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.
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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 in-band communications stack for a digital cable set-top and is discussed in Chapter 6, “The Digital Set-Top Converter,” in more detail.
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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