of 1080i video is sample number 1920, and the line ends at the following sample 1919. Note that this behavior differs somewhat from analog video interfaces, where the line transition is deemed to occur at the sync pulse, which occurs roughly halfway through the horizontal blanking region.