SMPTE ST 305.2, Serial Data Transport Interface.

In 1080i and 1080p standards documents, samples are numbered with respect to 0H (unlike SD standards documents, where samples are numbered with respect to the zeroth active sample of the line).

Further developments doubled the data rate of the 1.5 Gb/s interface to about 3 Gb/s (sometimes termed 3G-SDI). That rate is sufficient to carry 1080i30 at 4:4:4:4 (that is, no chroma subsampling and an alpha component) or 1080p60 at 4:2:2.

Interfaces for compressed video

Compressed digital video interfaces are impractical in the studio owing to the diversity of compression systems, and because compressed interfaces would require decompression capabilities in signal processing and display equipment. Compressed 4:2:2 digital video studio equipment is usually interconnected through uncompressed SDI interfaces.

Compressed interfaces can be used to transfer video into nonlinear editing systems, and to “dub” (duplicate) between VTRs sharing the same compression system. Compressed video can be interfaced directly using serial data transport interface (SDTI), to be described in a moment. The DVB ASI interface is widely used to convey MPEG-2 transport streams in network or transmission applications (but not in production). The IEEE 1394/DV interface, sometimes called FireWire or i.LINK, is widely used in the consumer electronics arena, and is beginning to be deployed in broadcast applications.

SDTI

SMPTE has standardized a derivative of SDI, serial data transport interface (SDTI), that transmits arbitrary data packets in place of uncompressed active video. SDTI can be used to transport DV25, DV50, DV100, and Sony MPEG IMX compressed datastreams. Despite DV bitstreams being standardized, different manufacturers have chosen incompatible techniques to wrap their compressed video data into SDTI streams. This renders SDTI useful only for interconnection of equipment from a single manufacturer.

SMPTE RP 168, Definition of Vertical Interval Switching Point for Synchronous Video Switching.

FIFO: First in, first out.

Switching and mixing

Switching or editing between video sources – “cutting” – is done in the vertical interval, so that each frame of the resulting video remains intact, without any switching transients. When switching between two signals in a hardware switcher, if the output signal is to be made continuous across the instant of switching, the input signals must be synchronous – the 0V instants of both signals must match precisely in time. To prevent switching transients from disturbing vertical sync elements, switching is done somewhat later than 0V; see SMPTE RP 168.

Timing in digital facilities

Modern digital video equipment has, at each input, a buffer that functions as a FIFO. The buffer at each

input accommodates an advance of timing at that input (with respect to reference video) of up to several line times. Timing a digital facility involves advancing each signal source so that signals from all sources arrive in time at the inputs of the facility’s main switcher. This timing need not be exact; it suffices to guarantee that no buffer overruns or underruns. When a routing switcher switches among SDI streams, a timing error of several dozen samples is tolerable; downstream equipment will recover timing within one or two lines after the instant of switching.

When a studio needs to accommodate an asynchronous video input – one whose frame rate is within tolerance, but whose phase cannot be referenced to house sync, such as a satellite feed – then a framestore synchronizer is used. This device contains a frame of memory that functions as a FIFO buffer for video. An input signal with arbitrary timing is written into the memory with timing based upon its own sync elements. The synchronizer accepts a reference video signal; the memory is read out at rates locked to the sync elements of the reference video. (Provisions are made to adjust system phase – that is, the timing of the output signal with respect to the reference video.) An asynchronous signal is thereby delayed up to one frame time, perhaps even a little more, so as to match the local reference. The signal can then be used as if it were a local source.

Some video switchers incorporate digital video effects (DVE) capability; a DVE unit necessarily includes a framestore.

Studio video devices commonly incorporate framestores, and exhibit latency of a field, a frame, or more. Low-level timing of such equipment is accomplished by introducing time advance so that 0V appears at the correct instant. However, even if video content is timed correctly with respect to 0V, it may be late by

a frame, or in a very large facility, by several frames. Attention must be paid to delaying audio by a similar time interval, to avoid lip-sync problems.

ASI

ETSI EN 50083-9, Cable networks for television signals, sound signals and interactive services – Part 9: Interfaces for CATV/SMATV headends and similar professional equipment for DVB/MPEG-2 transport streams.Standards are not clear on whether transformer coupling is required or whether capacitive coupling suffices.

Some people write 8B/10B; however, the elements involved are bits, not bytes, so lowercase b is apt.

The synchronous serial interface (SSI) was standardized by SMPTE for the purpose of conveying MPEG transport streams between equipment, but SSI has largely fallen into disuse.

Within a broadcast facility, an MPEG-2 transport stream can be serialized onto a dedicated asynchronous serial interface (ASI). A serialized ASI stream for broadcast has a payload bit rate of around 20 Mb/s; however, the ASI interface bit rate is 270 Mb/s, chosen so that SDI distribution infrastructure can be used. The ASI interface uses BNC connectors and coaxial cable. ASI is polarity sensitive (unlike SDI), though modern ASI receivers typically detect and correct polarity inversion.

Although the SDI physical layer is used, the serialized ASI stream has no TRS codes and the interface does not use SDI scrambling. Instead, channel data is encoded according to the 8b/10b scheme borrowed from Fibre Channel standards. (ASI interface data rate is therefore at most 216 Mb/s.) An 8b/10b bitstream never has more than four consecutive 0s or 1s, so clock recovery is simple. An 8b/10b encoder minimizes low frequency (“DC”) content on the media.

Since the ASI payload rate is typically far lower than the channel capacity, stuffing codes are inserted to occupy idle time. Stuffing codes – Fibre Channel comma codes, denoted K28.5 – are inserted either at the earliest opportunity (bytewise, “spaced byte mode”), or at the completion of the current packet (packetwise, “burst mode”). MPEG packets are separated by at least two comma codes.

It is increasingly common to convey transport streams using IP protocols across Ethernet.

Summary of digital interfaces

Table 38.7 summarizes SD and HD digital interface standards.