HD-SDI camera

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HD-SDI camera

What is HD-SDI?



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 colour 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. As of August 2014, 6G-SDI and 12G-SDI products are already in the market, although their corresponding standards are still in proposal phase.

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. Coaxial variants of the specification range in length but are typically less than 300 meters. Fibber optic variants of the specification such as 297M allow for long-distance transmission limited only by maximum fibber 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, 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 analogue 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 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

Standard

Name

Introduced

Bitrates

Example video formats

SMPTE 259M

SD-SDI

1989

270 Mbit/s, 360 Mbit/s, 143 Mbit/s, and 177 Mbit/s

480i, 576i

SMPTE 344M

ED-SDI

540 Mbit/s

480p, 576p

SMPTE 292M

HD-SDI

1998[

1.485 Gbit/s, and 1.485/1.001 Gbit/s

720p, 1080i

SMPTE 372M

Dual Link HD-SDI

2002

2.970 Gbit/s, and 2.970/1.001 Gbit/s

1080p

SMPTE 424M

3G-SDI

2006

2.970 Gbit/s, and 2.970/1.001 Gbit/s

1080p

SMPTE ST-2081*

6G UHD-SDI

6 Gbit/s

4Kp30

SMPTE ST-2082*

12G UHD-SDI

12 Gbit/s

4Kp60

* SMPTE standards for ultra-HD links are still being drafted, as of August 2014. Working group 32NF-70 is in the process of developing standards ST-2081, ST-2082, and ST-2083 for 6Gbit/s, 12Gbit/s and 24Gbit/s SDI respectively.

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.


HD-SD and IP






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