There is a big difference in fidelity between conventional voice telephony and an acceptable system that can be used for the enjoyment of music, which has led to the development of a number of sophisticated compression schemes for general audio that can utilize such narrow-band channels. They are based upon the concepts of psychoacoustics, that is, the study of the mechanisms used by the human ear for the perception of sound. The problems for the developers of codecs stem from the very different waveforms generated by, for example, the piccolo or the castanets. The quest to squeeze maximum fidelity from a channel started in the analog era before digital processing. Analog compression The compression schemes of analog audio originally were developed to counter the limited dynamic range of magnetic tape recorders. A typical recorder could have a range between peak level and the noise floor of 70 dB. High-quality recording demanded at least 90 dB. Dolby A Dolby A was the first commercially successful noise reduction product used by professional recording studios, dating back to 1965. It established the principal of splitting audio into sub-bands for processing, a system still used by MPEG codecs and by audio preprocessors. Multichannel recording added to the demands from recording studios for noise reduction. In a final mix-down, 40 or so tape tracks could be combined into a stereo signal. The noise from each channel adds, and without Dolby, the result would have been unusable. To get around the noise problem, the dynamic range of the input audio was compressed before recording on tape, then expanded back on replay, a process commonly referred to as companding. To make the process inaudible was not trivial, especially using analog computing elements. Earlier proposals for noise reduction systems had suffered from noise modulation and poor transient response. The Dolby system made use of psychoacoustics to mask the effects of compression. With the low-cost digital processing available now, much more complex algorithms can be used. Dolby B and C This was a low-cost noise reduction system aimed at consumer products like the compact cassette. At the low tape speeds used, the frequency-dependent noise (tape hiss) was a big problem. It could be reduced to acceptable levels Audio compression 103
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6 Audio compression Introduction For many consumers, Internet radio was their first exposure to streaming media. The other driver for interest in compressed audio has been the universal acceptance of the MP3 format for low data rate audio files. Although not a streaming format (it is for download and play), many popular streaming architectures can be used to wrap MP3 files for streaming. Although MP3 gradually is being replaced with the more efficient AAC codecs, it still retains a very wide user base. There are several audio-only architectures that come from the Internet radio arena: Nullsoft s Shoutcast and Winamp, and Audion for the Mac platform. The primary multimedia architectures MPEG-4, Windows Media, QuickTime, and Real all support audio-only streaming as well as video/audio content. To stream audio you have to go through three processes: Digitization Encoding (compression) Packetization The professional digital audio format, AES-3, is often the basis for digitization. The AES standard has a data rate of over 3 Mbit/s for stereo audio sampled at 48 kHz. To transmit this over a 28-kbit/s dial-up circuit, the data would have to be reduced by a factor of 100:1, and that excludes the overheads of packetization and error resilience. The compression schemes split between the speech (voice) coding developed by the telcos and the military, and the general audio or waveform encoders developed for multimedia applications. The speech coding algorithms generally use a model of the vocal tract and transmit voicing parameters for reconstruction by the decoder into intelligible speech. Much of the Internet radio encodes music that can be distributed over a 28- or 56-kbit/s voice circuit.
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Summary Which codec offers the best compression? This is a moveable feast codecs are improving all the time. Choosing a codec depends upon many factors. First there is the audience for your streams. What players do they have installed and what speed of connection do they use? You can make tests to see what codec gives the best quality at the required bit rate. There are commercial concerns: What is the cost of an end-to-end solution? Then the producer may have a view. Does the architecture support all the rich media content that he or she may want to use? Only you can answer these questions. To recommend a codec for a specific task depends upon many factors. Codecs are often compared to existing video formats for example, VHS quality. It is difficult to sensibly compare an analog tape format with DCT compression. We quite happily watch the poor quality of VHS (compared to broadcast television) because of the constant level of artifacts like noise. The brain ignores continuing distortions. DCT-based coding produces changing levels of artifacts. This means that stationary pictures with low levels of fine detail look better than fast-moving fine detail. So the picture quality changes as the picture content changes. This is visually disturbing as the blockiness comes and goes. So it is meaningless to compare DCT coding with VHS. To achieve a constant video quality you have to resort to variable bit rate encoding, as adopted with the DVD format. That way the fast-action sequences get a higher bit rate than the stationary scenes. This is now an option with some streaming codecs. Compression will always be a trade-off between bit rate and artifacts, but the use of perceptual coding now allows for compelling viewing over broadband circuits, DSL, and cable modems, using a fraction of the bandwidth of conventional television. Video compression 101
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video files (not live encoding) to give the optimum processing. This provides improved encoding quality with both constant and variable bit rates, but of course does lengthen the encoding time. RealVideo 10 now supports interlaced video as well as progressive scan. As well as support for personal computers on the Windows, Linux, and Mac platforms, RealNetworks support mobile platforms like Symbian running on the ARM processor. Windows Media 9 Series Part of Microsoft s Windows Media 9 Series is the Windows Media Video codec. It is similar in principle to the MPEG-4 part 2 visual codec, but has been optimized to give better compression efficiencies. Microsoft played an active part in the development of the MPEG-4 standards, but in the interests of improved quality, has adopted the proprietary route. Microsoft has also forwarded the core codec for standardization by the Society of Motion Picture and Television Engineers (SMPTE) as video codec 9 (VC-9). This would open the way for thirdparty developers to adopt the VC-9 compression codec. Windows Media is Microsoft s implementation of the VC-9. There are two video codecs, the basic codec and the Windows Media Video 9 Professional. The latter is for high-definition services, delivered by DVD or wide-band circuits, and supports line rates up to 1,080. The basic codec is more suited to regular streaming applications at bandwidths below 1 Mbit/s. The advanced profile will support 2,048 1,536 for digital film applications. Windows Media 9 supports constant and variable bit rate encoding (see Table 5.3). The video streams can be encoded at multiple bit rates for intelligent streaming over congested networks. 100 The Technology of Video and Audio Streaming Table 5.3 Windows Media 9 Encoding Options Bit rate Encoding method Notes constant (CBR) Stream data rate is constant one-pass For live encoding two-pass Improved quality over one-pass variable (VBR) Consistent video quality quality-based Bit rate varies to give constant quality bit-rate based Two-pass process, encodes for a set average bit-rate peak bit rate Maximum rate constrained to fit streaming data channel, a two-pass process
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Apple QuickTime QuickTime supports a large number of video formats, including uncompressed, but came late to streaming. Most of the codecs are for authoring CD-ROMs. As a general-purpose multimedia production tool in an editing environment, Quick- Time can be used to wrap DV content and motion JPEG. Apple does not offer their own streaming codecs, but now supports the MPEG-4 codecs. RealNetworks The RealVideo 10 codec replaces earlier Real codecs with improved compression efficiency and support for high-definition video at 720 and 1080 lines. RealVideo 10 files are backward-compatible with RealVideo 9 decoder. SureStream allows the content files to include video and audio encoded at different bit rates. The streaming server can optimize the delivery rate to suit the network conditions. Variable bit rate encoding can be used to maintain a constant video quality level. Two-pass encoding can be used for prerecorded Video compression 99 transaction use relationship authorization value exchange Identification & representation Digital item representation Content representation Intellectual property management & protection Content management & usage Terminals & networks User B User A Unique identifiers Content descriptors Natural & synthetic Scalability Storage management Content personalization Unique identifiers Content descriptors Encryption Authentication Watermark Container Item Resource content Figure 5.10 MPEG-21 framework.
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broadcast TV), baseline (for mobile/wireless), and extended for high-definition reproduction. Developments (fidelity range extensions (FREX)) are planned for the standard to handle 4:4:4 sampling up to 12 bits per color. The initial standard is 4:2:0, 8 bit. Other MPEG standards MPEG-7 This is a standardized system for describing audio-visual content. This enables fast and efficient searching. You can compare this with the facilities provided with a search engine to locate textual content. Images and sound could be described in many abstract ways: textures, text descriptions of content, color. Much of this is open to interpretation. Consider somebody searching for a clip of a happy person. Happiness is an emotion that is not obvious from a description of the shape, color, and texture of the images. Content description is not a trivial task, but essential to the efficient utilization of audio-visual archives. MPEG-21 MPEG-21 is a framework for integrating multimedia resources across a wide range of platforms and services. Streaming media is an important part of this framework. One focus of MPEG-21 is in value exchange, typically the sale of content. Key to this is the management of intellectual property rights. MP3 misnomers MP3 is the file extension for MPEG-1 and MPEG-2, layer 3 audio files, not MPEG-3. The latter was a proposed standard for HDTV that was dropped in favor of MPEG-2 high level. Proprietary architectures The proprietary architectures have been the most popular platforms for streaming. Windows Media has a very strong following, with other architectures moving toward compatibility with MPEG-4. There is the usual issue of proprietary versus international standard. Most vendors will claim that a proprietary solution brings higher quality encoding to the market quicker than the standards-based approach. Video codecs are a fast-moving product area. The list that follows gives the status as of 2004. It pays to use the latest versions of the encoders, since the quality is improving all the time, and often features like digital rights management are supported only in the latest versions. 98 The Technology of Video and Audio Streaming
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Simple For low-bandwidth wireless and low-latency applications like videoconferencing and telephony. Advanced Simple For interactive rich media applications, with bandwidth from 56 kbit/s up to high-definition at rates as high as 8 Mbit/s. These two are joined by a third option of the advanced video codec, AVC. This offers much improved efficiency, but needs considerably more processing resources to encode and decode. As processors get faster and lower cost, AVC becomes the favored option for most fixed applications (rather than mobile). AVC was created for conventional rectangular video, rather than the objectbased coding of the MPEG-4, part 2, visual codec. It was formalized in 2003, with implementation in software and hardware becoming available in 2004. It achieves the same quality as MPEG-2 at around half the data rate. These improvements are gained largely through improved prediction. AVC can use multiple reference frames within a GOP, rather than the bidirectional prediction of MPEG-2. The variable block size cuts down the mosquito effects, a common artifact of DCT block compression. AVC has three profiles: main (for Video compression 97 DCT video input ISO/IEC 14496 encoder intra-frame compression local decoder inter-frame compression encoded bitstream texture coding & codebook shape coding frame delay motion compensated prediction motion estimation mux IDCT IQ Quantizer (Q) - buffer bit rate control motion vectors shape mask + Figure 5.9 MPEG-4 encoder system.
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96 The Technology of Video and Audio Streaming same way as the earlier H.263 and MPEG-1 and -2 encoders. There are simpler ways of extracting objects that are suited to rich media presentations. The speaker could be shot on a blue-scene set, then extracted as an object. Figure 5.8 shows a typical application of objects for interactive content. The navigation elements and live video are combined over a background layer. Each object can be used as a hot-spot or link to other objects, giving the interactivity we are familiar with from the web page or DVD. The arrangement of the objects within a scene is described by the scene descriptor, the Binary Format for Scenes (BIFS). Both natural video (pixelbased) and synthetic two- and three-dimensional objects can be encoded. The player stacks the object layers together to render a two-dimensional representation of the scene. Shape coding A video object can be coded as an arbitrary shape. In the example the foreground flag could be coded as a separate video object. The shape of the object is defined by an alpha channel. To reproduce a complex edge structure the shape can be coded as a grayscale, giving an 8-bit resolution. A simple shape can be represented as a binary object (present or absent). Texture coding The texture of a video object is conventionally coded by the DCT. The codec is similar to H.263, MPEG-1, and MPEG-2. The texture of static objects can be coded using the wavelet transform. Motion coding The motion coding adds a global motion prediction. If an object is moving as a whole (like a camera pan), rather than code the motion vectors for each macroblock, the motion parameters of the entire object are transmitted. This can be very compact data. Video encoder Just like MPEG-1 and MPEG-2, the standard for MPEG-4 defines a standard player or terminal. The details of the encoder are left to the vendor, as long as the encoded bit stream can be played back on an MPEG-4 terminal. A typical MPEG-4 encoder is shown in Figure 5.9. The texture and shape coding are new components. The basic encoder architecture is similar to MPEG-1 and MPEG-2 encoders. The enhancements to the coding do offer a small increase in efficiency over MPEG-2, typically 10 15 percent for typical material with the advanced simple profile. The AVC gives much better efficiencies, up to twice MPEG-2, albeit with a much higher complexity for the encoder and decoder. MPEG-4 has a number of different profiles for natural video content. The most popular are:
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Video compression 95 Figure 5.8 MPEG-4 interactive objects.
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94 The Technology of Video and Audio Streaming original scene scene decomposed to objects scene rendered in media player background sprite video object video object player window Figure 5.7 MPEG-4 video objects.
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