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.
Standard-definition Serial Digital Interface (SD-SDI) is a serial link that can transmit uncompressed digital video or 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.