SDI is a one-way, multiplexed protocol designed to carry the highest quality video, audio and metadata over a coaxial cable or fiber. SDI is a professional video signal that is preferred in production environments because of its longer range (up to 300 feet) and reliability. If you’re in an environment in which your cable could be unplugged or tripped over (which should also be taped down anyway), SDI connections are ideal.
SDI is typically used to connect together different pieces of TV studio equipment such as recorders, monitors, PCs and vision mixers. SDI and HD-SDI are usually available only in professional video equipment. 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.
While the adoption of IP is clearly beginning to take off in many parts of the world, SDI is still heavily relied upon in thousands of existing broadcast products infrastructures—largely for the simplicity, security, reliability and familiarity the proven technology provides. SDI will remain a major part of video production infrastructures for several years to come, maybe a decade (broadcast equipment have life spans of 7 to 10 years).
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 standard definition TV. Nowadays various versions of the serial digital interface support numerous video formats.
With SDI, delay is kept to an absolute minimum as SDI is a synchronous distribution system meaning signals have the least delay possible when sent from the camera to the vision switcher. The transmission time through an SDI interface is fixed due to the synchronous
nature of the system. The SDI interface is unidirectional so that the signal source sends the signal to the line and the signal receiver receives it. There is no communications needed for the other direction.
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 traditionally used in analog video setups. Many old good quality 75 ohm video cabling can be used for SDI. Care needs to be take there are no impedance mismatched on the wiring, for example 50 ohms BNC connectors instead of correct 75 ohm BNC connectors.
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 300 meters (980 ft) without use of repeaters, but shorter lengths are preferred.
SDI has inherent near instantaneous plug-and-play ability. The learning curve for SDI is relatively low as broadcasters are familiar with using 10-bit SDI systems for three decades.
Uncompressed digital component signals are transmitted. Several color encodings are possible in the serial digital interface. The default (and most common case) is 10-bit linearly sampled video data encoded as 4:2:2 YCbCr (digital representation of the YPbPr colorspace). 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), and the two chrominance channels (Cb and Cr) are sub-sampled horizontally, and encoded at half bandwidth (6.75 MHz). The native resolution for all video components Y, Cb, Cr) is 10 bits.
The SDI interface is self-synchronizing and self-clocking. At low level the data is encoded in NRZI format where 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. A standard signal level 0.800 volt peak-to-peak is used for digital data transmission. Coaxial variants of the specification range in length but are typically less than 300 meters (980 ft). Fiber optic variants of the specification such as 297M allow for long-distance transmission limited only by maximum fiber length or repeaters.
Video payload (as well as ancillary data payload) may use any 10-bit word in the range 4 to 1,019 (00416 to 3FB16) inclusive; the values 0–3 and 1,020–1,023 (3FC16–3FF16) are reserved and may not appear anywhere in the payload. These reserved words are used for both for Synchronization packets and for Ancillary data headers.
Like SMPTE 259M, SMPTE 292M supports the SMPTE 291M standard for ancillary data that can be used for things such as embedded audio, closed captions, timecode, and other sorts of metadata. SD resolution SDI interfaces provide for 16 channels of embedded audio based on SMPTE 272M standard. Typically, 48 kHz, 24-bit (20-bit in SD, but extendable to 24 bit) PCM audio is stored, in a manner directly compatible with the AES3 digital audio interface. Audio data placed in the (horizontal) blanking periods where the SDI signal does not carry any useful video data.
The SDTV resolution SDI was one widely used for TV production, but it’s usage has dropped because nowadays more and more TV programs are produced in HDTV resolutions. SDI has been 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.
To be to transpor HDTV resolutions (720p,1080i) 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. SMPTE 292 was designed to accommodate 1080i HD television at 16:9 aspect ratio with bit rate of 1.485Gbits/s for PAL countries and 1.485/1.001Gbits/s for NTSC countries. SMPTE 296 further added support for 720p HD.
SMPTE 292M defines HD-SDI and is based on the constructs of SMPTE 259M, which defines standard definition serial digital. SMPTE 274M defines dimensions for all timing and digital video data description for the HD video formats. Moreover, it addresses image structure, colorimetry, raster structure, digital presentation, timing references, analog sync, and an analog interface.
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. At the point of creation, HD video is routed within the production environment uncompressed via HD-SDI at the 1.485 Gbps rate. After compression, the high definition video rate plummets to 19.4 Mbps (about 77:1)
Both the SDI and HD-SDI physical topologies are essentially identical except for operating rate. The HD bitrates have a shorter maximum run length though, typically 100 meters (330 ft). The range of operation for an HD-SDI receiver is specified in SMPTE 292M to at least -20 dB at one-half the data clock rate, or about 743 MHz. Therefore, a standard level 0.800 volt peak-to-peak digital transmission may be attenuated to as low as 0.080 volt, or 80 millivolts, while performing reliably. To perform a cable loss calculation, the designer should look for the attenuation in dB at 743 MHz, or a frequency very close to that value, on the cable specification loss chart. The SMPTE recommends the designer factor in about 10% less cable than the calculated run length so as to build in a safety margin for reliable operation.
The highest frequency required to send a series of one’s and zero’s for an HD SDI bit rate of 1.485 Giga-Bits, is 742.5MHz. However wider bandwidths must be used in a transmission system so that the phase shift at 743 MHz is held to a low level to reduce data bit errors. The frequencies below 742.5 MHz contains the actual picture information you wish to deliver, and any frequencies above 742.5 MHz are needed to keep the signal waveform square like enough that it can be handled properly by the receiver circuitry. The SDI signal standard indicates that signal transition should occur in no less than 270ps, that’s (270 pico seconds) and no faster. That equals a maximum frequency of 1.852GHz (Gigi-Hertz). This is the highest frequency that must pass through the transmission medium to deliver an SDI camera signal in an unchanged condition, without distortion.
The interface circuits that carry the HD-SDI signal are typically AC coupled with a suitable capacitor. Like SDI, HD-SDI employs a signal coding method called NRZI, Non-Return to Zero Inverted. NRZI is a coding method that facilitates recovery of the clock from the actual data transmission. NRZI coding minimizes residual DC component on the signal as well. In the high definition serial digital interface (and in dual-link HD), additional check words are provided to increase the robustness of the interface.
Due to the construction of this coding scheme, the bit rate is equivalent to the frequency component in MHz. In other words, 1.485 Gbps is equal to 1.485 Gigahertz. One HD-SDI feed operates at 1.485 Gbps over low-loss RG6-style video grade coaxial cable. Due considerable higher frequencies than SDI, the HD-SDI demand even better cabling and more accurately right impedance connectors to work well. Care should be taken to guarantee that only 75 Ohm cable and connectors are used when installing any HD CCTV camera. Beware of exceedingly small diameter cables of unknown origin, it has been my experience that these small diameter cables often do not conform to the 75 Ohm standard. Any mis-termination of the cable or connectors can cause “Return Loss” that will degrade the camera signal at the receiver.
HD-SDI compares favorably to other digital transports when considering its robustness for high data rate handling capability over hundreds of feet of coaxial cable. Passive HD-SDI Splitters can be used to split a single HD digital video source into two signals for distribution to several equipment when cable lengths are short. With fiber optic transmission technology, run distance is virtually limitless, depending more on cost than technical issues.
All digital television signals, including high definition rates up to 1920 x 1080 at 30 frames interlaced, are managed successfully over the HD-SDI. The component format allows transmission of HD because the luminance (Y channel) is the only full bandwidth channel. Several color encodings are possible in the serial digital interface. The default (and most common case) is 10-bit linearly sampled video data encoded as 4:2:2 YCbCr. (YCbCr is a digital representation of the YPbPr colorspace). The luminance channel (Y) is encoded at full bandwidth (~75 MHz), and the two chrominance channels (Cb and Cr) are sub-sampled horizontally, and encoded at half bandwidth (37.5 MHz).
Other color encodings are possible by treating the interface as a generic 10-bit data channel. SMPTE 348M describes a variation of HD-SDI called the high definition serial transport interface. HD-SDTI is a protocol whereby data other than video/audio may be transported using the HD-SDI constructs and a portion of the hardware utilized by HD-SDI. Utilization of HD-SDTI for data other than high definition video requires the proper custom formatting and de-formatting hardware for data loading and recovery.
HD-SDI has its variants which can transmit full-bandwidth, 10-bit RGB and an alpha channel. SMPTE 372M standardizes high definition serial digital for full bandwidth transmission; i.e. 4:4:4:4 sampling. Operating at dual-rate (two times the 1.485 Gbps rate) means that twice the information may be transmitted. At nearly 3 Gbps, component video can be accommodated without band-limiting the sample rate for chroma information. The fourth ’4′ in the sampling structure represents the ability to include an ‘alpha’ channel along with video data. The 4:4:4 color sub-sampling gives much higher color accuracy than the familiar 4:2:2 color sub-sampled system.
For audio HD-SDI uses the SMPTE 299M standard. The signal may contain up to sixteen audio channels (8 pairs) are embedded along with the video. Typically, 48 kHz, 24-bit PCM audio is stored, in a manner directly compatible with the AES3 digital audio interface. The audio data is placed in the (horizontal) blanking periods.
When the HD-SDI was not enough for even higher resolutions (like 1080p), first two parallel cables were used, but soon they could be replaced with one 3G-SDI cable. 3G-SDI (standardized in SMPTE 424M) consists of a single 2.970 Gbit/s serial link that allows applications such as digital cinema or HDTV 1080P. Moving to progressive 1080p formats needed the doubling of the bit rate from 1.485Gbits/s to 2.97Gbit/s. SMPTE 424M provided the specification for 3G-SDI to facilitate formats such as 1080p50/59. At nearly 3 Gbps, component video can be accommodated without band-limiting the sample rate for chroma information, meaning 4:4:4:4 sampling of component video or 10-bit RGB and an alpha channel. SMPTE 372M supports a wide variety of component formats and is but one of the stepping stones toward wideband digital cinema recording and support. Because faster data rate and higher frequencies used, 3G-SDI demands even more from video cabling quality than HD-SDI and SDI.
When HD-SDI and 3G-SDI use is limited to the professional devices, and consumer devices use interfaces like DVI and HDMI to transport HD video signals. RGB 8-bit image data (via 10-bit symbols) is transferred over the DVI using three digital data lines and one clock line. The High-Definition Multimedia Interface (HDMI®) combines uncompressed HD video, multichannel audio, and intelligent format/command data in a single cable. The HDMI basic technology can be though as expanded version of DVI. There are nowadays available quite inexpensive converters that can convert HDMI signal to HD-SDI and HD-SDI back to HDMI. When running HDMI signal over HD-SDI with converter, you need to keep in mind that SDI does not support HDCP (High-Bandwith Digital Content Protection) that is used for protecting HD video signal coming from Blue Ray or set-top-box or computer video card playing back HD video. Typically this means that you can’t usually transport the signal from those sources though SDI with simple converters. Several professional video and HD-video capable DSLR cameras and all uncompressed video capable consumer cameras use the HDMI interface, often called clean HDMI. The computer video output typically is clear HDMI when viewing normal desktop, but usually turns on HDCP when playing back HD video. With HDMI to SDI converters you usually need to keep in mind that SDI support only a small set of video resolutions (read more HDMI to SDI, SDI DA, SDI to HDMI. Pros: professional grade? Cons: potential color space (RGB vs YUV) or resolution mismatches).
4K and 8K interfaces
6G-SDI and 12G-SDI standards for higher resolutions were published on March 19, 2015. 6G became available in three versions; single link, dual link, and quad link. Single link (5.940Gbit/sec) supports distribution of 2160-line (4K) up to
30fps and 1080-line up to 60fps. Dual link (11.88 Gbit/ sec) can distribute 4Kp60 (4:2:2). Quad link (23.76Gbits/sec) provides 4Kp60 (4:4:4), 4Kp120 (4:2:2), and 8Kp30 (4:2:2). Broadcasters looking to move to 4K have up until recently relied on quad-link 3G-SDI, as the bit rate for 4Kp60 is just under 12Gb/s.
12G-SDI is capable of transporting 8Kp video with fabulously low latency. SMPTE released ST-2082 in 2015 to provide adata rate of 11.88Gbits/s and 11.88/1.001Gbits/sec. Now known as 12G-SDI, three versions are available; single- , dual- and quad-link. The single link can provide 4K 4:4:4 at 30fps or 4:2:2 at 60fps. Dual link (23.76Gbits/sec) provides 8K (4:2:2) at 30fps and as we move to quad link (47.52GBits/sec) 8K (4:2:2) at 60fps becomes available along with 4K at 120fps. ST2082-10 (single link) supports Mode 1 (4K/UHD up to 60fps) and Mode 2 (1080p 4:4:4 10 and 12-bit up to 120fps)
Although broadcasters are familiar with using 10-bit SDI systems, 12G-SDI is providing future proofing through the provision for 12-bit samples. As well as being backwards compatible and maintaining 10-bit samples, ST-2082 makes provision for 12-bit samples to enhance HDR images.
Key to the longevity of SDI is its inherent backwards compatibility. If you already have a 3G installation and then upgrade a part to be 12G, then any 3G signals routed through the 12G infrastructure should work reliably.
Serial Digital Interface (Wikipedia)
SDI or HDMI? Yes!
HDMI vs. SDI Video Connections: What’s the Difference?
HDMI over twisted pair vs. convert to HD-SDI
Despite Rise Of IP, SDI Is Alive And Well
Core Insights: Advances In 12G-SDI
HD-SDI: More Possibilities than Just Television
FREQUENCY CONTENT OF HD SDI CAMERA SIGNALS
Understanding blocking capacitor effects