- Written by Administrator
- Published on 27 November 2007
Digital Video Disc (DVD) used to be called High Density Compact Disc, but the abbreviation got confused with High Definition Compatible Digital (see above). Now it is called Digital Versatile Disc. DVD is a format released to consumers in the first quarter of 1997 (USA) which stores about 20 times as much information as a standard CD, and on the same size disc (5"). This allows several hours of movie (the DVD-Video or DVD-V) and multi-channel sound (such as AC-3 [Dolby Digital - DD] and DTS; see below) to be contained on a smaller disc than movies are on laserdiscs. Thousands of DVD movies have been released, with 5.1 DD surround sound. The audio and video are completely digital, rather than having some portions in analog form as is the case with laserdiscs. This produces home theater of profoundly improved quality over current broadcast TV, VHS tapes, and even laserdiscs.
Many DVDs are enhanced for 16:9 viewing. This means the image is stored in much the same way anamorphic films are stored. That is, the image is squeezed side-to-side. In order to use this feature, you must have a 16:9 television, which has the anamorphic mode. When playing a 16:9 enhanced DVD on a 16:9 TV in this mode, the final image occupies almost the entire viewing screen area rather than having so much of the screen taken up by blank bars at the top and bottom. This results in the same resolution side-to-side, but more resolution top-to-bottom, because more of the scanning lines are being used for the picture. A 16:9 TV can also just enlarge the image so that the widescreen movie fits into the screen area, but this discards the scanning lines above and below the picture, so it is not as high resolution as the 16:9 enhanced image. Not all manufacturers have jumped into the enhanced DVD production, because most consumers don't have 16:9 TVs.
The first DVDs (produced in 1997) had the widescreen version of the movie on one side of the disc, while the pan & scan version was on the other side. Also, films longer than about 2 hours had to be turned over to watch the last part of the movie. Now the trend is in having either just the widescreen version on the disc, with the entire film on one side, or having the widescreen as well as the pan & scan versions on one side. This is accomplished through a technique called Reverse Spiral Dual Layer (RSDL). For the first two hours, the laser beam is reading data on the top layer from the center of the disc to the outer edge. Then the laser beam shifts to read the deeper layer, from the outer edge of the disc back to the center. Because the disc continues to spin in the same direction, the spiral of the data track must be reversed. All DVD players, including the first generation, will read these RSDL discs.
DVD may also be used to store high sampling rate, long word length digital sound. DVD-A (DVD Audio) is DVD with music only, and the discs have 5.1 sound at 24 bit - 96 kHz sampling, or two-channels at 24 bit - 192 kHz sampling. There are some Dolby Digital DVDs, but DVD-A really just refers to uncompressed music discs (Dolby Digital is compressed). Meridian Lossless Packing (MLP) allows more data to be stored on the DVDs. Otherwise, 5.1 DVD-A would have been limited to 24 bit - 48 kHz.
Sony's latest format is called SACD (Super Audio Compact Disc), using 1 bit technology (Direct Stream Digital - DSD). The first discs are two-channel stereo only, but down the road, it will have 5.1 channels too. SACD needs no filters in the signal path, unlike PCM systems, and the lack of these filters should make for a higher sound quality. Keep in mind that SACD is a type of DVD-A, as the other music-only DVDs discussed above. DVD-V would refer to all DVDs that have video on them, and DVD-A is the category for DVDs with just music.
My own dream for DVD audio would be to have 8 channels of sound (Front left/center/right, Side left/right, Rear left/center/right) sampled at 500 kHz (the limit, at which no further audible improvements can be detected with increased sampling rate) with 24 bit word length. This would require a bitstream of 100 Megabits (Mb) per second just for the music data, not the overhead management data. A current technology DVD, using both layers and both sides, would only hold a few minutes of music in this form, and DVD is limited to 10 Mb per second anyway, so I guess we will have to wait until DVDs will hold a terabyte (1,000 gigabytes) or so, along with the ability to handle much higher bit rates.