Digital Video and Audio Interfaces

Professional video interfaces are undergoing a change, in part due to the age of the initial digital systems, and also because of the emergence of high-performance interconnects for consumer use. First, let’s summarize the existing solutions for high-bandwidth audio/video transfer.

Existing Interfaces
Standard-definition Serial Digital Interface (SD-SDI) is a serial link that can transmit uncompressed digital video and audio (usually up to eight channels) over 75Ω coaxial cable. Without repeaters, rates of up to 270Mb/s over 1000ft are customarily used. Digital Video Broadcasting, Asynchronous Serial Interface (DVB-ASI) was defined for the transmission of MPEG Transport Streams, and is electrically similar to SDI, with a data rate of 270Mb/s.

HD-SDI is the second-generation version of SDI and allows the transmission of HD (1080i and 720p) signals over the same 75Ω cables as SD-SDI. It can handle rates up to 1.485Gb/s. A dual-link HD-SDI provides up to 2.97Gb/s and supports 1080p resolution, but it is being replaced by the single-link 3G-SDI, the third-generation version of SDI that can reach a maximum bit rate of 2.97Gb/s over a 75Ω coax cable.

Consumer electronics are catching up with pro interfaces. Although driven from the non-professional side, evolving consumer electronics interfaces are affecting pro equipment, especially displays. The legacy analog VGA and hybrid analog/digital DVI interfaces used to interconnect PCs with displays could be obsolete by 2015, as chipset manufacturers have announced their intent to withdraw support by that year, and that means PC motherboard manufacturers will likely pull the functions from their new designs. Replacing them on PCs, DVDs and other consumer video devices are HDMI and DisplayPort.

HDMI 1.4a has a throughput of 8.2Gb/s, allowing it to carry up to 4096p24 video (or 1920p60) at 24 bits per pixel, as well as various 3-D formats, eight channels of audio, Consumer Electronics Control (CEC) and High-bandwidth Digital Content Protection (HDCP). DisplayPort 1.2 supports up to 8.6Gb/s, and thus can carry payloads similar to that of HDMI. Functionally, the interfaces differ in the way they handle video and audio, with HDMI using a raster-based protocol, and DisplayPort transporting content in packets. From a market standpoint, the main difference between HDMI and DisplayPort is that the first was designed primarily as a digital TV interface, while the second was intended as a PC-centric interface. The two interfaces also have license and royalty differences.

The USB 3.0 (also called Super Speed USB) specification is used almost exclusively as a PC (or tablet) interface to support peripherals. It supports transfer rates up to 5Gb/s, over a maximum distance of about 16ft. As a data-transfer protocol, USB is payload-agnostic, so the transfer of audio and video is essentially limited to the latency characteristics of the interface. IEEE 1394 was originally designed to support bit rates of up to 400Mb/s, but newer versions of the standard support speeds as high as 3.2Gb/s.

Thunderbolt is a newer 10Gb/s bidirectional serial interface. Developed by Apple/Intel, it provides full-bandwidth data and video transfer between a PC and peripheral and display devices, up to a distance of 10ft. Serving as the hardware layer below the PCI (bus used inside PCs) and DisplayPort stacks, the product utilizes a time-synchronization protocol that allows up to seven daisy-chained Thunderbolt products to synchronize their time within 8ns of each other. Like USB, Thunderbolt’s key differentiator from other display-interface technologies is its capability to supply power to the peripheral, at up to 10W, superseding USB 3.0’s 4.5W capacity.

HDBaseT is a recent standard that uses CAT-5e Ethernet cable to transmit 10Mb/s video and two-way control signals and power, with enough capacity for additional simultaneous 100BaseT Ethernet uses. The great attraction to this interface is that it can be deployed over existing Ethernet infrastructures, greatly reducing implementation cost. As with other data-based interfaces, the video can be conventional uncompressed HD, 3-D, 4K or high frame rate. The maximum specified distance for HDBaseT is 328ft, which can be extended through 8 hops, and the standard supports carrying up to 100W of power.

Wireless Video Products
There are several wireless standards that are vying for use driving displays. Wireless Home Digital Interface (WHDI) is an interface that uses the same 5GHz band as Wi-Fi, and is designed to transmit uncompressed HD video at data rates of up to 3Gb/s in a 40MHz channel. The range is said to be greater than 100ft, with a latency of less than 1ms.

WiGig (by the Wireless Gigabit Alliance) is a specification based on 802.11 that supports generic data transmission rates up to 7Gb/s. A different approach is being taken by WirelessHD, a specification that defines a wireless protocol that enables consumer devices to create a wireless video area network (WVAN) that can stream uncompressed audio and video up to Quad Full HD (QFHD, or4K) resolution, at 48-bit color and 240Hz refresh rates, with support for 3-D video formats. The specification, which is based on 802.15, supports data transmission rates at 10Gb/s to 28Gb/s.

The Wi-Fi Alliance has also announced a certification program, called Miracast, through which certified devices can make use of an existing Wi-Fi connection to deliver audio and video content from one device to another, without cables or a connection to an existing Wi-Fi network.

In another industry development, MHL is being used to connect tablets and smartphones to displays. MHL defines an HD video and digital audio interface optimized for connecting mobile phones and portable devices to HDTVs, displays and other home entertainment products. MHL features a single cable with a five-pin interface that is able to support up to 1080p60 HD video and 192kHz digital 7.1 channel audio, as well as simultaneously providing control and power (2.5W) to the mobile device.

Because MHL does not specify a unique connector, various mechanical interfaces have emerged, including five-pin and 11-pin MHL-USB connectors. MHL fully supports the HDCP specification (used elsewhere on DVI and HDMI interfaces) for the safeguarding of digital motion pictures, television programs and audio against unauthorized access and copying.

Maintaining High-Speed Networks
High-speed networks are challenging to maintain. When any of these high-speed interfaces are combined with long runs of cable, performance will degrade, primarily from inter-symbol interference caused by cable-based dispersion of different signal frequencies, as well as jitter caused by processing equipment, as shown in the figure below. The result will be an increase in error rate at the receiving end.

Binary digital signal with interference

To minimize this, video plants should be designed and maintained with equipment having low jitter and cable runs having the lowest length necessary, with repeaters used for lengths nearing maximum specifications. Adhering to these precautions will result in reliable operations.

By Aldo Cugnini, Broadcast Engineering