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[ Notes 7 ].doc [ Notes 7 ].doc Presentation Transcript

  • Introduction to Media Production Video Basics Topics cover ed in this lectur e  Video Quality  Frame Rate  Less bandwidth Resolution  Compression  Codec  How Video Cameras Work  Digital Video and Videotape Standards  Video Storage Media  Streaming Media 1/37
  • V ideo Quality Video quality derives from three factors: Frame rate, Resolution, and Compression. As the frame rate and resolution increase and the compression decreases, quality rises dramatically. However, it does so at a cost: bandwidth and storage. Bandwidth (also known as bit rate) Bandwidth refers to the amount of data that a system can push through some communications device at any time, e.g. bit/second. As the bandwidth of the Internet is limited, the bigger the bandwidth the video requests, the slower download time. 2/37
  • IEEE-1394 (FireWire or I-LINK) offers another demonstration of bandwidth efficiency. It allows you to transfer high-quality video in real time from your camera to your computer. The USB ports also allow near-1394 efficiency because they offer higher bandwidth capacity. Storage refers to the total space used to store your video. A small, highly compressed, low-frame- rate video might take up a few megabytes. In contrast, a longer, better-quality, high-frame-rate feature might occupy literally tens or hundreds of gigabytes. The better the video, the higher the bandwidth and storage demands will be. You must balance production values with your computer’s limitations. Fr ame Rate Many people have tried to explain the magic of movies and video. Some attribute it to a physiological feature called persistence of vision. In persistence of vision, our eyes and brain try to 3/37
  • hold onto a series of sequential images, forming the illusion of motion. Others point to more general cognitive functions that allow us to integrate and interpolate movement. When we’re shown a series of pictures at a certain rate, they begin to fuse together into an illusion of smooth movement. This rate is called the critical fusion frequency (CFF) which refers to the rate at which the screen needs to refresh in order to avoid flicker and allow the picture to appear steady to the viewer. As a rule of thumb, the higher the frame rate, the smoother the motion you’ll experience. (1) At 10fps offers the absolute lowest frame rate that lets us perceive smooth motion. Video that is slower than 10fps appears jerky with a strong jitter. (2) At about 24fps to 30fps, video runs smoothly without perceptible roughness. Motion pictures project film at 24fps. Most TV systems broadcast at approximately 30fps. 4/37
  • Of course, as the frame rate increases, so will the total number of frames you’ll need to process and store per second. More frames mean more disk space and, if you plan to broadcast over the Internet, more bandwidth to transmit your video. Figure 3.6: Higher frame rates require more bandwidth and storage. Reducing the number of frames per second reduces both storage and bandwidth requirements. Less Bandwidth Resolution 5/37
  • Resolution refers to the amount of picture detail in each video frame. The terms horizontal resolution and vertical resolution refer to the number of pixels that lie, respectively, along the width and the height of a frame. As the resolution increases, you see more picture detail, so the image becomes clearer and easier to view. Of course, as the resolution increases, so does the size of each frame, and that brings us back to bandwidth and storage. Large frames occupy a lot of bandwidth and storage space. If you increase frame resolution by as little as 10 percent, you end up increasing the size of your entire movie by 10 percent. A 10 percent increase in frame resolution isn’t really much improvement, but a 10 percent increase in movie size can be huge. 6/37
  • Compr ession Compression allows you to store video data more efficiently by squeezing the picture down to a manageable size. With compression, your video presents similar quality picture while using a smaller amount of storage. As the compression rates rise, storage requirements decrease. Pictures fit into a smaller space. While this might seem like an ideal solution for both bandwidth and storage issues, you must take two key compression factors into consideration: lossiness and decompression time. These factors greatly affect the quality of the final video. Lossiness refers to a quality decrease produced by the compression process. Lossiness means that the picture you see when you decompress won’t be exactly the same picture you originally compressed. 7/37
  • Time is the other factor associated with compression. A compressed video needs to be restored (decompressed) to be viewed. It takes time—often a lot of it—to decompress your video. If this is done before watching the video, there’s no problem. However, if you need to watch the video at the same time as you decompress it (watch it real time), decompression time makes a difference. Few things are harder to watch than a video that keeps pausing to decompress the next segment. Anyone who has attempted to watch a DVD on an old, slow computer with software decompression has had this experience. Standard digital video uses a 5:1 compression ratio. Web Video might be compressed as much as 50:1 or even 100:1. 8/37
  • Codec Codec refer to a large variety of algorithms/computer programmes (e.g. that COmpress and DECompress video and other types of images. There’s nothing magical or remarkable about codec. They’re simply a number of standards that people have developed to optimize some playback feature or another. Some codec produce highly compressed, fast-playing videos. Others offer excellent motion reproduction or good-quality stills. If we didn’t need to compress video, we wouldn’t need to use codec at all. As it is, each codec serves a different purpose. The table below lists some of the more common codec currently in use. 9/37
  • Common Codec Codec Application DV Hardware-encoded consumer video MPEG-1 Primarily VCDs MPEG-2 DVDs and VCDs MPEG-3 HDTV (High-definition television) MPEG-4 High-quality Web video Real Media Real-time streaming Web video Windows Media Video Real-time streaming Web video Cinepak High-motion video, works on older computers Intel Indeo 3 Low-motion video, works on older computers Sorenson High-quality video for CD-ROM Indeo Video Interactive High-quality video for CD-ROM MPEG-3, with a resolution of 1920 X 1080, was originally designed to support HDTV. However, it was soon discovered that similar results could be obtained through slight modifications to the MPEG-2 standard. Shortly thereafter, work on MPEG-3 was discontinued. 10/37
  • Compression and decompression times are key factors when using codec. Some codec, known as Asymmetric Codec, are built so that they take a long time to compress video, but they decompress video rapidly. This allows the viewer to watch movies in real time, even though it might have taken a long time to compress each minute. The various Motion Picture Experts Group (MPEG) formats are good examples of asymmetric codec. 11/37
  • Types of Compr ession Today’s technology offers two basic flavors of video compression: Intraframe and Interframe Intraframe compression squeezes down a single frame at a time, without reference to any other frame. This method, also called spatial compression, searches for solid blocks of color within an image and compresses those areas. It also compares the even and odd lines of the image (the fields) and compresses the picture using field similarity. 12/37
  • Frame 1 Frame 2 Frame 3 Interframe compression uses the similarity between sequential frames to save space. Rather than store entire frames, the computer stores just the differences between one frame and the next. Interframe compression is also called temporal compression. Frame 1 Frame 2 Frame 3 Encoding standards may use one or both of these methods. 13/37
  • Standard digital video, for example, uses only intraframe compression. The MPEG-2 standard, found on commercial DVDs, uses both. Table below summarizes some common video compression formats. Table 3.3: Common Video Compression Formats Format Resolution Compression Type MPEG-1 352 X 240 Intraframe MPEG-2 Ranges: Interframe and intraframe (1) 352 X240 (2) 720 X 480 (typically, DVD) (3) 1920 X 1080 (HDTV) DV (DV-25) 720 X 480 Intraframe Cinepak Varies Interframe and intraframe Intel Indeo 3.2 Varies Interframe and intraframe 14/37
  • MPEG-2 Interfr ame Compr ession Basics MPEG-2 is used so widely that it helps to know how it works. MPEG-2 uses three types of frames: I frame (intraframe) This is a type of key frame. Key frames derive directly from the video and are not calculated from other frames. I frames are the largest frames and must store the most data. P frame (predictive) This frame is derived from the frame before it and specifies how it differs from the previous frame. P frames are smaller than I frames, requiring much less data storage. P frames are a type of difference frame. All difference frames are calculated from other frames, so they store much less data per frame. B frame (bidirectional) This frame is computed from both the frames before and after it. B frames are the smallest of the three frame types. Like P frames, B frames are difference frames. 15/37
  • MPEG-2 frame sequences can include any combination of I, P, and B frames. Most encoders use a prebuilt fixed pattern such as IBBPBBPBBIBBPBBPBB. Technologically advanced encoders, in contrast, will try to optimize the frame placement based on the quality and features of the video itself. The key frame frequency helps determine both the size and the quality of the compressed video, as illustrated below. More I key frames preserve image quality but at the cost of greater size and bandwidth. 16/37
  • With MPEG-2 compression, as key frame rates increase, so do image quality, storage, and bandwidth. MPEG-2 compares successive frames by dividing pictures into blocks. When successive blocks prove sufficiently dissimilar, the change is recorded onto a difference frame, either a B or P frame. As illustrated in Figure 3.10, the difference frame stores only those parts of the picture that have changed. 17/37
  • Difference frames store only those video blocks that have changed between successive frames. 18/37
  • Digital V ideo Basics We have discussed the video technology basics. We now discuss how digital video cameras work, digital video standards, and some drawbacks of digital video. How Video Camer as Wor k All video cameras operate in much the same way. Light enters the camera. The camera lens focuses this light onto a detector. The detector creates an electrical signal. A magnetic tape or hard-disk records this signal. Nearly every video camera on the market uses the same type of detector, called a charge couple device (CCD). CCDs transform light into levels of electricity that 19/37
  • can be recorded onto a tape. It is how this information is recorded that differentiates analog and digital video cameras. Almost all single-CCD DC use a Bayer mask over the CCD. Each square of four pixels has one filtered red, one blue, and two green. The result of this is that luminance information is collected at every pixel, but the color resolution is lower than the luminance resolution. Three-CCD cameras have three separate CCDs, each one taking a separate measurement of red, green, and blue light. Light coming into the lens is split by a trichroic prism assembly, which directs the appropriate wavelength ranges of light to their respective CCDs. By taking a separate reading of red, green, and blue values for each pixel, three-CCD cameras achieve much better precision than single-CCD cameras. 20/37
  • 21/37
  • Three-CCD cameras are generally regarded to provide superior image quality to cameras with only one CCD. By taking a separate reading of red, green, and blue values for each pixel, three-CCD cameras achieve much better precision than single-CCD cameras. The very best cameras use larger CCDs to stabilize your image. Analog video cameras record a signal that represents image brightness directly to tape. Signals range as low or as high as the detector can measure, and they can include any level within that range. Digital video cameras, in contrast, can record only specific levels. You might wonder why this is considered an advantage. After all, an analog camera can record an almost infinite range of values representing the signal, while the digital camera plods along with whole levels such as 15, 16, 17, 18, and so on. 22/37
  • The advantages of using digital format for video: 1) Digital signals allow you to transfer your video data directly to a computer. Once you import your video, you can edit, enhance, and play with it using non-linear editing. 2) Digital signals guarantee data accuracy. If you record level 252, you’re going to get level 252 played back. There’s no approximation or round-off error with digital. Digital video cameras also use built-in error correction to make sure that the data you record is as perfect as can be. 3) Digital produces no generation effects. Each time you record to a new copy, you’re guaranteed to reproduce the original exactly. You do not lose any information or gain unwanted noise. 4) If you add a tiny bit of noise to an analog signal, it sticks. You cannot get rid of it, no matter how you try. With digital, you change the digital value of the audio or video. 23/37
  • 24/37
  • Digital Video and Videotape Standar ds A consortium of electronic companies joined together to form a standards committee for consumer- grade digital video. This group, called the DVC Consortium, included many high-profile companies such as Sony, Philips, Panasonic, Hitachi, and Sharp. As of today, more than 60 companies have joined. DV-25 features:  Stores video information using a 25-megabit per second (Mbps) data stream. This streaming technology is better known as DV-25.  All DV25 formats record what is called 4:1:1 video, which is compressed at a 5 to 1 ratio and recorded on a narrow tape only 1/4" wide. This results in a data rate of 25 megabits per second. 25/37
  •  The actual data stream associated with DV-25 can run as high as 36Mbps after you add in audio, time codes, error correction, and other features.  Uses intraframe compression to store data. This means that each video frame is compressed individually, without using data from the previous frame or next frame.  Includes interfield compression, because each video frame is composed of two halves, called fields. This allows tighter compression at the expense of rapidly moving objects. Because of this, the standard sometimes produces a slight blockiness effect near moving edges.  Uses 720 samples per scan line. You’ll find 525 of these lines on a standard TV screen (625 if you live outside the U.S. in a country that supports the PAL format). In the U.S., you’ll encounter video frames whose sizes are at best 720 pixels wide by 525 pixels high. 26/37
  • MiniDV The DVC Consortium also developed a tape standard called MiniDV. MiniDV provides the most widely used standard for digital video cameras. MiniDV offers excellent resolution and color- recording capabilities. Many consider MiniDV recordings to be of broadcast quality. Digital8 is a standard introduced by Sony to offer backward compatibility with the older Hi8 and Video8 formats. Digital8 cameras can play these older tapes, automatically converting them to digital video format. Digital8 tapes can prove a bit harder to find than the pervasive MiniDV tapes and often cost a bit more. 27/37
  • Digital V ideo Dr awbacks For all its promise, digital video technology does have some drawbacks. (1) Feathering and dot cr awl When you look at fine detail on digital video, you may find a slight feathering or crawl effect. You’ll see this effect near the sharp edges of text, along natural diagonal lines, and on other edges. This effect is caused by compression and is most visible when you place white lettering on a blue background. (2) Motion blocking This effect occurs when an on-scene object moves quickly. Recall that the camera first records one field of a frame, and then records the next field. When your digital video camera 28/37
  • compresses these two fields after a rapid change, it needs to balance the difference in fields with the amount of space it has available to store the picture. When fields are similar, the data compresses easily. When fields differ due to movement, the digital video format must save space to fit in that extra information. It does this by using a cruder compression scheme; hence, you get the blockiness that borders the object and tends to travel with it across the screen. (3) Banding Banding results from a dirty camera record head. Digital video cameras have two record heads. When one record head becomes soiled and cannot read data, you end up with a striped image. You can diagnose this problem right away. You’ll see 10 bands across your screen. Every other band will show a live picture. When this occurs, try using a head-cleaning tape. If that doesn’t work, you may need to service your camcorder. 29/37
  • V ideo Stor a ge Media V ideotape Videotape may seem “old hat,” but it provides a cheap, convenient, and easy-to-distribute medium for your videos. Of all the media, videotape allows you to share with the widest audience. VCRs are an almost universal technology. VCDs Video compact discs (VCDs) are CDs that store video. VCDs have proven popular in the Far East. VCDs come in three flavors: 30/37
  • • Standard VCDs use MPEG-1 compression to store video, allowing a single CD to hold approximately 74 minutes. • Super VCDs (SVCDs) use a variable bit rate and can store video using either MPEG-1 or MPEG-2 compression. With a higher resolution—480 X 480 for NTSC and 480 X 560 for PAL —SVCDs generally store less video, typically around 30 minutes’ worth. • Extra VCDs (XVCDs or XSVCDs) are like SVDs with higher bit rates, up to 3.5Mbps. This standard is fairly new and not widely supported. Many people use the term XVCD to refer to any nonstandard VCD format. DVDs and Recor dable DVDs DVD stands for digital versatile disc. In many ways, DVDs can store video as well as audio and data. DVDs use MPEG-2 compression to provide high-quality video and audio. The DVD player has found broad acceptance, becoming the most rapidly adopted consumer-electronics product ever. 31/37
  • Early in 2001, Compaq and Apple led the pack, introducing Pioneer’s DVD-R drives. Soon after, Philips introduced a competing DVD+RW standard, which has not competed well against the more successful DVD-R/DVD-RW standard. A DVD can hold 4.7GB per side of the disc, and 9.4GB data on a double-sided DVD disc. HD DVD HD DVD (High-Definition DVD) is a optical disc format designed for high-definition video (up to such as 720p, 1080i and 1080p) . The HD DVD disc is designed to be the successor to the standard DVD format. It can store 15 GB data per layer (a dual-layer capacity of 30 GB). The HD DVD standard was jointly developed by a group of consumer electronics and PC companies, spearheaded by Toshiba. 32/37
  • HD DVD can be mastered with up to 7.1 channel surround sound. Audio can be encoded using linear (uncompressed) PCM, Dolby Digital, Dolby Digital EX, DTS and DTS ES formats also used on DVDs. In addition, it also supports the new lossy formats Dolby Digital Plus and DTS-HD High Resolution Audio, also the lossless formats Dolby TrueHD and DTS-HD Master Audio. Blu-r ay Blu-ray Disc is the next-generation disc format that is jointly developed by the Blu-ray Disc Association (BDA). The BDA include a group of the world's leading consumer electronics, personal computer and media manufacturers (including Apple, Dell, Hitachi, HP, JVC, LG, Mitsubishi, Panasonic, Pioneer, Philips, Samsung, Sharp, Sony, TDK and Thomson). 33/37
  • The format was developed for HD video, as well as storing large amounts of data. The Blu-ray can hold up to 25GB on a single-layer disc and 50GB on a dual-layer disc. The Blu-ray format uses a blue-violet laser which has a shorter wavelength than a red laser (used by DVD), which makes it possible to focus the laser spot with even greater precision. This allows data to be packed more tightly and stored in less space. Despite the different type of lasers used, Blu-ray products can easily be made backwards compatible with CDs and DVDs through the use of a BD/DVD/CD compatible optical pickup unit. Blu-ray is currently supported by more than 180 of the world's leading consumer electronics, personal computer, recording media, video game and music companies. The format also has broad support from the major movie studios as a successor to today's DVD format. In fact, seven of the eight major movie studios (Disney, Fox, Warner, Paramount, Sony, Lionsgate and MGM) are 34/37
  • supporting the Blu-ray format and five of them (Disney, Fox, Sony, Lionsgate and MGM) are releasing their movies exclusively in the Blu-ray format. Str eaming Media With streaming media, you can broadcast your movies to others in real time thru the Internet. A host, called a server, stores your movie online, to be transmitted upon request. Three standards dominate this arena: Real Networks It has made a name for itself with its Real Video offerings. Real Networks focuses exclusively on delivering streaming content over the Internet in real time. Real Video uses a variety of data compression techniques and works with both normal IP connections as well as IP Multicast 35/37
  • connections Its SureStream technology provides excellent-quality delivery for both live and on- demand product. QuickTime It is a video and animation system developed by Apple Computer. QuickTime is built into the Macintosh operating system and is used by most Mac applications that include video or animation. PCs can also run files in QuickTime format, but they require a special QuickTime driver. QuickTime supports most encoding formats, including Cinepak, JPEG, and MPEG. QuickTime is competing with a number of other standards, including AVI and ActiveMovie. In February 1998, the ISO standards body gave QuickTime a boost by deciding to use it as the basis for the new MPEG-4 standard. 36/37
  • Windows Media Microsoft proves to be the third, and arguably the biggest, player in the streaming video world. It supports Windows-based movies. These technologies promise flexible, powerful, and exciting methods for sharing video. 37/37