Video distribution was historically a simple concept involving analog distribution amplifiers to send live video to multiple devices. In the broadest sense, distribution today typically entails the complicated transport of digital content from one place to another. Broadcast technology organizations like DVB and ATSC specifically refer to distribution as one part of the media chain that includes contribution, distribution and transmission. Contribution describes how to aggregate content, distribution describes how to transport it in an extended video infrastructure, and transmission describes how to send it to consumer receivers.
Figure 1. Broadcasters have a range of video distribution technologies at their disposal to suit different requirements.
This article will extend the definition of “distribution” to include ways of moving content within destinations like homes, businesses and campuses, because the associated technologies bring value to broadcast operations. (See Figure 1.)
In the plant
Whereas the handling of video had at one time been divided between “live” and “recorded” media, the infrastructure to transport content now includes that of handling file-based content — and that means that the interfaces between sources, storage and destinations are becoming increasingly IT-based.
Twisted-pair wiring is now becoming a viable alternative to the widely-used coaxial cable of the video industry. Baluns (balanced-to-unbalanced transformers) enable operators to distribute HD video over an existing Cat 6/7 twisted-pair infrastructure, in either analog or digital form. Full analog-SD video can be provided at up to 1970ft or a distsance of 1.24mi, and up to 6562ft with active baluns; power and bidirectional signaling can also be carried over the twisted pair. Digital video can also be carried over twisted-pair cable such as Cat 5e, ordinarily used for Ethernet data networks. Passive baluns can support distances of up to 150ft for 3G-SDI signals.
Twisted-pair cables are classified by the ISO/IEC 11801 standard depending on their electrical performance characteristics, which include various factors that depend on the transmission frequency, such as insertion loss, inter-pair cross talk and return loss. Class D carries up to 100MHz using Cat 5 cable; Class E runs up to 250MHz using Cat 6, and so forth, up to Class-FA, supporting up to 1000MHz over Cat 7A.
Cat 5 uses the EIA-T568 standard for wiring, which specifies up to four pairs per cable. T-568 specifies two different color-coding conventions, for compatibility with different existing and legacy communications equipment. In its most basic form, a Cat 5 interconnect consists of two twisted pairs of 100V characteristic impedance, with each pair carrying a unidirectional signal downstream or upstream. When assembling connectors, correct pairing must be observed, or there will be a large cross-talk component induced between the downstream and upstream signals — as can happen with inexperienced cable installers.
One confusion arising with Ethernet data cables has come from the need for “cross-over” connections, providing a connection between “source” (also called Data Communications Equipment or DCE) and “sink” components. As networks have become more complex, however, devices have increasingly adopted automatic cross-over detection (also called automatic MDI/MDI-X configuration), so that the need for cross-over cables has been reduced.
10BASE-T and 100BASE-TX Ethernet (which support data rates of 10Mb/s and 100Mb/s, respectively) each use two pairs of Cat 5/6 cable, which allows carrying two complete circuits in one cable with appropriate adapters. GigE (or 1000BASE-T), which operates at 1Gb/s, and 10GBASE-T (10Gb/s) use all four pairs. The maximum length for an Ethernet cable in a network environment is limited to 328ft.
Locations outside the plant
Microwave and satellite links continue to be the preferred method of distributing mission-critical video in the broadcast industry.
Digital video-over-fiber is typically based on the SMPTE 297 standard, using Wavelength-Division Multiplexing (WDM), which uses different wavelengths (colors) of light to carry multiple signals. SDI per SMPTE 424/292/259, (3G/HD/SD) with embedded audio and data, as well as DVB-ASI, can be carried over one single-mode fiber. Three or more SDI channels can be transported over a minimum distance of 6.2mi, compared with a maximum distance of about 2045ft with SD-SDI or 570ft with 3G-SDI signals.
Although video streaming over the Internet to viewers is appropriately the subject of a separate discussion, the use of the Internet to distribute file-based video to affiliate stations, as well as storage archives, is becoming practical and merits consideration by planners given the explosive amount of content now being generated. The newly developed MPEG HEVC can form part of such a solution, thanks to a lossless coding mode that provides perfect fidelity with an average bit-rate reduction of more than 13 percent. Significantly outperforming existing lossless compression solutions such as JPEG2000, the artifact-free performance is achieved by bypassing transform, quantization and in-loop filters, while keeping the lossless entropy-coding elements. Combining this with an appropriate encryption wrapper makes Internet video distribution attractive and secure.
Video in the home
Although many video technologies have been developed specifically for consumer use in the home, these same technologies can make production and monitoring facilities more economical to build and maintain. HDBASE-T is a consumer-facing standard that uses Cat 5E/6 cable to transmit 10Mb/s video, plus audio, two-way control signals and 100W of power, at up to 328ft. With a total capacity of 10.2Gb/s, HDBASE-T provides an extension to HDMI, with Ultra HD video at up to 4K resolution, and can also support multiple simultaneous 100BASE-T Ethernet users. A distance of up to 2624ft can be supported with repeaters.
Optical fiber and converter devices are now available to consumers, too, capable of relaying HD video, audio and control signals through one fiber cable at distances up to 1000ft Essentially HDMI extenders, these products use no compression, and support HDCP content protection.
When transporting video to displays, several wireless technologies are now becoming practical, including Wireless USB, Wireless HD (WiHD) and Wireless Home Digital Interface (WHDI). Although some of these are intended to work over a relatively short distance of about 10ft for near-lossless video, others, like WiHD, can support uncompressed digital transmission of HD video and audio and data signals over DisplayPort or HDMI interfaces, with theoretical data rates as high as 25Gb/s at distances up to 30ft.WiHD is reported to support 1080p/60Hz HD with deep-color video content at a distance of 108ft.
Another cross-over application is that for professional A/V and digital signage. Low-cost fiber-optic extenders and matrix switchers are now available that can transport SDI/HDMI/DisplayPort/VGA video, audio, RS-232, IR-remote and USB signals at a distance of more than 6600ft for multi-mode operation, and even up to 18.75mi for single mode.
Web integration
Management, delivery and tracking of digital content now require extensive integration of repurposed content across multiple channels. Digital distribution now extends to the cloud for production, affiliate management and consumption. Broadcasters must continually adapt their infrastructure and workflow to keep pace.
—Aldo Cugnini is a consultant in the digital television industry.
