Serial Digital Interface
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Serial Digital Interface (SDI), standardized in ITU-R BT.656 and SMPTE-259M, is a digitized video interface used for broadcast grade video. A related standard, known as High Definition Serial Digital Interface (HD-SDI) is standardized in SMPTE-292M. An emerging interface, commonly known in the industry as dual link, is SMPTE 372M; a 3Gb/s nominal interface used in applications (such as digital cinema) which require greater fidelity and resolution than standard HDTV can provide.
These standards are used for transmission of uncompressed, unencrypted digital television signals (optionally including audio) within television facilities. They are designed for operation over short distances; due to their high bitrates they are inappropriate for long-distance transmission. SDI and HD-SDI are currently only available in professional video equipment; various licensing agreements, restricting the use of unencrypted digital interfaces to professional equipment, prohibit its use in consumer equipment. (There are various mod kits for existing DVD players and other devices, which allow a user to add a serial digital interface to these devices).
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Technical details of the various standards
Electrical interface
The various serial digital interface standards all use one (or more) coaxial cables with BNC connectors, with a nominal impedance of 75 ohm. This is the same type of cable used in analog video setups, which potentially makes for easier upgrades (though higher quality cables may be necessary for long runs at the higher bitrates). 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 equalisation at the receiver, it is possible to send 270 Mbit/s SDI over 300 metres without use of repeaters, but shorter lengths are preferred. The HD bitrates have a shorter maximum run length, typically 100 meters.
Uncompressed digital component signals are transmitted. Data is encoded in NRZ 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.
Bitrates
Several bitrates are used in serial digital video:
- For standard definition applications, as defined by SMPTE 259M, the possible bitrates are 270Mb/s, 360Mb/s, 143Mb/s, and 177Mb/s. 270Mb/s is by far the most commonly used; though the 360 Mb/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 540Mb/s interfaces defined. These are rarely encountered.
- For HDTV applications, the serial digital interface is defined by SMPTE 292M. Two bitrates 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 upwards compatible with existing NTSC systems. The 1.485 Gb/sec version of the standard supports other frame rates in widespread use, including 60Hz, 50Hz, 30Hz, 25Hz, and 24Hz. It is common to collectively refer to both standards using a nominal bitrate of 1.5 GBit/sec.
- 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.
Other interfaces
SMPTE 292M defines an optical interface as well as an electrical one; this interface is widely considered to be obsolete. An 8-bit parallel digital interface is defined by CCIR 601, this is also obsolete (however, many clauses in the various standards accommodate the possibility of an 8-bit interface).
Data Format
In SD and ED applications, the parallel 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:
- Cb Y Cr Y' Cb Y Cr Y'
whereas the HD datastreams are arranged like this:
- Y
- Y Y' Y Y' Y Y' Y Y'
- C
- Cb Cr Cb Cr Cb Cr Cb Cr
In both cases, the video is encoded in 4:2:2 format, meaning that the luminance channel is encoded at full bandwidth (13.5 Mhz in 270Mbit/s SD, ~75Mhz in HD), and the two chrominance channels are subsampled horizontally, and encoded at half bandwidth (6.75 MHz or 37.5 MHz). The Y, Cr, and Cb samples are cosited (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.
Video payload (as well as anciliary data payload) may use any 10-bit word in the range 4 to 1019 (004 to 3FB in hexadecimal) inclusive; the values 0-3 and 1020-1023 (3FC - 3FF) are reserved and may not appear anywhere in the payload. These reserved words have two purposes, for synchronization packets, and for anciliary data headers.
Synchronization packets
A synchonization 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 syncronization 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 anciliary 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 (typically 525, 625, 750, or 1125). Determination of line 1 is somewhat aribrary; 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 (625 and 1125-line), 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 (the first EAV sample in that format), and the line ends at the following sample 1919 (the last active sample in that format). 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.
Link numbering
Link numbering is only an issue in dual-link interfaces. The first link (the primary) link, is assigned a link number of 1, subsequent links are assigned increasing link numbers; so the second (secondary) link in a dual-link system is link 2. The link number of a given interface is indicated by a VPID packet located in the vertical anciliary data space.
Note that the data layout in dual link is designed so that the primary link can be fed into a single-link interface, and still produce usable (though somewhat degraded) video. The secondary link generally contains things like additional LSBs (in 12-bit formats), non-cosited samples in 4:4:4 sampled video (so that the primary link is still valid 4:2:2), and alpha or data channels. If the second link of a 1080P dual link configuration is absent, the first link still contains a valid 1080i signal.
Ancillary data
Template:Main Like SMPTE 259M, SMPTE 292M supports the SMPTE 291M standard for ancillary data. Ancillary data is provided as a standardized transport for non-video payload within a serial digital signal; it is used for things such as embedded audio, closed captions, timecode, and other sorts of metadata. Ancillary data is indicated by a 3-word packet consisting of 0, 3FF, 3FF (the opposite of the synchronization packet header), followed by a two-word identification code, a data count word (indicating 0 - 255 words of payload), the actual payload, and a one-word checksum. Other than in their use in the header, the codes prohibited to video payload are also prohibited to ancillary data payload.
Specific applications of anciliary data include embedded audio, EDH, VPID, and SDTI.
Embedded audio
Both the HD and SD serial interfaces provide for 16 channels of embedded audio. The two interfaces use different audio encapsulation methods--SD uses the SMPTE 272M standard, whereas HD uses the SMPTE 299M standard. In either case, a SDI signal may contain up to sixteen audio channels (8 pairs) embedded 48kHz, 24bit audio channels along with the video. Typically, 48kHz, 24-bit (20-bit in SD) PCM audio is stored, in a manner directly compatible with the AES3 digital audio interface. These are placed in the (horizontal) blanking periods, when the SDI signal carries nothing useful, since the receiver generates its own blanking signals from the TRS.
EDH
As the standard definition interface carries no checksum, CRC, or other data integrity check, an EDH (Error Detection and Handling) packet may be optionally placed in the vertical interval of the video signal. This packet includes CRC values for both the active picture, and the entire field (excluding those lines at which switching may occur, and which should contain no useful data); equipment can compute their own CRC and compare it with the received CRC in order to detect errors.
EDH is typically only used with the standard definition interface; the presence of CRC words in the HD interface make EDH packets unnecessary.
VPID
VPID (or video payload identifier) packets are increasingly used to describe the video format. In early versions of the serial digital interface, it was always possible to uniquely determine the video format by counting the number of lines and samples between H and V transitions in the TRS. With the introduction of dual link interfaces, and segmented-frame standards, this is no longer possible; thus the VPID standard (defined by SMPTE 352M) provides a way to uniquely and unambiguously identify the format of the video payload.
Video payload
Within the active portion of the video, the data words correspond to signal levels of the respective video components. The luminance (Y) channel is defined such that a signal level of 0 mV is assigned the codeword 64 (40 hex), and 700 millivolts (full scale) is assigned the codeword 940 (3AC) . For the chroma channels, 0 mV is assigned the code word 512 (200 hex), -350mV is assigned a code word of 64 (0x40), and +350mV is assigned a code word of 960 (3C0). Note that the scaling of the luma and chroma channels is not identical. The minimum and maximum of these ranges represent the preferred signal limits, though the video payload may venture outside these ranges (providing that the reserved code words of 0 - 3 and 1020 - 1023 are never used for video payload).
For portions of the vertical and horizontal blanking regions which are not used for anciliary data, it is recommended that the luma samples be assigned the code word 64 (40 hex), and the chroma samples be assigned 512 (200 hex); both of which correspond to 0 mV. It is permissible to encode analog vertical interval information (such as vertical interval timecode or vertical interval test signals) without breaking the interface, but such usage is nonstandard (and anciliary data is the preferred means for transmitting metadata). Conversion of analog sync and burst signals into digital, however, is not recommended--and neither is necessary in the digital interface.
Supported video formats
The various versions of the serial digital interface support numerous video formats.
- The 270 Mbit/s interface supports 525-line, interlaced video at a 59.94 Hz field rate (29.97 Hz frame rate), and 625-line, 50Hz interlaced video. These formats are highly compatible with NTSC and PAL respectively; and the terms NTSC and PAL are often (incorrectly) used to refer to these formats. (NTSC and PAL are composite color encoding schemes; and the serial digital interface--other than the obsolete 143 Mbit/sec and 177 Mbit/sec forms, is a component standard).
- The 360 Mbit/s interface supports 525i and 625i widescreen. It can also be used to support 525p, if 4:2:0 sampling is used.
- The various 540 Mbit/s interfaces support 525p and 625p formats.
- The nominal 1.5 Gb/s interfaces support most high definition formats. Supported formats include 1080i60, 1080i59.94, 1080i50, 1080p30, 1080p29.97, 1080p25, 1080p24, 1080p23.98, 720p60, 720p59.94, and 720p50. In addition, there are several 1035i formats (an obsolete Japanese television standard), half-bandwidth 720p standards such as 720p24 (used in some film conversion applications, and unusual because it has an odd number of samples per line), and various 1080psf (progressive, segmented frame) formats. Progressive Segmented frames formats appear as interlace video but contain video which is progressively scanned. This is done to support analog monitors and televsions, many of which are incapable of locking to low field rates such as 30Hz and 24Hz.
- The dual link HD interface supports 1080p60, 1080p59.94, and 1080p50, as well as 4:4:4 encoding, greater color depth, RGB encoding, alpha channels, and nonstandard resolutions (often encountered in computer graphics or digital cinema).
Related interfaces
In addition to the regular serial digital interface described here, there are several other similar interfaces which are similar to, or are contained within, a serial digital interface.
SDTI
Template:Main There is an expanded specification called SDTI (Serial Data Transport Interface), which allows compressed (i.e. DV, MPEG and others) video streams to be transported over an SDI line. This allows for multiple video streams in one cable or faster-than-realtime (2x, 4x,...) video transmission. A related standard, known as HD-SDTI, provides similar capability over a SMPTE 292M interface.
The SDTI interface is specified by SMPTE 305M. The HD-SDTI interface is specified by SMPTE 348M.
SMPTE 349M
The standard SMTPE 349M: Transport of Alternate Source Image Formats through SMPTE 292M, specifies a means to encapsulate non-standard and lower-bitrate video formats within a HD-SDI interface. This standard allows, for example, several independent standard definition video signals to be multiplexed onto a HD-SDI interface, and transmitted down one wire. This standard doesn't merely adjust EAV and SAV timing to meet the requirements of the lower-bitrate formats; instead, it provides a means by which an entire SDI format (including synchronication words, anciliary data, and video paylaod) can be encapsulated and transmitted as ordinary data payload within a 292M stream.
External links
- Society of Motion Picture and Television Engineers: SMPTE 274M-2005: Image Sample Structure, Digital Representation and Digital Timing Reference Sequences for Multiple Picture Rates
- Society of Motion Picture and Television Engineers: SMPTE 292M-1998: Bit-Serial Digital Interface for High Definition Television
- Society of Motion Picture and Television Engineers: SMPTE 291M-1998: Anciliary Data Packet and Space Formatting
- Society of Motion Picture and Television Engineers: SMPTE 372M-2002: Dual Link 292M Interface for 1920 x 1080 Picture Raster
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