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time. The DCT generally has been adopted because it approaches the performance of the KLT, but with a lower processing overhead. Any encoder that is to be used for video-conferencing or other live applications must have a short processing delay. So the DCT was chosen as the best compromise. Wavelet compression Although the DCT has been very successful, there has long been a demand for other compression schemes that do not suffer from the blocking artifacts that often are visible in DCT compression. Wavelet compression often has been cited as an alternative. It provides a time-frequency representation of the image and can achieve the same image quality as DCT at much higher compression ratios. It has been employed with some success in desktop video-editing systems as an alternative to motion JPEG. It is now becoming a more mainstream technology since it has been adopted as the core technology of the JPEG2000 standard for still image coding. Video applications include the QuVIS product line for the distribution of digital cinema files. Model-based compression This is an alternative to waveform coding. The codec attempts to model the scene and then transmit descriptors, rather than a representation, of the spatial image. Fractal The fractal compression technique relies on the fact that, in certain images, parts of the image resemble other parts of the same image. Similar sections of an image are located, and then the fractal algorithm is applied. Patents have restricted its use, and since wavelet compression offers better efficiency it has been the focus of more intense development effort. Object-based coding Object-based coding has been adopted as the basis for MPEG-4 coding. Ascene which can be in two or three dimensions is represented by a number of video objects. Each object is described by its shape, texture, and motion. Conventional algorithms like the DCT and wavelet can be used to compress the objects. Entropy coding Entropy coding can yield a much shorter image representation with average content by using short code words for more likely bit sequences, and longer code words for less likely sequences. One of the first examples of entropy coding was Morse code. Video compression 83

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Intraframe coding or spatial compression To compress, the data required to recover each pixel must be reduced below the bit-depth of each pixel. Each pixel can be quantized separately spatial quantization but this would lead to posterization artifacts with any continuoustone images. An alternative is vector quantization, where a group of pixels are quantized together. This theoretically offers the most efficient compression, but in practice it is computationally intensive and has a long processing delay. The alternative is transform coding, where a block of pixels is transformed to the frequency domain, and then quantized. Entropy coding can then be used to further reduce the data rate. The result can be as efficient as vector quantization, but without the processing overheads. 82 The Technology of Video and Audio Streaming scalar quantization transform coding bits per pixel entropy coding Figure 5.1 Compressing the bits. Transform coding Transform coding converts the spatial array of pixel blocks, a bit map, to the frequency domain. Although several different transforms have been proposed, the discrete cosine transform has been the most widely adopted for video codecs. Some other transforms investigated include Discrete Fourier (DFT), Karhunen Lo eve (KLT), and Walsh Hadamard (WHT). The DFT was ruled out because it suffers discontinuities at the block boundaries. For typical real-world images, the DCT outperforms the WHT and DFT in the energy compaction. The KLT has the optimal decomposition but requires a very large number of operations, so it has the longest processing

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Perceptual Statistical Compression can be lossless or lossy. If all the original information is preserved, the codec is called lossless. A typical example for basic file compression would be ZIP. To achieve the high levels of compression demanded by streaming codecs, the luxury of lossless codecs is not possible the data reduction is insufficient. Spatial redundancy occurs where neighboring pixels in a frame of a video signal are related; it could be an object of a single color. If consecutive pictures also are related there is temporal redundancy. The human visual system has psychovisual redundancy; not all the visual information is treated with the same relevance. An example is lower acuity to color detail than luminance. Finally, not all parameters occur with the same probability in an image. This statistical redundancy can be used in the coding of the image parameters. For example, frequently occurring parameters can be coded with fewer bits (Huffman coding). The goal with compression is to avoid artifacts that are perceived as unnatural. The fine detail in an image can be degraded gently without losing understanding of the objects in a scene. As an example we can watch a 70-mm print of a movie or a VHS transfer and in both cases still enjoy the experience. If too much compression is applied, and the artifacts interfere with the image perception, the compression has become unnaturally lossy. Table 5.1 lists some of the more popular technologies that have been used for encoding streaming files. The techniques may be combined within codecs. For example, MPEG-2 divides the picture into blocks. Each block is encoded using a spatial transform, and the data is then run-length encoded. Blocks that repeat from frame to frame have temporal redundancy. Motion from frame to frame is approximated by the motion of a repeated block, which is encoded as the block with a motion vector. Video compression 81 Table 5.1 Compression Techniques Spatial compression Other techniques Vector quantization Frame differencing Block Discrete Cosine Transform (DCT) Motion estimation/compensation Discrete Wavelet Transform (DWT) Run-length encoding (RLE) Fractal image compression Contour-based image coding

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Intraframe compression The compression of a single image in the spatial domain is called intraframe within the frame. Video editing systems have used an extension to JPEG called motion JPEG, where the video sequence is compressed as a sequence of individual frames. When you are editing you need random access to each frame, and the ability to reorder the frame sequence. Once the material has been edited into the final clip and is ready for dissemination, we no longer need the ability to freely cut a sequence on any frame. A streamed clip is delivered in a fixed order of frames. We now can take advantage of the redundant information in scene content that repeats from frame to frame. Video and interframe compression A sequence of video images has little change from one picture to the next, except at scene changes. This redundancy can be exploited by transmitting only the difference between successive pictures; by this means, a large reduction in the data rate can be achieved. This temporal or interframe compression allows the data rate for video sequences to be reduced much more than a sequence of unrelated still images. It typically allows a further 3:1 reduction over any initial spatial (intraframe) compression. Visual perception The human retina and visual cortex together process the visual scene in a way that detects edges and lines. This allows objects (like potential prey or predators) to be rapidly separated from the background. A consequence of this is that codecs that destroy or create edges will be viewed as creating perceptible distortions. Another feature is that fine detail near the edges of objects is not perceived with great acuity. So there are opportunities for perceptual redundancies and potential pitfalls where distortions are easily noticed. A good compression architecture has to exploit the mechanisms of visual perception. Compression algorithms Compression algorithms aim at lowering the total number of parameters required to represent the signal, while delivering a reasonable quality picture to the player. These parameters are then coded into data packets for streaming. There are four main redundancies present in the video signal: Spatial Temporal 80 The Technology of Video and Audio Streaming

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Below 60 fps this becomes noticeable. To avoid flicker in the movie theater each film frame is shown twice, giving a display rate of 48 fps. The perception of flicker is reduced at low luminance levels. At the low levels of light used for projection in the theater, flicker is not a problem, even though it is lower than the optimal rate. At the higher luminance levels used with television and VDU displays, refresh rates of at least 60 Hz must be used to avoid flicker. Television uses a similar system to film, but rather than repeating each frame twice it splits each picture into two one half of the odd lines and the other of the even lines in a process called interlaced scanning. These half-pictures are called fields; two fields make a picture of frame. Because European television systems use a 50-Hz field rate, under certain viewing conditions the flicker can be apparent. Compression is always a trade-off between the level of artifacts and the bandwidth. Note that the shooting or encoding rate (chosen for realistic motion portrayal) can be different from the final display rate. We watch a film shot at 24 fps on a television receiver at either 25- or 30-Hz frame rates (50- or 60-Hz field rates). Compression basics A video clip is a sequence of pictures or frames. Each picture can be processed in isolation, much like a still image. A typical example is the JPEG standard. Still images JPEG The JPEG format is widely used to compress continuous-tone grayscale and color images. JPEG compresses the file size by selectively discarding picture data. This is called lossy compression. The final picture is a representation of the original, but the original can never be restored from the JPEG file. The standard also supports a lossless compression with about a 3:1 reduction in data, but it is more commonly used in the lossy mode with ratios of 20:1 or greater. JPEG compression is based on a technique called the discrete cosine transform. A lower compression ratio results in less data being discarded, but the JPEG compression algorithm will degrade any fine detail in an image. As the compression is increased, more artifacts become apparent. Wave-like patterns and blocky areas become visible, and there is ringing on sharp edges. The levels of artifacts that are considered acceptable depend upon the application. A picture reproduced in a magazine should have no visible artifacts, whereas minor distortions are to be expected with a thumbnail on a web page. It is more important to have a small file size with a short download time. Video compression 79

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5 Video compression Introduction Compression reduces the number of bits used to represent each pixel in the image. The algorithms used to compress the images introduce, to a lesser or greater extent, visible artifacts. In general, the more complex algorithms introduce fewer of these artifacts. The processing power available to us increases with time, so more sophisticated algorithms always are being introduced into the codec products. This has to be done in a way that uses realizable computation, and has a short processing delay. The processing should not distort the image so that visible artifacts are generated. As the level of artifacts decrease, so does our acceptance of the distortions. Once we were happy to watch monochrome television; now it must be color. Families were thrilled with the flickering home movies shot on 8 mm film; now a DV camcorder produces almost broadcast-quality results. Even so, the acceptable level of artifacts will depend on the application. If you paid to go to a movie theater to see the 70 mm print with Dolby Digital and they showed you a projection from a VHS tape, you would consider the picture unacceptable. Compression systems exploit the mechanisms of human visual perception to remove redundant information, but still produce a compelling viewing experience. A typical example of the process is the persistence of vision, where an instantaneous view of a scene (for example, through a shutter) fades over about one-tenth of a second. This allows us to portray the continuum of time by a series of discrete pictures or frames. As an example, viewing a motion picture shot at 24 frames per second (fps) gives a good approximation of motion in the original scene. Now there are artifacts a classic is aliasing; for example, the wagon wheels of the stagecoach appearing to turn backward. Another potential artifact is flicker, where the brain is conscious of the discrete frames. If the refresh rate of the display drops below a certain threshold, the viewer will observe flicker.

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Summary Video formats have evolved from the standard definition color format developed by the NTSC in the late 1940s. This standard established the interlaced scan and the composite color encoding. Later, videotape recorders were developed. The pace of new formats has quickened, but the DV family has stood the test of time and has proved very popular for streaming video production. Future camcorders will use optical disk or memory cards as alternatives to tape, while retaining DV or MPEG compression. Analog television is being replaced by digital formats, although analog composite is still the most used delivery format for the terrestrial receiver. The defining standard is BT601; this set the rules for the sampling structure that we see in MPEG compression. Digital capture has a great advantage for streaming: there is less noise than analog systems. This makes for a better picture quality at a given bit rate. The compression codec and noise do not mix. Domestic television formats are moving to higher definition standards, up to 1080 lines, and electronic cinematography is adopting a frame rate of 24 to be compatible with film, so there is ever-increasing diversity in standards and in tape formats. For the streaming video producer, this means cross-format conversion is likely to be part of the production workflow. Video formats 77

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Which interface to use The priority for streaming interconnects should be as follows: 1. BT601 2. Analog S-video 3. Analog composite If the source were using DV compression, then IEEE 1394 would be the first choice for a file interchange or streaming. High definition Everything in this chapter so far relates to standard-definition (SD) television. Although standard definition is great for small and medium-sized displays, once the screen diagonal gets above 750 mm the lines become very visible. Projection receivers and home theater systems demand higher resolution to equal the experience of watching 35-mm film. The Advanced Television Systems Committee (ATSC) set about defining a new standard for high-definition television. The end result was a family of possible resolutions, leaving the broadcaster to make the choice. Both progressive and interlaced scanning are supported with 720 and 1080 being the line standards. At the same time, studios were using digital processing for visual effects sequences in movies. Often using 2,048 1,556 or 4,096 3,112 pixels per frame (called 2k and 4k, respectively), such sequences can be composited seamlessly with the film stock. There had long been a demand to shoot productions on videotape rather than using film that is, electronic cinematography. Rather than shoot at 30 frames per second, the same rate as film is used, which is 24 frames per second. What has all this to do with streaming? Not much. An ATSC transmission uses a data rate of 18 Mbit/s, a long way from the requirements of a streaming file for less than 1 Mbit/s. If you are given HD source material you will have to convert the material to SD for encoding, a process called down-rezzing lowering the resolution. Maybe one day we will be able to view HD streaming, but that will have to wait for fiber-to-the-home and improved compression techniques. 76 The Technology of Video and Audio Streaming

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Video formats 75 IEEE 1394 (FireWire, i.LINK) BNC RCA phono XLR S-Video Figure 4.10 Video and audio connectors. chained to a maximum of 17 devices. Each device regenerates the signal. If any device in a chain is turned off, the internal IEEE 1394 buffer is powered from the cable. The four-pin does not carry power and is used on self-powered cameras. It can be hot-plugged, and likened to a high-speed USB connection in its simplicity of use. The data rates that are supported are 100, 200, and 400 Mbit/s for the original standard, and 800 Mbit/s for IEEE 1394b. The interface carries not only audio/video data, but also remote control. So the capture card can stop, start, and cue the VTR over the IEEE 1394 cable. This makes for a very simple interconnection between the recorder and the encoding workstation. Note that IEEE 1394 is not DV, although often called such, but it can carry DV compressed signals from a DV format camcorder.

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Interconnection standards Television companies have always preferred simple, robust interfaces that use coaxial cable and connectors. This is different from the computer industry, which has migrated toward the use of twisted pairs. Broadcasters prefer a single circuit, whereas computer interconnects frequently use multiple pairs. Any standard that departs from the single coaxial cable tends to be used for short interconnects; for example, a few meters. This policy is due partly to cost coax can be terminated very easily and quickly and partly due to a vast legacy of coaxial cable around the television plant. Analog composite S-Video or Y/C SDI (BT601) IEEE 1394 Analog composite Analog composite uses the familiar 75 W coaxial format, with BNC connectors. Some consumer equipment uses RCA connectors. Analog composite is the defacto standard interconnect; even today many low-cost video monitors feature only composite inputs. S-Video, Y/C S-Video is a spin-off from the S-VHS format. The chroma component, modulated on the subcarrier, is carried on a separate circuit from the luminance. This is to avoid the artifacts that result from the frequency interleaving of composite video. It uses a small four-pin connector. BT601 BT601 uses 75W coaxial cable, but the high frequency (data rate 270 Mbit/s) demands better quality cable than analog composite. Belden makes cable designed especially for 601. IEEE 1394 Originally developed by Apple as FireWire, it was adopted by IEEE as a standard and now is used by many other manufacturers. Sony uses the tradename i.LINK. There are two versions of the connector, a six-pin and a four-pin. The cable uses two pairs for data and one for power. The system can be daisy- 74 The Technology of Video and Audio Streaming

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