• The Differences Between DVD and CD\VCD
  • DVD has many differences between CD/VCD mainly in four aspects:

    1. Technology
    DVD and CD are of the same principles, all the required data will be stored in the in a very small groove in the track of CD-ROM, and then through the drive laser beam to read the CDs. However, DVD has a larger density than that of CD / VCD, so while reading the DVD data; it needs a much shorter wavelength of the laser beam. Only in this way can make a more accurate laser beam focusing and positioning on the disc. The minimum pit length on the CD is 0.834μm, the track pitch of 1.6μm, with a wavelength of 780 ~ 790nm infrared laser to read data, while the DVD has a minimum pit length of only 0.4μm, channel spacing of 0.74μm, using wavelength for the 635 ~ 650nm infrared laser to read data.

    2. Video Playing
    VCD can only reach the standard of 240 lines while the DVD can reach the standard of 720 lines, so DVD has a much better definition.

    3. Capacity
    CD/DVD can only hold data about 650-700MB, while DVD can hold at least 4.7 GB data. DVD can be divided into several types: single-side single-layer, single-side double-layer, double-side single-layer, double-side double-layer four structures. The single side, single layer DVD has a capacity of 4.7 GB which is 7 times much as that of CD-ROM. While the double side, double layer DVD has a capacity of 17 GB that is 26 times as that of CD-ROM.
    The single side DVD disc may have one or two recording layers. As that of CD, laser head shall read data on the single side disc from the below side, while the double side DVD has data stored on both upper and lower side.
    There are two ways to read data on double side disc: To read the data on double-sided disc, there are two methods:

    (1) After playing the first side of the plate program, it will remove the disc from the player, upside down and then into the players continue to play second surface of the program;
    (2) Players are set with two read lasers, respectively, from the disk to read data up and down both sides, or players, only installed a read laser, but read the disc after the first plane can automatically jump to the disc The other side continues to play.
    If we use the second method, users shall need not to get out the disc and turn it over, but the DVD player should have this function.
    The double layer disc is to join the two layer disc together, the lower layer is reflective layer, and laser head can read data from the upper layer through reflective layer.While reading the lower layer, the laser head shall read from the inner rings to outer rings, and then read the upper layer. There are mainly two ways:
    (1)OTP(Opposite Optical track Path) that is to read the layer1 from the outer side rings and from the outer rings to the inner rings.
    (2) PTP(Parallel Optical track Path)That is to read the first layer from the inner side and then move outwards. The optical points shall change while the laser read the data on the lower layer.
    No matter single layer disc or double layer disc, there are two film bases, each is 0.6 mm, so the thickness of DVD is 1.2 mm. For single layer disc, data is only stored on the lower layer, and the upper base has no data, while the double layer disc has data on both sides.

    The DVD has a larger capacity than that of CD/VCD:
    1. Smaller length of pit shall make the capacity larger: 0.83/0.4 ＝2.075 (fold).
    2. Smaller intervals between tracks shall increases the capacity:1.6/0.74 ＝2.162 (fold).
    3. Increased area of discs shall increase the capacity: 87.5/86.0 =1.017(fold).
    4.The change of channel code, from 17 bit to 16 bit shall increase the capacity:17/16 ＝1.062(fold).
    5. More effective error correction shall increase the capacity: 1.32 fold.
    6. Smaller amount of section over head shall increase the capacity: 1.06 fold.

    4、Functionality
    DVD can provide multi-audio track, multi-language, multi-subtitle and multi-dimension and much more advanced functions. All these functions can not be achieved with CD and DVD.
    The above are the differences between DVD and CD/VCD. But this does not mean that DVD can take the place of CD/VCD. The latter ones have advantages in price, so they share a large amount in market. However, DVD is the trend in the near feature.

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