Differences between DVDs, VCDs, and CDs: Common distinctions in disc types
On the surface, DVD discs appear very similar to CD/VCD discs, but in reality, there is a fundamental difference between the two. If you believe that the disparity between DVD and CD/VCD lies solely in playback effects, such a perspective seems overly simplistic.
- From a technical standpoint:
DVD/CD are fundamentally similar as both store required data in the tracks of the disc within small grooves, subsequently read through the optical drive’s laser beam. However, in terms of optical disc density, DVDs surpass CD/VCD significantly. Consequently, a shorter wavelength laser beam is required to read DVD data compared to reading CD/VCD data. This ensures more accurate laser beam focus and positioning on the disc. The minimum pit length of a CD is 0.834μm with a track pitch of 1.6μm, read by an infrared laser with a wavelength of 780-790nm. In contrast, the minimum pit length of a DVD is only 0.4μm with a track pitch of 0.74μm, read by a 635 ~ 650nm infrared laser.
- When playing video:
VCD can only achieve a standard of 240 lines, while DVD can reach up to 720 lines, giving DVD a clear advantage in definition.
- In terms of data capacity:
CD/VCD can only hold around 650MB-700MB of data, whereas a DVD can hold at least 4.7GB. DVDs can be categorized into four physical structures based on single/double-sided and single/double-layer configurations. For instance, a single-sided single-layer DVD has a capacity of 4.7GB (approximately 7 times that of a CD-ROM), while a double-sided double-layer DVD can reach 17GB (about 26 times that of a CD-ROM).
A single-sided DVD disc may feature one or two recording layers. Similar to a CD, the laser reads data on a single-sided disc from the underside, while data on a double-sided DVD is stored on both sides.
There are two methods for reading data on a double-sided disc:
- After playing the program on the first side, take the disc out, turn it over, and insert it back into the player for continued playback on the second side.
- Install two reading lasers in the player, one for each side, or use a single read laser that automatically jumps to the other side after reading the first side.
For a double-layer disc, the lower layer is half-reflective, allowing the data of the upper layer to be read. Reading the upper layer can be done through Opposite Track Path (OTP) or Parallel Track Path (PTP) methods.
- In terms of function:
DVDs offer various features such as multi-sound track (multi-language), multi-text support, and multi-angle viewing, which are not available on CDs and VCDs. These differences make DVDs outperform CDs and VCDs in the market, and it’s anticipated that DVDs will replace CDs and VCDs as the mainstream in the near future.
Additional information:
DVDs store more data than CDs due to:
- Higher density data storage
- Smaller error correction area and larger storage area
- Multi-layer storage
The capacity increase in DVDs, despite adding a second layer, is due to slightly longer pits on both layers to avoid interference and errors during playback. A single-sided, single-layer DVD can store about seven times more data than a CD, primarily due to smaller pits and tracks on DVDs.
The tighter physical spacing of pits on DVDs allows for 4.5 times as many pits compared to CDs. DVDs also achieve higher capacity by employing more efficient error correction methods and encoding data onto a slightly larger area than CDs.
Moreover, DVDs can have up to four layers, two on each side, with the laser capable of focusing through the first layer onto the second layer for increased storage capacity.