Digital interfaces
Mar 1, 2010 12:00 PM, By Aldo Cugnini
High-speed data connections for production environments are continuing to evolve.
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
Table 1. (caption: Table 1. Digital A/V interfaces)
Select figure to enlarge.
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.
Figure 1. Digital video processing pipeline
Select figure to enlarge.
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.
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