Video interfaces

Professional digital interfaces continue to evolve in parallel with higher-resolution professional and consumer video formats. The once simple arena of analog connections now encompasses both compressed and uncompressed serial digital interfaces (SDI), with ever-higher transfer speeds. (See Figure 1.)

The SDI is a serial link originally standardized by SMPTE 259M. It is used to transmit uncompressed digital video over 75Ω coaxial cable and has a maximum data rate of 360Mb/s. HD-SDI is the second-generation version of SDI and allows transmission of HD (1080i and 720p) signals over the same 75Ω cables as SD-SDI. It handles rates up to 1.485Gb/s, and is defined by SMPTE 292M and ITU-R BT.656. Dual-link HD-SDI, defined by SMPTE 372M, uses two coaxial cables to provide up to 2.97Gb/s throughput while supporting up to 1080p resolution.

3Gig-SDI is the latest version of SDI and allows transmission of 1080p60 HD signals (with a 4:2:2 sampling structure at 10 bits per sample) over a single 75Ω coax cable. Defined by SMPTE 424M, the single-link studio interface can reach a maximum bit rate of 2.97Gb/s and can be thought of as a multiplexed version of HD-SDI. A companion standard, SMPTE 425M, specifies the mapping of different video signals to the 3G physical interface, as well as the supported video formats, which are listed in Table 1 on page 26. A single, high bit rate video signal can be carried over the 3Gig-SDI link and is referred to by some users as Level A, for direct image format mapping. Alternatively, two lower bit rate signals could be carried, such as two SMPTE 292 HD-SDI, and referred to as Level B. However, these specific levels are not strictly a part of the standard.

This single-coaxial-cable solution, however, is currently limited to a cable length of about 90m, so SMPTE 435-3-2007 defines a 10.692Gb/s optical fiber interface for longer runs. Nonetheless, a demonstration in February at the Hollywood Post Alliance showed a 140m length of coax using 3Gig-SDI. This suggests the potential for new circuit designs to advance the state of the art.

An alternative high-speed solution, proposed by the BBC, is to apply a mild, mezzanine compression to the HD-SDI signal, compressing a 1080p60 signal by a factor of about 2.5:1, so that it can be mapped into a legacy 1.5Gb/s HD-SDI infrastructure. This approach has led to the development of a related SMPTE standard, currently in the ballot process (at press time). The compression is essentially artifact-free, but it introduces a small delay of a few video lines and requires a simple encoder and a decoder at each interface point.

An interesting feature of the proposed mezzanine codec is that the two most-significant bits of the 10-bit output can be used to carry a compatible interlaced version of the input picture. In this way, the compressed signal can be viewed as if it were standard 1080i video, making the content recognizable, although corrupted by noise. This makes it possible to monitor the signal (without a decoder) for the purposes of identification, and to provide a confidence check that the encoder is working. With full decoding, of course, the noise is not present.

Crossover applications

While the aforementioned interfaces are used almost exclusively in professional equipment, several interfaces that originated in consumer equipment are beginning to find their way into broadcast and production equipment. While these are not likely to replace SDI and similar interfaces, their growing popularity in display products are making them appear on an increasing number of professional displays.

IEEE 1394 (sometimes trademarked as i.Link or Firewire) 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. Designed as a networked interface, IEEE 1394 is not limited to carrying video, and had been the interface of choice for DV-based cameras and prosumer HD equipment. IEEE 1394 now appears to be losing traction, however, to HDMI (High Definition Multimedia Interface) and USB.

HDMI, Digital Visual Interface (DVI) and DisplayPort are becoming de-facto display standards in consumer electronics and appear to be replacing IEEE 1394 for short-distance interconnects. HDMI1.3a supports a bandwidth of up to 10.2Gb/s, and optionally supports Deep Color with 30-bit, 36-bit — and 48-bit Y'CBCR , xvYCC and sRGB — signals. HDMI has gotten a strong foothold in the consumer market, and this is a big plus, considering the long lead times of CE companies investing in new technologies. And while HDMI runs are usually limited to about 15m, twisted-pair (Cat 5) can extend this to about 50m, and fiber can push this to over 330m.

DVI was developed to support high-resolution PC monitors. A single DVI link consists of four twisted pairs of wires, each carrying one red, green, blue and clock signal, supporting up to 24 bits per pixel. In addition, DVI is probably the only popular standard that optionally supports both analog and digital signals over the same connector.

With a single DVI link, the largest resolution possible at 60Hz is 2.75 megapixels. Higher resolutions are possible by using a dual-DVI link (over a single cable), with alternate pixels transmitted on each link. The maximum bit rate in single link mode is 3.96Gb/s and in dual link mode is 7.92Gb/s. A single link can therefore carry a 1920 × 1080p60 signal, and a dual link can support a 2560 × 1600p60 (WQXGA) display. Most high-end PC graphics boards supply one or more single or dual-link DVI connections.

DisplayPort is a relatively recent license-free digital audio/video interconnect, proposed by VESA to succeed DVI; it's primarily intended to be used between a computer and its display monitor. Because it's based on a micro-packet protocol, DisplayPort may have an advantage over other interfaces by easing future expansion of the standard. The interface consists of a unidirectional main link for connecting A/V streams from source devices to sink devices, and a half-duplex bidirectional auxiliary channel used for realizing plug-and-play features such as equipment control. The main link may have one, two or four differential signal pairs (or lanes), offering a total raw capacity of up to 10.8Gb/s. Computer giant Dell is a strong DisplayPort supporter, and rumors have it that the company will include the interface on all products within two years.

Traditional USB 2.0 (high-speed USB) has already become the interface of choice for many computer peripherals, including storage media. It supports a maximum transfer rate of 480Mb/s. The new USB 3.0 specification (Super Speed) has already been completed, and USB 3.0 products are expected in late 2009. This version will increase transfer rates up to 5Gb/s.

Where's it all going?

Most production and broadcast video systems now include SD/HD-SDI, which supports up to 720p/1080i (uncompressed), and marketplace evidence suggests that video equipment is transitioning from HD-SDI to 3Gig-SDI. With 1080p video becoming the dominant video format in the future (and possibly the current de-facto production standard), transmission speed will be the bottleneck in any video interface. Research over the last two years has shown that 1080p can be done at an equivalent bit rate to current 1080i broadcasts, provided MPEG-4 (H.264/AVC) is used.

Despite the proliferation of displays with HDMI or DVI interfaces, studios will still need to test video quality on legacy analog TVs and monitors. So, don't count out analog component video interfaces — yet.

Aldo Cugnini is a consultant in the digital television industry.

Picture format Signal format Progressive frame rate Interlaced field rate 1920 × 1080 10-bit
4:2:2 (Y'C'BC'R) 60, 59.94, 50 10-bit*
4:4:4 (R'G'B')
4:4:4:4 (R'G'B' + A)
4:4:4 (Y'C'BC'R)
4:4:4:4 (Y'C'BC'R + A) 30, 29.97, 25, 24, 23.98 60, 59.94, 50 12-bit
4:4:4 (R'G'B')
4:4:4 (Y'C'BC'R)
4:2:2 (Y'C'BC'R)* 1280 × 720 10-bit
4:4:4 (Y'C'BC'R)
4:4:4:4 (Y'C'BC'R + A)
4:4:4 (R'G'B')
4:4:4:4 (R'G'B' + A) 60, 59.94, 50, 30, 29.97, 25, 24, 23.98 2048 × 1080
(D-Cinema) 12-bit
4:4:4 (X'Y'Z')* 24 * Includes PsF (Progressive segmented Frame) format

Table 1. 3Gig-SDI supported video formats, per SMPTE-425M