A video processing pipeline encompasses end-to-end audio and video processing, commencing with A/V capture and culminating at a video display. As shown in Figure 1, this pipeline typically includes A/V capture, post processing, storage, distribution and playback. These fundamental steps, in existence since the early days of video processing, form the backbone of this pipeline. On the other hand, the media, methods and interfaces associated with each step have changed over the years.
Digital video interfaces
Digital formats have now all but replaced analog, and hard disk drives and other digital storage media have replaced analog videotapes. Most video post processing can be handled on a standard PC, and the ubiquitous Internet has become as important a video distribution medium as traditional off-the-air, cable and satellite broadcasting.
Along with their numerous benefits, digital TV and digital cinema also present new challenges throughout the video processing pipeline, and increased video resolution dictates higher bit-rate requirements. Digital rights management (DRM) intentionally imposes limitations on digital content usage via copy protection or access control at all stages of the processing pipeline. Modern video equipment needs to support both the new digital, as well as legacy analog, interfaces. Professional and consumer digital audio and video formats afford higher resolutions, wide acceptance and convenience. It is only natural to see the underlying digital interfaces evolve in parallel with these formats. Table 1 on page 22 summarizes the key features of some of the common digital audio and digital video interfaces used in professional or consumer equipment.
SD-SDI is a serial link specified by the ITU-R BT.656 and SMPTE 259M standards. It can be used to transmit uncompressed digital video or audio (usually up to eight channels) over 75Ω coaxial cable. Without repeaters, rates of up to 270Mb/s over 300m are serviceable. Defined by the European Community standard EN50083-9, DVB-ASI (Digital Video Broadcasting, Asynchronous Serial Interface) 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 and is defined by SMPTE 292M. A dual-link HD-SDI (defined by SMPTE 372M) provides up to 2.97Gb/s and supports 1080p resolutions, but is expected to be replaced by the single-link 3G-SDI. 3G-SDI is the third-generation version of SDI and allows the transmission of HD 1080p signals over a 75Ω coax cable. It is defined by SMPTE 424M and can reach a maximum bit rate of 2.97Gb/s.
Consumer electronics equipment continues to push the envelope of data rates and usability as well. Traditional USB 2.0 (High Speed) is already enjoying widespread use on most computer peripherals, including storage media, and supports a maximum transfer rate of 480Mb/s. The new USB 3.0 (Super Speed) specification supports transfer rates up to 4.8Gb/s; products using the interface to carry video to PC monitors are expected this year.
HDMI is becoming the de-facto standard in consumer electronics and seems to be replacing legacy digital interfaces for short-distance interconnects. HDMI supports, on a single cable, both an SD or HD uncompressed video stream (up to 4K × 2K resolutions with version 1.4), up to eight channels of audio, and consumer electronics control (CEC). High bandwidth Digital Content Protection (HDCP) provides a robust mechanism for authentication and copy protection over the interface.
Starting with version 1.3, HDMI supports two other important features in digital video processing: the xvYCC color space and Deep Color. Above this, HDMI 1.4 brings the theoretical total upper bandwidth to nominally 10.2Gbs/s, with a maximum video bandwidth of 8.16Gb/s and maximum audio bandwidth of 36.86Mb/s. HDMI 1.4 supports the resolutions and features used in digital cinema, such as a 4K × 2K (3840 × 2160p) video resolution at 24/25/30Hz, or 4096 × 2160p at 24Hz. A 100Mb/s Ethernet link, an audio return channel (similar to S/PDIF) and 3-D video are also supported in version 1.4.
The DisplayPort interface, defined by VESA and incompatible with both DVI and HDMI, supports up to 17.2Gb/s. Security for the interface is provided by DisplayPort Content Protection (DPCP), based on 128-bit AES (Advanced Encryption Standard) encryption. While it has been adopted by Apple, Dell and other PC manufacturers for high-end monitors, the interface seems to be struggling to gain major acceptance in the marketplace when compared to HDMI.
Digital audio interfaces
In a broadcast plant, increasing flexibility while minimizing wiring, complexity and maintenance are strong drivers to adopting new standards and technologies, including the associated physical interfaces required to support those technologies. Audio distribution over data networks, such as Ethernet, is one example of such an emerging technology.
Mature point-to-point digital audio interconnects within the video processing pipeline include AES3, MADI and SDI. First published in 1985, the AES/EBU interface, AES3, was primarily designed to carry 44.1kHz or 48kHz PCM audio. It supports a variety of cabling and connectors, including 75Ω coax with BNC or DB25 connectors; AES3 can easily support eight channels of uncompressed PCM audio. The Sony/Philips Digital Interconnect Format (S/PDIF), essentially a consumer derivative of the AES/EBU specification, supports both uncompressed PCM (up to four channels) and compressed audio, such as 5.1 Dolby Digital. S/PDIF uses RCA, BNC and TOSLINK connectors, the latter commonly used for consumer audio applications using a digital optical interface. The ADAT Optical Interface uses cabling similar to the optical S/PDIF. Unlike S/PDIF, ADAT was developed for the pro-market and can support up to eight 24-bit, 48kHz uncompressed audio channels. MADI is a further enhancement to the AES/EBU interface that supports transmission of digital audio using both coax and fiber-optic lines, and up to 64 uncompressed audio channels (up to 96kHz and 24 bits each). MADI uses a basic data rate of about 100Mb/s, and enables audio transmission using cables over 100m long and up to 3000m, making it desirable to the professional audio sector.
The ever-evolving video processing pipeline
Over the past few years, video has transitioned from SD, 480i video and 2-channel audio, to 1080p video with multichannel audio. New display technologies allow for wider color gamuts. 3-D video is transitioning from the movie theater to the home, and may force the development of new interfaces, especially on graphics cards. As standards bodies continue to upgrade existing audio and video interfaces and develop new ones, video production studios need to merge traditional video capturing and editing equipment with powerful PC and video servers and use a combination of Gigabit Ethernet, HDMI, and legacy audio and video interfaces, such as SDI and HD-SDI. Count on continuing evolution as the norm!
Aldo Cugnini is a consultant in the digital television industry.
Table 1. Digital A/V interfaces Interface Standard Max bit rate (Mb/s) Examples SD-SDI SMPTE 259M 360 480i video, two-channel audio SMPTE 344 M 540 480p video, two-channel audio HD-SDI SMPTE 292 M 1485 720p video, 16-channel audio 3G-SDI SMPTE 424M 2970 1080p video AES/EBU AES3 1.5 (two channel) Eight-channel audio S/PDIF IEC 60958 type II 1.5 (two channel) Four-channel PCM DVB-ASI EN50083-9 270 MPEG 480p TS ADAT optical 9.2 Eight-channel PCM MADI AES10-2003 100 64-channel PCM USB USB 2.0 480 480p video or compressed ATSC USB 3.0 4800 1080p video HDMI HDMI v1.4 10,200 1080p video, 7.1 DTS-HD master audio DisplayPort VESA 17200 (v 1.2) 1080p video
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