Digital video compression is already an important technology in virtually every type of video application. As the trend toward media convergence continues, compression and interoperability will only become more critical.
Among the most prominent digital video applications are DVD, HDTV, video telephony/ teleconferencing, and, more recently, video surveillance. Each of these consumer system-level technologies, however, has a different historical background. As a result, each has adopted different compression algorithms (see "Video Processing Brings New Meaning To Motion," Electronic Design , Sept. 1, 2006, ED Online 13291) .
Over the last few years, the number of standards in the market has grown significantly, particularly with the introduction of new codecs like H.264 (MPEG-4 Part 10) Advanced Video Coding (AVC) with its various profiles and Windows Media Video version 9 (WMV9) and its profiles. At the same time, engineers designing video systems still must deal with legacy equipment that supports only a few of the older standards like H.261, H.263, MPEG-2, or MPEG-4 part 2. But depending on the application, this equipment must be interoperable with the latest equipment integrating the newest algorithms.
Algorithm development is another part of the challenge. As new, more powerful algorithms are developed and standardized, they must be compatible with all pre-existing algorithms — a daunting task at best — or some powerful, universal transcoding scheme must be developed.
From an intellectual-property perspective, the problem becomes even more complex. Although many video coding algorithms (e.g., MPEG-2, MPEG-4, H.263, and H.264) are published standards, others (e.g., On2 and Real Video) are proprietary. Sometimes, proprietary algorithms become standards. WMV9, for example, began as a proprietary algorithm and was ultimately adopted by the Society of Motion Picture and Television Engineers (SMPTE) as the public VC1 standard.
What Is Video Transcoding? Video transcoding converts one video format to the other. It has two distinct and important roles:
Transcoding enables communication between legacy and emerging devices. Many existing video-conference systems, for instance, are based on the legacy video coding standard H.263. More recent video-conference systems utilize the H.264 baseline profile. Real-time video transcoding is necessary to realize communication between them.
Networks, particularly the Internet, place bandwidth constraints on the video transmission. Most movies today, for example, are stored in DVD with MPEG-2 format. The bandwidth limits of video-on-demand and video-streaming-over-IP systems require the video data to be converted to a more bandwidth-efficient format by real-time video transcoding before transmission.
For a video-conferencing system, legacy and emerging video streams need to be converted between formats using video transcoding. For a video-on-demand application, the conversion is usually from a video stream encoded in a legacy video coding standard (MPEG-2, H.263) to a stream encoded in a new and advanced video coding standard (H.264 or VC1). The rationale for transcoding is that it saves up to 50% network bandwidth without losing any video quality.
Background of a Video Transcoding System From an operational perspective, video transcoding is typically employed in the video infrastructure of a system central office. The most common implementation calls for the host processor to handle network traffic while multiple DSPs handle the video encoding and decoding involved in the task of transcoding. Usually, a single video multiport control unit (MCU) is powerful enough to take care of multiple video-transcoding channels simultaneously.
As an example, Figure 1 illustrates the basic video transcoding requirements and dataflow in a video conferencing system. DSP2 decodes the input video stream and generates a reconstructed video frame, which is transmitted to DSP1 over the Serial RapidIO interface. Another DSP will encode the reconstructed video frame into the target format. The most common scenario is that one end of the video conference uses H.263-based equipment while the other party uses H.264-based equipment.
Here, the host processor communicates with multiple DSPs (four in this instance) over a PCI connection. A critical feature for processor intercommunication in this example is the sRIO connections between the four DSPs. Since the data being transferred between DSPs are uncompressed video data typically at 30 frames/s, the bandwidth requirement between devices is huge.
Taking NTSC standard resolution (720 by 480) with color space YUV 4:2:0 video as an example, the size of each frame is 720 × 480 × 1.5 = 518,400 bytes. Transcoding at 30 frames/s means each channel requires approximately 124 Mbits/s. The choice of sRIO is key due to the video bandwidth requirements and the support for a flexible switching fabric. In turn, the advantages of sRIO become a critical factor in the choice of a DSP for this application.
An ac-coupled interface, sRIO offers three data rates at 1.24, 2.5, and 3.125 Gbits/s. The interface utilizes a serializer/deserializer (SERDES) interface to perform clock recovery from the data stream and incorporates 8/10-bit coding. The serial specification supports one-lane (1 ×) and four-lane (4 ×) port sizes. Its physical layer defines the mechanism for handshaking between link partners and handling error detection based on CRC. It also defines packet priority, which is used for routing within the switch fabric.
To take full advantage of sRIO's bandwidth capability, the DSPs must have sRIO interfaces. The built-in sRIO interface in Texas Instruments' TMS320C6455 DSP realizes four simultaneous links and enables peak data transmission at 20 Gbits/s bidirectionally.
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