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Compression Primer Introduction This primer is for anyone learning how to prepare high-quality compressed video files. It is for web and video professionals who are new to compression, and for people who want to improve their skills. Compression is often performed at the last minute, so feel free to skip to the section covering the format and codecs you care about. WHITE PAPER TABLE OF CONTENTS 1 Introduction 2 Getting started with compression 3 Understanding the fundamentals of c
  Compression Primer Introduction This primer is for anyone learning how to prepare high-quality compressed video files. It is for web and video professionals who are new to compression, and for people who want to improve their skills. Compression is often performed at the last minute, so feel free to skip to the section covering the format and codecs you care about. WHITE PAPER TABLE OF CONTENTS 1 Introduction2 Getting started with compression3 Understanding the fundamentals of compression4 Types of compression5 Frame types and compression6 Color modes6 Color depths7 Color sampling8 Audio compression9 Compression efficiency9 Authoring video for future compression12 Capturing video: choosing formats and connections13 Choosing the right capture codec14 Exporting video15 Understanding preprocessing17 Cropping17 Scaling18 Noise reduction19 Image processing20 Audio processing21 Encoding25 Delivery methods for video27 Choosing a compression format28 Exporting to Windows Media format29 Windows Media players30 Windows Media encoding modes32 Windows Media video codecs33 Windows Media audio codecs34 Exporting to RealMedia format34 RealMedia video codecs35 RealMedia audio codecs36 Exporting to QuickTime format37 QuickTime video codecs42 QuickTime audio codecs  2 Compression Primer Compression efficiency  The top two rows of uncompressed video are substantially larger in size and shorter in duration than the compressed video in the last five rows. Getting started with compression The best starting point for learning about how to achieve great results from compression is to understand why and how it’s done. While the mathematical bases of compression are very com-plex, the basic principles are quite approachable. The main reason you need to compress video is because uncompressed video is far too large to transport effectively. Bitstream typeData rate (Kilobits per second)Time on one 700 MB CD-ROM Uncompressed high definition (1920 x 1080 29.97 fps)7457507.5 secondsUncompressed standard definition (720 x 486 29.97 fps)16779433 secondsDV25 (miniDV, DVCAM, DVCPRO)250003 min, 44 seconds Typical DVD500018 min, 40 secondsVideo CD116780 minutesBroadband web video100-2000(500 Kbps)3 hours, 8 minutesModem web video18-48(32 Kbps)48 hours, 37 minutes  The ratios between data rates used for authoring video and for distributing compressed video are huge. For example, converting from digital video (DV) to DVD typically entails a 5:1 com-pression, and converting an uncompressed source to a modem-compatible video uses about 9000:1 compression. That leaves only about 0.1% of the srcinal data left. It’s almost amazing that video on the web looks and sounds as good as it does!In addition, different types of compressed video vary in bit rate. For example, in web video, there is about a 100:1 ratio between the low end of modem rates to the high end of broadband rates. Compression and DVD formats More Americans watch compressed video on DVD than on any other media. (Digital broadcasts over cable and satellite are close runners up.) Modern DVD-R burners can put 4.7 GB of data on a disc (called a DVD-5). Manufactured DVDs can use a dual-layer format that can hold 8.54 GB (called DVD-9) of data, which burners do not support yet. Even with a DVD-5, it’s possible to put a high-quality two-hour movie on a disc.The kind of DVD that plays in a DVD player is more accurately known as DVD-Video. There is also DVD-ROM, which is a computer-formatted disc that requires a computer to play it—in effect, it’s a large capacity CD-ROM. There are also hybrid DVD-Video/DVD-ROM discs, which play in normal DVD players, but also include information only available for computer playback.DVD-Video discs are mastered in either NTSC (the format used in the U.S. and Japan) or PAL (the format used in Europe and Asia). A set-top player (such as your home DVD player) for one format typically won’t play discs in the other format; however, computers can typically play discs of both types. DVDs can also be region coded, so they only play in DVD players or soft-ware intended for a certain region. Region coding is an optional feature. 45 Exporting to MPEG-1 format46 MPEG-1 video and audio codec46 Exporting to DVD and MPEG-2 formats47 MPEG-2 audio codecs48 Exporting to MPEG-4 format50 MPEG-4 audio codecs51 Exporting to AVI format51 AVI video codecs52 AVI audio codecs  3 Compression Primer Compression and VideoCD formats VideoCD is an older disc-based format. It has a much lower resolution and data rate than DVD, and it provides lower quality. However, it uses standard CD media, so VideoCD discs can play in a vast variety of devices, including most personal computers. A manufactured VideoCD can play in any DVD player.Like DVD, VideoCD discs can be either NTSC or PAL. VideoCD is very popular in PAL countries, especially in Asia. Also like DVD, software players support both formats, and many set-top players only support one. Compression and CD-ROM formats Starting in the early 1990s, CD-ROM video was a popular medium. While the web has largely replaced CD-ROM as a medium for interactive content, people still use CD-ROM video for many applications, like computer games and kiosks. Compared to web video, CD-ROM offers much higher bandwidth than broadband connections and is capable of DVD quality on fast computers. Compression and Internet formats Video on the web is the hottest area for ongoing development of compressed video technologies. The main reason is that the bandwidth available on the web is much less than on discs, so it is critical to make every bit count. Another factor is that it’s possible to distribute updated player software to computers, which isn’t possible with consumer electronic devices, like a DVD.Web video can use either the real-time streaming mode or progressive download mode. Real-time streaming files play in real time but require specialized streaming server software and are limited in quality by the available bandwidth. Progressive download files come off standard web servers and may take a long time to download, but they may provide higher and more reliable video and audio quality. Understanding the differences between these modes, and the strengths and weaknesses of each, is a major theme of this Primer. Compression and mobile devices Mobile devices, such as personal digital assistants (PDAs) and modern cell phones create a new area for compressed video. These devices have small screens and slow processors, even compared to a laptop, but the ability to play video back on a device with a weight measured in ounces (not pounds) can have a lot of value for some markets.Most formats for compressed video have players for some mobile devices, including MPEG-4, Microsoft® Windows Media, and RealMedia. Dedicated mobile formats, like Kinoma, are also available.In many ways, video on mobile devices, especially cell phones, is reminiscent of the early days of video on desktop computers. The technology is still evolving, but it’s amazing to see it work. Today, there aren’t any universal formats or even settings for mobile devices. Instead, content has to be tuned to a small class of devices or even a particular model. But when that works, the results can be great. Mobile video is rapidly maturing and should become easier to compress at a higher quality over the next few years. Understanding the fundamentals of compression Compression tools and formats hide the mathematical complexity of compression from the user, but to achieve the best results, you need to understand the fundamentals of why and how compression tools and formats operate.  4 Compression Primer How we see and hear Lacking video adapters and audio ports in our heads, we don’t directly perceive video or audio, compressed or not. Instead, we use our senses of sight and hearing, which are powerful, but have limitations that make compression possible.The eye is often described as a camera, but the human visual system includes both the eye and the brain and is massively more complicated and intertwined than a simple CCD-to-tape device. The whole sensory system is tuned according to the evolutionary environment of our ancestors. Thus, humans are skilled at noticing motion, like a saber-toothed tiger jumping out of a jungle, or the difference between ripe and unripe fruit by color. But we aren’t skilled at see-ing high motion and detail at the same time, or high motion and color at the same time. In the same way, hearing is powerful, but it has its limitations. We may easily hear a sound in a quiet environment but not hear it in a loud one. Human ears are best attuned to the frequencies of human speech and are less aware of higher or lower tones. Compression technologies take advantage of the strengths and weaknesses of human senses to spend the available bits on the information that humans perceive more acutely. How compression works Fundamentally, compression works by summarizing the sequences of images and sounds as efficiently as possible. Files encoded for delivery don’t provide an identical representation of the source back to the user—the data rates would be far too high. Instead, the encoded files describe the important details of the content as tersely as possible while still providing a reproduction that, to the human sensory system, provides the experience of the srcinal as accurately as possible.A matched pair of a compressor and a decompressor, better known as a codec  , performs the compression. It’s important that the pair be matched because the decoder (decompressor) needs to understand the encoder’s (compressor’s) summary of the data. Types of compression Video codecs can use spatial compression, temporal compression, or a combination of both. Spatial compression Spatial compression affects only a single frame at a time, also called intraframe compression . Most authoring codecs only use spatial compression, so there aren’t any interdependencies among frames. This type of compression makes random access and editing easy.Spatial compression finds redundancy within a frame—either in areas of flat or similar colors or areas of a frame that look like other areas of the frame.Many codecs are based around a technique called Discreet Cosine Transformation (DCT). These codecs deal very well with smooth gradations in an image, but they can have trouble encoding random details or sharp edges at lower bit rates. DCT, or similar techniques trying to encode overly complex images at too low of a data rate, cause two of the most common kinds of artifacts: ringing and blocking. One common area of trouble is small text with sharp edges. Some codecs use very different base technologies, like wavelet, and therefore deal better with sharp edges. Temporal compression Temporal compression adds the ability to use other frames as a reference for the current frame. In most video, a majority of the frames are similar enough to the frame previous to them that temporal compression is applicable.


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