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`Digital Broadcast Technologies
`Date: Aug 20, 2001 By Michael Adams. Article is provided courtesy of Cisco Press.
`
`Digital technology is becoming pervasive in all types of services. As computing power
`continues to increase, more and more functions can be tackled in the digital domain. An
`excellent example is the transmission of television pictures. This sample chapter from
`OpenCable™ Architecture, winner of NCTA Book of the Year Award, introduces a number
`of key digital broadcast technologies.
`
`Digital technology is becoming pervasive in all types of services. As computing power
`continues to increase according to Moore’s Law (see Chapter 1, "Why Digital Television?"),
`more and more functions can be tackled in the digital domain. An excellent example is the
`transmission of television pictures.
`
`Nevertheless, digital technology is not a panacea. The complexity of the techniques can
`introduce reliability and quality issues. In addition, only a few engineers thoroughly
`understand all these techniques, making us more reliant on a smaller number of de facto
`standard chip-sets.
`
`This chapter introduces a number of key digital technologies. If you are familiar with these
`technologies, you might want to skim through or skip over this chapter. There are many
`excellent texts (see the list of references at the end of the chapter) that explain how these
`digital technologies work. This chapter does not attempt to repeat them, but instead provides
`a commentary on why these techniques are so important and what they mean in practical
`terms. This chapter discusses
`
`Video compression—The basic principles of the video compression algorithms
`commonly used for entertainment quality video, the importance of choosing the
`correct parameters for video encoding, and some alternative video compression
`algorithms.
`
`Audio compression—The basic principles of MPEG-2 audio compression and Dolby
`AC-3 audio compression.
`
`Data—Arbitrary private data can be carried by the underlying layers.
`
`System information—Tabular data format used by the digital receiver device to drive
`content navigation, tuning, and presentation.
`
`Timing and synchronization—A mechanism is required to recover the source timing at
`the decoder so that the presentation layer can synchronize the various components
`and display them at exactly the intended rate.
`
`Packetization—The segmentation and encapsulation of elementary data streams into
`transport packets.
`
`Multiplexing—The combining of transport streams containing audio, video, data, and
`system information.
`
`Baseband transmission—The various mechanisms for carrying digital transport
`streams: the Digital Video Broadcast (DVB) asynchronous serial interface (ASI),
`Synchronous Optical Networks (SONET), Asynchronous Transfer Mode (ATM), and
`Internet Protocol (IP).
`
`Broadband transmission—The digital transmission payload must be modulated
`before it can be delivered by an analog cable system. Quadrature Amplitude
`Modulation (QAM) has been selected as the best modulation for downstream
`transmission in cable systems. Other modulation techniques include quaternary
`phase shift keying (QPSK) and vestigial side band (VSB) modulation.
`
`Figure 4-1 shows how each of these techniques are layered. This diagram illustrates the
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`in-band communications stack for a digital cable set-top and is discussed in Chapter 6, “The
`Digital Set-Top Converter,” in more detail.
`
`Figure 4-1 Layered Model for Digital Television
`
`Video Compression
`
`Image compression has been around for some time but video compression is relatively new.
`The processing requirements to compress even a single frame of video are large—to
`compress 30 frames (or 60 fields) per second of video requires massive processing power
`(delivered by rapid advances in semiconductor technology).
`
`Nonetheless, digital video must be compressed before it can be transmitted over a cable
`system. Although other compression algorithms exist, the dominant standard for video
`compression is MPEG-2 (from Moving Picture Experts Group). Although MPEG-2 video
`compression was first introduced in 1993, it is now firmly established and provides excellent
`results in cable, satellite, terrestrial broadcast, and digital versatile disk (DVD) applications.
`
`This section discusses
`
`MPEG-2 video compression—The basics of the MPEG-2 video compression
`algorithm and why it has become the dominant standard for entertainment-video
`compression
`
`Other video compression algorithms—Why other video compression algorithms have
`their applications and why they are unlikely to challenge MPEG-2 video compression
`in the entertainment world for some time
`
`Details on MPEG-2 video compression—Some more details on the use of MPEG-2
`video compression and its parameters
`
`MPEG-2 Compression
`
`MPEG-2 video compression is the de facto standard for entertainment video. MPEG-2 video
`compression is popular for a number of reasons:
`
`It is an international standard [ISO/IEC IS 13818-2].
`
`MPEG-2 places no restrictions on the video encoder implementation. This allows
`each encoder designer to introduce new techniques to improve compression
`efficiency and picture quality. Since MPEG-2 video encoders were first introduced in
`1993, compression efficiency has improved by 30 to 40%, despite predictions by
`many that MPEG-2’s fundamental theoretical limitations would prevent this.
`
`MPEG-2 fully defines the video decoder’s capability at particular levels and profiles.
`Many MPEG-2 chip-sets are available and will work with any main level at main
`profile (MP@ML)–compliant MPEG-2 bit-stream from any source. Nevertheless,
`quality can change significantly from one MPEG-2 video decoder to another,
`especially in error handling and video clip transitions.
`
`MPEG-2 video compression is part of a larger standard that includes support for
`transport and timing functions.
`
`Moreover, MPEG-2 is likely to remain as the dominant standard for entertainment video
`because it has been so successful in establishing an inventory of standard decoders (both in
`existing consumer electronics products and in the chip libraries of most large semiconductor
`companies). Additional momentum comes from the quantity of real-time and stored content
`already compressed into MPEG-2 format. Even succeeding work by the MPEG committees
`has been abandoned (MPEG-3) or retargeted to solve different problems (MPEG-4 and
`MPEG-7).
`
`MPEG-2 is a lossy video compression method based on motion vector estimation, discrete
`cosine transforms, quantization, and Huffman encoding. (Lossy means that data is lost, or
`thrown away, during compression, so quality after decoding is less than the original picture.)
`Taking these techniques in order:
`
`Motion vector estimation is used to capture much of the change between video
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`frames, in the form of best approximations of each part of a frame as a translation
`(generally due to motion) of a similar-sized piece of another video frame. Essentially,
`there is a lot of temporal redundancy in video, which can be discarded. (The term
`temporal redundancy is applied to information that is repeated from one frame to
`another.)
`
`Discrete cosine transform (DCT) is used to convert spatial information into frequency
`information. This allows the encoder to discard information, corresponding to higher
`video frequencies, which are less visible to the human eye.
`
`Quantization is applied to the DCT coefficients of either original frames (in some
`cases) or the DCT of the residual (after motion estimation) to restrict the set of
`possible values transmitted by placing them into groups of values that are almost the
`same.
`
`Huffman encoding uses short codes to describe common values and longer codes to
`describe rarer values—this is a type of entropy coding.
`
`The foregoing is a highly compressed summary of MPEG-2 video compression (with many
`details omitted). However, there are so many excellent descriptions of MPEG compression
`(see DTV: The Revolution in Electronic Imaging, by Jerry C. Whitaker; Digital Compression
`for Multimedia: Principles and Standards, by Jerry D. Gibson and others; Testing Digital
`Video, by Dragos Ruiu and others; and Modern Cable Television Technology; Video, Voice,
`and Data Communications, by Walter Ciciora and others) that more description is not
`justified here. Instead, the following sections concentrate on the most interesting aspects of
`MPEG:
`
`What are MPEG-2’s limitations?
`
`What happens when MPEG-2 breaks?
`
`How can compression ratios be optimized to reduce transmission cost without
`compromising (too much) on quality?
`
`MPEG Limitations
`
`If MPEG-2 is so perfect, why is there any need for other compression schemes? (There are a
`great many alternative compression algorithms, such as wavelet, pyramid, fractal, and so
`on.) MPEG-2 is a good solution for coding relatively high-quality video when certain
`transmission requirements can be met. However, MPEG-2 coding is rarely used in Internet
`applications because the Internet cannot generally guarantee the quality of service (QoS)
`parameters required for MPEG-2–coded streams. These QoS parameters are summarized in
`Table 4-1.
`
`Table 4-1 MPEG-2 QoS Parameters for Entertainment Quality Video
`
`As you can see from the table, for entertainment-quality video, MPEG-2 typically requires a
`reasonably high bit rate, and this bit rate must be guaranteed. Video-coding will, in general,
`produce a variable information rate, but MPEG-2 allows for CBR transmission facilities (for
`example, satellite transponders, microwave links, and fiber transmission facilities). As such,
`MPEG-2 encoders attempt to take advantage of every bit in the transmission link by coding
`extra detail during less-challenging scenes. When the going gets tough—during a car chase,
`for example—MPEG-2 encoders use more bits for motion and transmit less detail. Another
`way to think of this is that MPEG-2 encoding varies its degree of loss according to the source
`material. Fortunately, the human visual system tends to work in a similar way, and we pay
`less attention to detail when a scene contains more motion. (This is true of a car chase
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`whether you are watching it or you are in it!)
`
`MPEG-2 coded material is extremely sensitive to errors and lost information because of the
`way in which MPEG-2 puts certain vital information into a single packet. If this packet is lost
`or corrupted, there can be a significant impact on the decoder, causing it to drop frames or to
`produce very noticeable blocking artifacts. If you think of an MPEG-2 stream as a list of
`instructions to the decoder, you can understand why the corruption of a single instruction can
`play havoc with the decoded picture.
`
`Finally, MPEG-2 is extremely sensitive to variations in transmission delay. These are not
`usually measurable in synchronous transmission systems (for example, satellite links)
`because each bit propagates through the system according to the clock rate. In packet- or
`cell-based networks, however, it is possible for each packet-sized group of bits to experience
`a different delay. MPEG-2 was designed with synchronous transmission links in mind and
`embeds timing information into certain packets by means of timestamps. If the timestamps
`experience significant jitter (or cell delay variation), it causes distortions in audio and video
`fidelity due to timing variations in the sample clocks—for example, color shifts due to color
`subcarrier phase variations.
`
`MPEG-2 Artifacts
`
`What are MPEG artifacts? In practice, all lossy encoders generate artifacts, or areas of
`unfaithful visual reproduction, all the time; if the encoder is well designed, all these artifacts
`will be invisible to the human eye. However, the best laid plans sometimes fail; the following
`are some of the more common MPEG-2 artifacts:
`
`If the compression ratio is too high, there are sometimes simply not enough bits to
`encode the video signal without significant loss. The better encoders will
`progressively soften the picture (by discarding some picture detail); however, poorer
`encoders sometimes break down and overflow an internal buffer. When this happens,
`all kinds of visual symptoms—from bright green blocks to dropped frames—can
`result. After such a breakdown, the encoder will usually recover for a short period
`until once again the information rate gets too high to code into the available number
`of bits.
`
`Another common visible artifact is sometimes visible in dark scenes or in close-ups of
`the face and is sometimes called contouring. As the name suggests, the image looks
`a little like a contour map drawn with a limited set of shades rather than a
`continuously varying palette. This artifact sometimes reveals the macro-block
`boundaries (which is sometimes called tiling). When this happens, it is usually
`because the encoder allocates too few quantization levels to the scene.
`
`NOTE
`
`Macro-blocks are areas of 16-by-16 pixels that are used by MPEG for DCT and
`motion-estimation purposes. See Chapter 3 of Modern Cable Television Technology;
`Video, Voice, and Data Communications by Walter Ciciora and others, for more
`details.
`
`High-frequency mosquito noise will sometimes be apparent in the background.
`Mosquito noise is often apparent in surfaces, such as wood, plaster, and wool, that
`contain an almost limitless amount of detail due to their natural texture. The encoder
`can be overtaxed by so much detail and creates a visual effect that looks as if the
`walls are crawling with ants.
`
`There are many more artifacts associated with MPEG encoding and decoding; however, a
`well-designed system should rarely, if ever, produce annoying visible artifacts.
`
`MPEG-2 Operating Guidelines
`
`To avoid visible artifacts due to encoding, transmission errors, and decoding, the entire
`MPEG-2 system must be carefully designed to operate within certain guidelines:
`
`The compression ratio cannot be pushed too high. Just where the limit is on
`compression ratio for given material at a certain image resolution and frame rate is a
`subject of intense and interminable debate. Ultimately, the decision involves
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`engineers and artists and will vary according to encoder performance (there is some
`expectation of improvements in rate with time, although also some expectation of a
`law of diminishing returns). Table 4-2 gives some guidance based on experience.
`
`Table 4-2 MPEG-2 Resolution Versus Minimum Bit Rate Guidelines
`
`The transmission system must generate very few errors during the average viewing
`time of an event. For example, in a two-hour movie, the same viewers may tolerate
`very few significant artifacts (such as frame drop or green blocks). In practice, this
`means that the transmission system must employ forward error correction (FEC)
`techniques.
`
`Other Video Compression Algorithms
`
`There are a great many alternative video compression algorithms, such as wavelet, pyramid,
`fractal, and so on (see Chapter 7 of Digital Compression for Multimedia: Principles and
`Standards by Jerry D. Gibson and others). Many have special characteristics that make them
`suitable for very low bit rate facilities, for software decoding on a PC, and so on. However, it
`is unlikely that they will pose a significant threat to MPEG-2 encoding for entertainment video
`in the near future.
`
`Compression Processing Requirements
`
`Let’s take a full-resolution video frame that contains 480 lines, each consisting of 720 pixels.
`The total frame, therefore, contains 345,600 pixels. Remember that a new frame arrives from
`the picture source every 33 milliseconds. Thus, the pixel rate is 10,368,000 per second.
`Imagine that the compression process requires about 100 operations per pixel. Obviously, a
`processor with a performance of 1,000 million instructions per second (mips) is required.
`
`In practice, custom processing blocks are often built in hardware to handle common
`operations, such as motion estimation and DCT used by MPEG-2 video compression.
`
`Details of MPEG-2 Video Compression
`
`The following sections detail some of the more practical aspects of MPEG-2 video
`compression:
`
`Picture resolution—MPEG-2 is designed to handle the multiple picture resolutions
`that are commonly in use for broadcast television. This section defines what is meant
`by picture resolution and how it affects the compression process.
`
`Compression ratio—MPEG-2 can achieve excellent compression ratios when
`compared to analog transmission, but there is some confusion about the definition of
`compression ratios. This section discusses the difference between the MPEG
`compression ratio and the overall compression ratio.
`
`Real-time MPEG-2 compression—Most of the programs delivered over cable systems
`are compressed in real-time at the time of transmission. This section discusses the
`special requirements for real-time MPEG-2 encoders.
`
`Non–real-time MPEG-2 compression—Stored-media content does not require a
`real-time encoder, and there are certain advantages to non–real-time compression
`systems.
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`Statistical multiplexing—This section explains how statistical multiplexing works in
`data communications systems and what special extensions have been invented to
`support the statistical multiplexing of MPEG-2 program streams.
`
`Re-multiplexing—Re-multiplexing, or grooming, of compressed program streams is
`discussed, including a recent technique that actually allows the program stream to be
`dynamically reencoded to reduce its bit rate.
`
`Picture Resolution
`
`MPEG-2 compression is a family of standards that defines many different profiles and levels.
`(For a complete description of all MPEG-2 profiles and levels, see Chapter 5 of DTV: The
`Revolution in Electronic Imaging by Jerry C. Whitaker.) MPEG-2 compression is most
`commonly used in its main profile at its main level (abbreviated to MP@ML). This MPEG-2
`profile and level is designed for the compression of standard definition television pictures
`with a resolution of 480 vertical lines.
`
`The resolution of a picture describes how many pixels are used to describe a single frame.
`The higher the resolution, the more pixels per frame. In many cases, the luminance
`information is coded with more pixels than the chrominance information.
`
`NOTE
`
`The retina of the human eye perceives more detail with rod cells, which are sensitive only to
`the intensity of light—the luminance—and perceives less detail with cone cells, which are
`sensitive to the color of light—the chrominance.
`
`Chroma subsampling takes advantage of the way the human eye works by sampling the
`chrominance with less detail than the luminance information. In the MPEG-2 main profile
`(MP), the chrominance information is subsampled at half the horizontal and vertical resolution
`compared to the luminance information. For example, if the luminance information is sampled
`at a resolution of 480 by 720, the chrominance information is sampled at a resolution of 240
`by 360, requiring one-fourth the number of pixels. This arrangement is called 4:2:0 sampling.
`The effect of 4:2:0 sampling is to nearly halve the video bandwidth compared to sampling
`luminance and chrominance at the same resolution.
`
`Compression Ratio
`
`The compression ratio is a commonly misused term. It is used to compare the spectrum used
`by a compressed signal with the spectrum used by an equivalent NTSC (National Television
`Systems Committee) analog signal. Expressed this way, typical compression ratios achieved
`by MPEG-2 range from 6:1 to 14:1. Why is the term confusing?
`
`If you take the same video signal and modulate it as an analog signal (uncompressed) and
`compress it using MPEG-2, you have two very different things. The analog signal is an
`analog waveform with certain bandwidth constraints so that it fits into 6 MHz, whereas the
`MPEG-2 elementary stream is just a string of bits that cannot be transmitted until further
`processing steps are taken. These steps include multiplexing, transport, and digital
`modulation, and they all affect how much bandwidth is required by the compressed signal.
`
`To compare apples with apples, you must take the same video signal and convert it to an
`uncompressed digital signal (this is actually the first step in the compression process and is
`termed analog-to-digital conversion or simply sampling). You can now compare the
`uncompressed digital signal with the MPEG-2 compressed elementary stream for a true
`comparison of the input bit rate and the output bit rate of the compression process. A
`full-resolution uncompressed video signal sampled in 4:2:0 (see the previous section,
`“Picture Resolution”) requires 124.416 Mbps. MPEG-2 can squeeze this down to about 4
`Mbps with little or no loss in perceived quality; this is a true compression ratio of 124:4 or
`31:1. This is very different than the commonly quoted range of 6:1 to 14:1.
`
`To continue the math, take the 4 Mbps video elementary stream and add an audio stream at
`192 Kbps to create a program stream at 4.192 Mbps. Add information to describe how the
`streams are synchronized and place the data into short transport packets for efficient
`multiplexing with other streams. You now have a payload of approximately 4.3 Mbps. Using
`64-QAM modulation (see the section Broadband Transmission in this chapter), six 4.3 Mbps
`streams fit into its 27 Mbps payload. Thus, we could express this as a 6:1 compression ratio.
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`This is all very confusing! In this example, a video signal with a 31:1 MPEG-2 video
`compression ratio is roughly equivalent to an overall compression ratio of 6:1. (If the example
`employs 256-QAM and statistical multiplexing, you might achieve an overall compression
`ratio of 12:1 although the MPEG-2 video compression ratio is still 31:1.)
`
`In this book, the terms MPEG-2 video compression ratio and overall compression ratio will
`be used to distinguish these very different measures.
`
`Real-Time MPEG-2 Compression
`
`Real-time compression is commonly used at satellite up-links to compress a video signal into
`a digital program stream as part of the transmission (or retransmission) process. Very often,
`the encoder runs for long periods of time without manual intervention. There must be
`sufficient headroom in the allocated bit rate to allow the encoder to operate correctly for all
`kinds of material that it is likely to encode. (Headroom refers to available, but normally
`unused, bits that are allocated to allow for the video compression of difficult scenes.) Each
`channel requires a dedicated encoder, so price is a definite issue for multichannel systems.
`The encoder must also be highly reliable, and in many cases automatic switching to a
`backup encoder is required.
`
`Non–Real-Time MPEG-2 Compression
`
`Non–real-time encoders are technically similar to real-time encoders, but have very different
`requirements. In fact, they may encode in real-time but their application is to encode to a
`stored media (such as a tape or disc), and a highly-paid compressionist usually monitors the
`compression of each scene. (Compressionists are studio engineers who not only understand
`how to operate the encoding equipment but also apply their artistic judgment in selecting the
`best trade-off between compression ratio and picture quality.) Therefore, encoder price is
`less of an issue and performance is extremely important because the compressed material
`will be viewed over and over again. In the case of digital versatile disks (DVDs), no annoying
`visible artifacts, however subtle, can be tolerated, because the picture quality will be carefully
`evaluated by a magazine reviewer.
`
`Statistical Multiplexing
`
`Statistical multiplexing is a technique commonly used in data communications to extract the
`maximum efficiency from a CBR link. A number of uncorrelated, bursty traffic sources are
`multiplexed together so that the sum of their peak rates exceed the link capacity. Because
`the sources are uncorrelated, there is a low probability that the sum of their transmit rates will
`exceed the link capacity. However, although the multiplex can be engineered so that periods
`of link oversubscription are rare, they will occur. (See Murphy’s law!) In data communications
`networks, periods of oversubscription are accommodated by packet buffering and, in extreme
`cases, packet discard. (The Internet is a prime example of an oversubscribed, statistically
`multiplexed network where packet delay and loss may be high during busy periods.)
`
`Video material has a naturally varying information rate—when the scene suddenly changes
`from an actor sitting at a table to an explosion, the information rate skyrockets. Although
`MPEG-2 is designed to compensate by encoding more or less detail according to the amount
`of motion, the encoded bit rate may vary by a ratio of 5 to 1 during a program.
`
`This makes MPEG-2 program streams excellent candidates for statistical multiplexing, except
`for the fact that MPEG-2 is extremely sensitive to delay and loss. As such, statistical
`multiplexing cannot be used for MPEG-2 if there is any probability of loss due to
`oversubscription.
`
`Therefore, statistical multiplexing has been specially modified for use with MPEG by the
`addition of the following mechanisms:
`
`A series of real-time encoders are arranged so that their output can be combined by a
`multiplexer into a single multi-program transport stream (MPTS). Each encoder has a
`control signal that instructs it to set its target bit-rate to a certain rate.
`
`The multiplexer monitors the sum of the traffic from all the encoders as it combines
`them, and in real-time decides whether the bit rate is greater or lower than the
`transmission link capacity.
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`When one encoder has a more challenging scene to compress, it requests that its
`output rate be allowed to rise. The hope is that one of the other encoders will have
`less-difficult material and will lower its output rate.
`
`However, there is a significant probability that all the encoders could be called upon to
`encode a challenging scene at the same time. When this happens, the aggregate bit rate will
`exceed the link capacity. A conventional statistical multiplexer would discard some packets,
`but in the case of MPEG-2, this would be disastrous and almost guarantee poor-quality video
`at the output of the decoders.
`
`Instead, the multiplexer buffers the additional packets and requests that the encoders lower
`their encoded bit rate. The buffered packets are delayed by only a few milliseconds, but
`MPEG-2 is extremely sensitive to delay variation. The multiplexer can fix this within limits; as
`long as the decoder pipeline does not underflow and the timestamps are adjusted to
`compensate for the additional time they are buffered, the decoder continues to function
`normally.
`
`Some statistical multiplexers use a technique called look ahead statistical multiplexing
`(pioneered by DiviCom—see http://www.divi.com/). In this technique, the material is encoded
`or statistics are extracted in a first pass, the information is passed to the multiplexer (while
`the original input video is passing through a pipeline delay), and bit rates are assigned for
`each encoder; so when the real encoding happens, a reasonable bit rate is already
`assigned. This solves some of the nasty feedback issues that can happen in less
`sophisticated designs.
`
`Reencoding
`
`Until recently, it was impossible to modify an encoded MPEG-2-bit stream in real-time. It is
`now possible, however, to parse the MPEG-2 syntax and modify it to reduce the bit rate by
`discarding some of the encoded detail. This technique was pioneered by Imedia Corporation
`(http://www.imedia.com/) and allows the feedback loop between the MPEG encoders and the
`multiplexer to be removed. In a reencoding (or translation) system, the multiplexer is used to
`combine a number of variable bit rate MPEG-2 streams. If, at any instant in time, the
`aggregate bit rate of all of the streams exceeds the transmission link capacity, the multiplexer
`will reencode one or more of the streams to intelligently discard information to reduce their bit
`rate. Unlike statistical multiplexing, where the multiplexer could not discard any bits, the
`multiplexer reduces the bit rate by discarding some information—for example, fine detail.
`
`Reencoding is a very useful technique to use whenever a number of multi-program transport
`streams are groomed—that is, a new output multiplex is formed from program streams taken
`from several input multiplexes. Without some means of adapting the coded rate,
`re-multiplexing would result in considerable inefficiency and the output multiplex would
`contain fewer channels.
`
`A second application of reencoding is in Digital Program Insertion (DPI). DPI splices one
`single-program transport program stream into another so that the viewer is unaware of the
`transition. It can be used to insert local advertisements into a broadcast program.
`Reencoding allows the inserted segment to be rate-adapted to the segment that it replaces.
`DPI is discussed in more detail in Chapter 15, "OpenCable Headend Interfaces."
`
`Although reencoding techniques are extremely useful, feedback-controlled statistical
`multiplexing is superior from a compression-efficiency perspective when it is possible to
`collocate encoders and multiplexers. Hence, feedback-controlled statistical multiplexing tends
`to dominate at original encoding sites that include statistical multiplexing, whereas
`reencoding is appropriate at nodes where grooming of statistically multiplexed signals needs
`to be performed.
`
`Audio Compression
`
`Audio compression is a companion to video compression, but the techniques are very
`different. First, the audio signal requires much less bandwidth than the video signal. For
`example, a stereo audio pair sampled at 48 KHz and using 16-bit samples requires 1.536
`Mbps (compared to 124 Mbps for the video signal). It would be quite possible to send
`uncompressed audio. Moreover, audio compression cannot achieve the same compression
`ratio as video; a typical rate for the stereo audio pair is 192 Kbps, an 8:1 compression ratio.
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`11/26/2013 5:50 PM
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`Virginia Innovation Sciences, Ex. 2003.
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`Articles
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`http://www.ciscopress.com/articles/printerfriendly.asp?p=106971
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`Nevertheless, it is easy to make an economic argument for audio compression. For example,
`in a 40 Mbps transmission link, an additional two video channels can be carried if audio
`compression is used (assuming 4 Mbps video).
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`There are two leading contenders for audio compression for entertainment quality audio:
`MPEG audio compression and Dolby AC-3.
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`MPEG-1 Layer 2 (Musicam)
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`MPEG-1 Layer 2 audio compression, also known as Musicam, is specified in ISO/IEC IS
`13818-3. MPEG audio compression delivers near–CD quality audio using a technique called
`sub-band encoding. MPEG audio compression is used mainly in Europe and is used by most
`direct-to-home satellite providers in the United States.
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`MPEG audio compression is a two-channel system, but it can encode a Dolby Pro-Logic
`signal, which includes two additional channels for rear and center speakers. (This is
`analogous to the way in which a Dolby Pro-Logic signal is carried by a BTSC [Broadcast
`Television System Committee] signal as part of an NTSC transmission).
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`Dolby AC-3
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`Dolby AC-3 is a more advanced system than MPEG audio compression. AC-3 was selected
`as the audio compression system for digital television in North America and is specified by
`ATSC Standard A/52. AC-3 encodes up to six discrete channels: left, right, center, left-rear,
`right-rear, and sub-woofer speakers. The sub-woofer channel carries only low frequencies
`and is commonly referred to as a 0.1 channel because it has such a limited frequency range.
`Thus, AC-3 5.1 gets its name from five full channels and a 0.1 channel. In addition, AC-3 has
`a two-channel mode that can be used to carry stereo and Dolby Pro-Logic encoded signals.
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`AC-3 also uses sub-band encoding