SMPTE 292M
Encyclopedia
SMPTE 292M is a standard
published by SMPTE which expands upon SMPTE 259M
and SMPTE 344M
allowing for bit-rates of 1.485 Gbit/s, and 1.485/1.001 Gbit/s. These bit-rates are sufficient for and often used to transfer uncompressed High Definition
video.
This standard is usually referred to as HD-SDI; it is part of a family of standards that define a Serial Digital Interface
based on a coaxial
cable, intended to be used for transport of uncompressed digital video and audio in a television studio environment.
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.
The standard also defines nominal bitrates of 3 Gbit/s, for 1080P applications. This version of the interface is not used (and has not been commercially implemented); instead, a dual-link extension of SMPTE 292M known as SMPTE 372M
is used for very-high-definition applications.
and optical
interfaces were defined by SMPTE, over concerns that an electrical interface at that bitrate would be expensive or unreliable, and that an optical interface would be necessary. Such fears have not been realized, and the optical interfaces are seldom if ever used, and are likely to be deprecated in future revisions of the standard.
The cabling used for the SMPTE 292M electrical interface is coaxial
cable with a nominal impedance
of 75 Ω
. 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-clocking. Framing is done by detection of a special synchronization
pattern, which appears on the (unscrambled) serial digital signal to be a sequence of twenty ones followed by forty zeroes; this bit pattern is not legal anywhere else within the data payload.
The SMPTE 292M digital interface is known to be reliable (without use of repeaters) at cable lengths of 100 m or greater.
video samples, the other ("C") which encodes chrominance
information. The C channel is further time-multiplexed into two half-bandwidth channels, known as Cr (the "red color difference" channel), and Cb (the "blue color difference" channel). The nominal datarate of the Y channel is 75 Mwords/sec (1.5 Gbit/s divided by 20), and the nominal datarate of each of the two chroma channels is 37.5 Mwords/sec.
Video payload (as well as ancillary 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 ancillary data headers.
ing 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.
Synchronization packets must occur simultaneously in both the Y and C datastreams.
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 (lines are deemed to start 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 progressive 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.
Other than the fact that synchronization packets occur in parallel in two datastreams (Y and C), their behavior is virtually identical to the packet types defined in CCIR 601
and SMPTE 259M
, the digital interface commonly used for SDTV.
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 preceding line.
, 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.
For portions of the vertical and horizontal blanking regions which are not used for ancillary 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 ancillary 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.
Standardization
Standardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....
published by SMPTE which expands upon SMPTE 259M
SMPTE 259M
SMPTE 259M is a standard published by SMPTE which "... describes a 10-bit serial digital interface operating at 143/270/360 Mb/s." The goal of SMPTE 259M is to define a Serial Digital Interface , called SDI or SD-SDI....
and SMPTE 344M
SMPTE 344M
"SMPTE 344M" is a standard published by SMPTE which expands upon SMPTE 259M allowing for bit-rates of 540 Mbit/s, allowing EDTV resolutions of 480p and 576p.This standard is part of a family of standards that define a Serial Digital Interface....
allowing for bit-rates of 1.485 Gbit/s, and 1.485/1.001 Gbit/s. These bit-rates are sufficient for and often used to transfer uncompressed High Definition
High-definition video
High-definition video or HD video refers to any video system of higher resolution than standard-definition video, and most commonly involves display resolutions of 1,280×720 pixels or 1,920×1,080 pixels...
video.
This standard is usually referred to as HD-SDI; it is part of a family of standards that define a Serial Digital Interface
Serial Digital Interface
Serial digital interface is a family of video interfaces standardized by SMPTE. For example, ITU-R BT.656 and SMPTE 259M define digital video interfaces used for broadcast-grade video...
based on a coaxial
Coaxial
In geometry, coaxial means that two or more forms share a common axis; it is the three-dimensional linear analogue of concentric.Coaxial cable, as a common example, has a wire conductor in the centre a circumferential outer conductor and an insulating medium called the dielectric separating...
cable, intended to be used for transport of uncompressed digital video and audio in a television studio environment.
Technical details
The SMPTE 292M standard is a nominally 1.5 Gbit/s interface. Two exact 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 NTSCNTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
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.
The standard also defines nominal bitrates of 3 Gbit/s, for 1080P applications. This version of the interface is not used (and has not been commercially implemented); instead, a dual-link extension of SMPTE 292M known as SMPTE 372M
SMPTE 372M
SMPTE 372M is a standard published by SMPTE which expands upon SMPTE 259M, SMPTE 344M, and SMPTE 292M allowing for bit-rates of 2.970 Gbit/s, and 2.970/1.001 Gbit/s over two wires. These bit-rates are sufficient for 1080p video....
is used for very-high-definition applications.
Electrical interface
Originally, both electricalElectronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
and optical
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
interfaces were defined by SMPTE, over concerns that an electrical interface at that bitrate would be expensive or unreliable, and that an optical interface would be necessary. Such fears have not been realized, and the optical interfaces are seldom if ever used, and are likely to be deprecated in future revisions of the standard.
The cabling used for the SMPTE 292M electrical interface is coaxial
Coaxial
In geometry, coaxial means that two or more forms share a common axis; it is the three-dimensional linear analogue of concentric.Coaxial cable, as a common example, has a wire conductor in the centre a circumferential outer conductor and an insulating medium called the dielectric separating...
cable with a nominal impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
of 75 Ω
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
. Data is encoded in NRZ
Non-return-to-zero
In telecommunication, a non-return-to-zero line code is a binary code in which 1's are represented by one significant condition and 0's are represented by some other significant condition , with no other neutral or rest condition. The pulses have more energy than a RZ code...
format, and a linear feedback shift register
Linear feedback shift register
A linear feedback shift register is a shift register whose input bit is a linear function of its previous state.The most commonly used linear function of single bits is XOR...
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-clocking. Framing is done by detection of a special synchronization
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
pattern, which appears on the (unscrambled) serial digital signal to be a sequence of twenty ones followed by forty zeroes; this bit pattern is not legal anywhere else within the data payload.
The SMPTE 292M digital interface is known to be reliable (without use of repeaters) at cable lengths of 100 m or greater.
Data format
The corresponding parallel data formats, defined by SMPTE 274M, SMPTE 296M, and several other standards, are 20-bit standards; thus SMPTE 292M uses a 20-bit word size. Each 20-bit word consists of two 10-bit datums, coming from two logical (and parallel) data channels, one ("Y") which encodes luminanceLuminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
video samples, the other ("C") which encodes chrominance
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
information. The C channel is further time-multiplexed into two half-bandwidth channels, known as Cr (the "red color difference" channel), and Cb (the "blue color difference" channel). The nominal datarate of the Y channel is 75 Mwords/sec (1.5 Gbit/s divided by 20), and the nominal datarate of each of the two chroma channels is 37.5 Mwords/sec.
Video payload (as well as ancillary 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 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 (and before the start of the horizontal blankHorizontal blank
Horizontal blanking interval refers to a part of the process of displaying images on a computer monitor or television screen via raster scanning. CRT screens display images by moving beams of electrons very quickly back and forth from the left to right side of the screen...
ing 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.
Synchronization packets must occur simultaneously in both the Y and C datastreams.
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 (lines are deemed to start 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 progressive segmented frame
Progressive segmented frame
Progressive segmented Frame is a scheme designed to acquire, store, modify, and distribute progressive-scan video using interlaced equipment and media....
formats to indicate whether the line comes from the first or second field (or segment). In progressive scan
Progressive scan
Progressive scanning is a way of displaying, storing, or transmitting moving images in which all the lines of each frame are drawn in sequence...
formats, the F bit is always set to zero.
Other than the fact that synchronization packets occur in parallel in two datastreams (Y and C), their behavior is virtually identical to the packet types defined in CCIR 601
CCIR 601
ITU-R Recommendation BT.601, more commonly known by the abbreviations Rec. 601 or BT.601 is a standard published in 1982 by International Telecommunication Union - Radiocommunications sector for encoding interlaced analog video signals in digital video form...
and SMPTE 259M
SMPTE 259M
SMPTE 259M is a standard published by SMPTE which "... describes a 10-bit serial digital interface operating at 143/270/360 Mb/s." The goal of SMPTE 259M is to define a Serial Digital Interface , called SDI or SD-SDI....
, the digital interface commonly used for SDTV.
Line counter and CRC
To provide additional robustness, the four samples immediately following the EAV packets (but not the SAV packets) contain a cyclic redundancy checkCyclic redundancy check
A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
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 preceding line.
Ancillary data
Like SMPTE 259MSMPTE 259M
SMPTE 259M is a standard published by SMPTE which "... describes a 10-bit serial digital interface operating at 143/270/360 Mb/s." The goal of SMPTE 259M is to define a Serial Digital Interface , called SDI or SD-SDI....
, 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
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
, closed captions, timecode, and other sorts of metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
. 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.
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 ancillary 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
Vertical interval timecode
Vertical Interval TimeCode is a form of SMPTE timecode embedded as a pair of black-and-white bars in a video signal. These lines are typically inserted into the vertical blanking interval of the video signal...
or vertical interval test signals) without breaking the interface, but such usage is nonstandard (and ancillary 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.
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: Ancillary 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