Serial digital interface (SDI) is a family of digital video interfaces first standardized by SMPTE (The Society of Motion Picture and Television Engineers) 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 (HD-SDI), 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 (HD, UHD and beyond), frame rates, stereoscopic (3D) 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 (such as digital cinema or HDTV 1080P) that require greater fidelity and resolution than standard HDTV can provide. 3G-SDI (standardized in SMPTE 424M) 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 (optionally including embedded audio and time code) 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 (one or more) 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 (although, at high bitrates and/or long distances, it may be necessary for older, oxidising, or lower-grade cable to be replaced with optical fibre). The specified signal amplitude at the source is 800 mV (±10%) 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 (unscrambled) serial digital signal to be a sequence of ten ones followed by twenty zeroes (twenty ones followed by forty zeroes in HD); this bit pattern is not legal anywhere else within the data payload.
Standards StandardNameIntroducedBitrates (Mbit/s)Example video formatsSMPTE 259MSD-SDI1989270, 360, 143, and 177480i, 576iSMPTE 344MED-SDI2000540480p, 576pSMPTE 292MHD-SDI19981485 and 1485/1.001720p, 1080iSMPTE 372MDual Link HD-SDI20022970 and 2970/1.0011080p60SMPTE 424M3G-SDI20062970 and 2970/1.0011080p60SMPTE ST 20816G-SDI20155940 and 5940/1.0011080p120, 2160p30SMPTE ST 208212G-SDI201511880 and 11880/1.0012160p60SMPTE ST 208324G-SDI202023760 and 23760/1.0012160p120, 4320p30
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 (used for widescreen standard definition) is sometimes encountered. The 143 and 177 Mbit/s interfaces were intended for transmission of composite-encoded (NTSC or PAL) video digitally and are now considered obsolete. For enhanced definition applications (mainly 525P), 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 (more accurately, 2.97 Gbit/s, but commonly referred to as "3 gig") 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 SMPTE 297-2006 defines an optical fiber system for transmitting bit-serial digital signals. It is intended for transmitting SMPTE ST 259 signals (143 through 360 Mbit/s), SMPTE ST 344 signals (540 Mbit/s), SMPTE ST 292-1/-2 signals (1.485 Gbit/s and 1.485/1.001 Gbit/s) and SMPTE ST 424 signals (2.970 Gbit/s and 2.970/1.001 Gbit/s). In addition to optical specification, ST 297 also mandates laser safety testing and that all optical interfaces are labelled to indicate safety compliance, application and interoperability.
An 8-bit parallel digital interface is defined by ITU-R Rec. 601; this is obsolete (however, many clauses in the various standards accommodate the possibility of an 8-bit interface).
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 (known as Y and C). The SD datastream is arranged like this:
whereas the HD datastreams are arranged like this:
Y C
For all serial digital interfaces (excluding the obsolete composite encodings), the native color encoding is 4:2:2 YCbCr format. The luminance channel (Y) is encoded at full bandwidth (13.5 MHz in 270 Mbit/s SD, ~75 MHz in HD), and the two chrominance channels (Cb and Cr) are subsampled horizontally and encoded at half bandwidth (6.75 MHz or 37.5 MHz). The Y, Cr, and Cb samples are co-sited (acquired at the same instance in time), 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 (as well as ancillary data payload) may use any 10-bit word in the range 4 to 1,019 (004 to 3FB) inclusive; the values 0–3 and 1,020–1,023 (3FC–3FF) 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 (commonly known as the timing reference signal or TRS) occurs immediately before the first active sample on every line, and immediately after the last active sample (and before the start of the horizontal blanking region). 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. (Some delay between the two cables in a dual link interface is permissible; equipment which supports dual link is expected to buffer the leading link in order to allow the other link to catch up). 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 (commonly known as the XYZ 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 (lines are deemed to start at EAV) 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 (or segment). In progressive scan formats, the F bit is always set to zero.
Line counter and CRC In the high definition serial digital interface (and in dual-link HD), additional check words are provided to increase the robustness of the interface. In these formats, the four samples immediately following the EAV packets (but not the SAV packets) contain a cyclic redundancy check field, and a line count indicator. The CRC field provides a CRC of the preceding line (CRCs are computed independently for the Y and C streams) 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