The storage capacity. Despite having the same format and size, a DVD can store much more information than a CD. This was possible thanks to advances in recording and optical reading techniques. Both types of disk have a long spiral sequence filled with microscopic valleys and hills, which help transmit sound or visual information. Until then, it’s all the same for both of them. The big difference is that, in the DVD sequence, these bumps and indentations are much smaller and so is the distance between them. Thus, each square centimeter of disk has 4.5 times more data than on CD. Another advance is that, on the CD, the same data is recorded several times to avoid reading errors – if, for example, a tiny scratch causes an equally microscopic stretch to be read wrongly, the device compares it with another stretch that should be the same . The DVD doesn’t need so much data repetition, because the device’s reading method is much more accurate, thus saving even more space. Finally, the DVD can be recorded on both sides, like the old vinyl records, and each side has two layers of superimposed data, totaling, in practice, four sides. CDs only allow recording on a single layer. The summary of all this is that a DVD disc can receive 17 gigabytes of information against only 0.6 of a CD. It’s so much capacity that you can’t even use it all. Most DVDs today only use one of their four recording layers, enough to store a full-length movie with subtitles in several languages.
The devices for both types of discs follow the same principle.
1. The device’s laser source emits the rays that will scan the CD or DVD disc
2. A system of lenses reflects and focuses the rays towards the disk
3. The motor rotates the disc. The faster the rotation, the faster the data readability.
4. The disc reflects the laser at different intensities if the beam passes through one of the microscopic hills or valleys of the embossed sequence
5. A sensor detects successive variations in laser intensity and sends electronic signals to the computer, which will decode them into sound or visual information
6. The positioning system moves the laser, following the sequence in high relief, which starts from the center to the ends of the disk
The CD has only one layer of information. The DVD has four
CD
It has only a thin layer of aluminum, which contains sound and visual information recorded in a sequence composed of microscopic hills and valleys. This layer, which gives the discs their metallic shine, is surrounded by transparent plastic.
DVD
With the same thickness as the CD, it can have up to four layers of aluminum, two on each side. That’s because, on the DVD, the microscopic hills and valleys are half the size of those on the CD and twice as tight. A DVD player’s laser can be precisely aimed at any of the four layers