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DVD video details

We will look at the details of video and audio in general in Chapter 6 and compression (MPEG formats) in Chapter 7. These concepts are mentioned below and will be explained in further detail in these later chapters.

DVD-Video is an application of DVD-ROM. DVD-Video is also an application of MPEG-2. This means the DVD format defines subsets of these standards to be applied in practice as DVD-Video. DVD-ROM can contain any desired digital information, but DVD-Video is limited to certain data types designed for television reproduction.

A disc has one track (stream) of MPEG-2 constant bit rate (CBR) or variable bit rate (VBR) compressed digital video. A limited version of MPEG-2 Main Profile at Main Level (MP@ML) is used. MPEG-1 CBR and VBR video is also allowed. 525/60 (NTSC, 29.97 interlaced frames/sec) and 625/50 (PAL, 25 interlaced frames/sec) video systems are supported. Coded frame rates of 24 fps progressive or interlaced from film, 25 fps interlaced from PAL video, and 29.97 fps interlaced from NTSC video are supported. In the case of 24 fps source, the encoder embeds MPEG-2 repeat_first_field flags into the video stream to make the decoder either perform 3-2 pulldown for 60 (59.94) Hz displays or 2-2 pulldown (with 4displays. In other words, the player doesn't really know what the encoded rate is, it simply follows the MPEG-2 encoder's instructions to arrive at the predetermined display rate of 25 fps or 29.97 fps. (No current players convert from PAL to NTSC or NTSC to PAL.) See the Chapter 7 for more information on MPEG-2 video.

Picture dimensions are max 720x480 (29.97 frames/sec) or 720x576 (25 frames/sec). Pictures are subsampled from 4:2:2 ITU-R 601 down to 4:2:0, allocating an average of 12 bits/pixel. (Color depth is still 24 bits, since color samples are shared across 4 pixels.) The uncompressed source is 124.416 Mbps for video source (720x480x12x30 or 720x576x12x25), or either 99.533 or 119.439 Mbps for film source (720x480x12x24 or 720x576x12x24). Using the traditional (and rather subjective) television measurement of lines of horizontal resolution DVD can have 540 lines on a standard TV (720/(4/3)) and 405 on a widescreen TV (720/(16/9)). In practice, most DVD players provide about 500 lines because of filtering. VHS has about 230 (172 w/s) lines and laserdisc has about 425 (318 w/s).

Different players use different numbers of bits for the video digital-to-analog converter. (Sony and Toshiba use 10 bits, Pioneer and Panasonic use 9 bits.) This has nothing to do with the MPEG decoding process. It provides more headroom and more analog signal levels which supposedly give a better picture.

Maximum video bitrate is 9.8 Mbps. The average bitrate is 3.5 but depends entirely on the length, quality, amount of audio, etc. This is a 36:1 reduction from uncompressed 124 Mbps (or a 28:1 reduction from 100 Mbps film source). Raw channel data is read off the disc at a constant 26.16 Mbps. After 8/16 demodulation it's down to 13.08 Mbps. After error correction the user data stream goes into the track buffer at a constant 11.08 Mbps. The track buffer feeds system stream data out at a variable rate of up to 10.08 Mbps. After system overhead, the maximum rate of combined elementary streams (audio + video + subpicture) is 10.08. MPEG-1 video rate is limited to 1.856 Mbps with a typical rate of 1.15 Mbps.

Still frames (encoded as MPEG-2 I-frames) are supported and can be displayed for a specific amount of time or indefinitely. These are generally used for menus. Still frames can be accompanied by audio.

A disc also can have up to 32 subpicture streams that overlay the video for subtitles, captions for the hard of hearing, captions for children, karaoke, menus, simple animation, etc. These are full-screen, run-length-encoded bitmaps limited to four pixel types. For each group of subpictures, four colors are selected from a palette of 16 (from the YCrCb gamut), and four contrast values are selected out of 16 levels from transparent to opaque. Subpicture display command sequences can be used to create effects such as scroll, move, color/highlight, and fade. The maximum subpicture data rate is 3.36 Mbps, with a maximum size per frame of 53220 bytes.

Video can be stored on a DVD in 4:3 format (standard TV shape) or 16:9 (widescreen). The 16:9 format is anamorphic, meaning the picture is squeezed horizontally to fit a 4:3 rectangle then unsqueezed during playback. DVD players can output video in four different ways:

Video stored in 4:3 format is not changed by the player. It will appear normally on a standard 4:3 display. Widescreen systems will either enlarge it or add black bars to the sides. 4:3 video may have been formatted in various ways before being transferred to DVD. For example, it may have been letterboxed to hold video with a wider shape. Or it may have been panned and scanned from film composed for a wider theatrical presentation. All formatting done to the video prior to it being stored on the disc is transparent to the player. It merely reproduces the signal in standard form.

For automatic letterbox mode, the player creates black bars at the top and the bottom of the picture (60 lines each for NTSC, 72 for PAL). This leaves 3/4 of the height remaining, creating a shorter but wider rectangle. In order to fit this shorter rectangle, the picture is squeezed vertically using a letterbox filter that combines every 4 lines into 3. This compensates for the original horizontal squeezing, resulting in the movie being shown in its full width. The vertical resolution is reduced from 480 lines to 360.

For automatic pan and scan mode, the video is unsqueezed to 16:9 and a portion of the image is shown at full height on a 4:3 screen by following a `center of interest' offset that's encoded in the video stream according to the preferences of the people who transferred the film to video. The pan and scan window is 75% of the full width, which reduces the horizontal pixels from 720 to 540. The pan and scan window can only travel laterally. This does not duplicate a true pan and scan process in which the window can also travel up and down and zoom in and out. Therefore, most DVD producers choose to put a separate pan and scan version on the disc in addition to the widescreen version.

For widescreen mode, the anamorphic video is stretched back out by widescreen equipment to its original width. If anamorphic video is shown on a standard 4:3 display, people will look like they have been on a crash diet. Widescreen mode is complicated because most movies today are shot with a soft matte. (The cinematographer has two sets of frame marks in her viewfinder, one for 1.33 (4:3) and one for 1.85, so she can allow for both formats). A few movies are even wider, such as the 2.35 ratio of Panavision. Since most movies are wider than 1.78 (16:9), one of at least 4 methods must be used during transfer to make it fit the 1.78 rectangle: 1) add additional thin black bars to the top and bottom; 2) include a small amount of extra picture at the top and bottom from the soft matte area; 3) crop the sides; 4) pan and scan with a 1.78 window. With the first two methods, the difference between 1.85 and 1.78 is so small that the letterbox bars or extra picture are hidden in the overscan area of most televisions. Nevertheless, and especially with 2.35 movies, many DVD producers put 16:9 source on one side (or layer) of the disc and 4:3 source on the other. This way the full-frame version of the film can be used for a horizontal and vertical pan and scan, and zoom process with no letterbox bars and no reduction in resolution.

Anamorphosis causes no problems with line doublers, which simply double the lines before they are stretched out by the widescreen display.

For anamorphic video, the pixels are fatter. Different pixel aspect ratios (none of them square) are used for each aspect ratio and resolution. 720-pixel and 704-pixel sizes have the same aspect ratio because the first includes overscan. Note that conventional values of 1.0950 and 0.9157 are for height/width (and are tweaked to match scanning rates). The table below uses less-confusing width/height values (y/x * h/w).

       720x480   720x576
       704x480   704x486   352x480   352x576
4:3     0.909     1.091     1.818     2.182
16:9    1.212     1.455     2.424     2.909

Playback of widescreen material can be restricted. Programs can be marked for the following display modes:

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Next: DVD audio Up: DVD Previous: Sizes and capacities of
Dave Marshall