6. Encoder hardware:
MPEG2 and other algorithms intended for consumer use, are designed so that most of the sound and video compression effort is done by a single expensive and complex encoder or transmit-only codec, located at the signal transmit location. This in turn allows the subscriber's set-top box, the integrated receiver-decoder, (IRD), to be a relatively simple and inexpensive device.
A bit of history regarding this complex technology is worth a quick once-over. DirecTV's broadcast centers were initially equipped with more than 200 Compression Labs, Inc. (CLI, San Jose, CA) Magnitude MPEG2 encoders. These encoders each used an array of video signal processor chips from Milpitas, CA semiconductor manufacturer C-Cube Microsystems. An important part of the encoder hardware included was a dynamic statistical multiplexer, allowing the operating companies to best tailor the image quality including motion-handling to the demands of the program material, helping to optimize bandwidth utilization and video quality.
In 1996, CLI's broadcast division (including DBS coders and multiplexers) was sold to General Instrument (GI) which added the former CLI products to its product line. Later, the GI operation was sold to Motorola who continue to actively market these and related products.
In the spring of 1997, the balance of CLI (mainly videoconferencing products) was merged with VTEL, an Austin, Texas videoconferencing company without interest in broadcast-related products. In 2001, part of VTEL became known as Forgent.
DirecTV's, and most other DBS services use the MPEG2 compression standard, although the more capable and computationally more intensive MPEG4 is gradually being introduced. MPEG2 provides high quality video and stereo audio transmission at a variety of bit rates from 1.5 Mbps to 15 Mbps. Initially, DirecTV used two basic encoding rates centered at 3 Mbps and 7.5 Mbps. High-motion program material such as sports was allocated a nominal 7.5 Mbps while more sedate material such as talk shows used about 3 Mbps. Several channels were then combined using a statistical multiplexer, a device combining a number of digital television signals into a single transponder signal.
At this stage, it's important to understand that the video encoder or transmit codec produces variable amounts of data over relative short periods of time, depending upon the amount of motion, the amount of image detail and other factors relating to the television signal. The bit rates can vary between just a few hundred kilobits per second up to the system's programmed maximum (in this example, 3.0 or 7.5 Mbps).
Within the DBS context, there is only 23 Mbps available per transponder shared by a group of encoders. The statistical multiplexer can help maximize the utilization of each transponder by controlling each codec's bit rate. For example, if a codec only needs half of its available nominal assigned bandwidth for a period of time, the rest of its allotted bandwidth can be borrowed by another (or several) encoder(s). If the system has been correctly programmed, this will provide better motion handling and/or picture definition for one or more of the other signals. Such dynamic bandwidth allocation is a very effective way of providing what seems like more bandwidth than the actual 23 Mbps available, but only if the types of services sharing each transponder have been chosen with care. On the other hand, if all the channels on a given transponder are assigned to sports, gains from bandwidth sharing could be virtually non-existent.
Picture resolution is programmable in each codec, allowing fine-tuning of the available bandwidth for any given type of program. Most programs are encoded at the ITU-R 601 standard's Nyquist sampling rate and at a resolution of 544 pixels by 480 lines (NTSC) (576 lines for PAL), a good-quality picture. However, resolution can be adjusted from a minimum of 352 pixels x 240 lines (NTSC), (288 lines for PAL), about the same picture quality as H.320/H.323 videoconferencing. In high-quality mode, a snappy, full ITU-T 601 resolution of 720 pixels by 480 lines (NTSC), (576 lines for PAL) is realized. Incidentally, use of s-video connections from your receiver to TV set and VCR are generally only beneficial with the 544 pixel and 720 pixel horizontal resolutions. Otherwise, at lower resolutions, a standard composite NTSC signal works just fine.
Most program signals are MPEG2 coded in real time, a process that can introduce some coding errors. However, an MPEG2 option which is often used for pay-per-view (PPV) movies for example, involves digitally encoding the video off-line, prior to transmission. A digital recording is produced that is later played back from a video server at film's native 24 frames per second (fps) rather than TV's 30 fps, resulting in increased quality and reduced bandwidth requirements.
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