Organizing storage on the CD-ROM
The basic unit of data stored on the CD-ROM is the frame. Each frame contains 24 bytes of input data. One byte of the source data occupies 17 bits on the disk: 14 bits - the code of the byte of the source data and 3 bits of the merge. For error correction, 180 bits are used. Thus, one frame on the disk occupies 17 • 24 + 180 = 588 bits. The frames are combined into sectors. The sector contains 2352 data bytes (98 frames) and 882 bytes for error correction and control. The use of error correction algorithms allows providing a qualitative reading of information with an error probability of 10-10 bits.
There are over 17,000 tracks on the working surface of the CD. Each track contains 32 sectors with ordinal numbers (addresses) from 0 to 31. Each sector stores user data and service information part with a total of 9996 bits. The sector is divided into 49 segments by 204 bits each. Segments are numbered from 0 to 48, with:
• in the zeroth segment is the sector index, which determines its beginning;
• The second segment contains the data providing the clock adjustment for playback (reading the disc);
• The third segment stores the service information: disk type and side, sector number on the track, etc.
• Segments 4 through 48 are used for data.
General principles of writing/reading
The processes of recording information on a CD (and reading from it) are based on geometric phenomena (reflection, absorption, transmission and refraction of light) and wave (interference, diffraction and polarization of light) optics. The CD contains a recording (working) layer, on which a signalogram is plotted with the help of a laser beam in the form of certain alternations of its states corresponding to a logical zero and a logical unit. In this case, the properties and optical characteristics of the recording layer material change. During the reading process, the laser beam captures these changes and converts them into a digital signal. The most widely used method of recording, leading to a change in the reflection coefficient of the material.
Optical recording achieves a high density of data on the disk. Its limit is due to diffraction of light and is determined by the minimum size of the label. The smaller the wavelength of the light flux emitted by the laser, the higher the recording density.
Optical recording (and reading) of data is carried out through a transparent substrate, which is also used to protect the disk from damage. The following recording methods are known:
• ablation (ablation), a method of recording once, at which by heating the material of the recording layer by a laser beam its individual regions are removed;
• bubble a write-once-recording method in which the recording layer is expanded. When reading, the ray reflected from the expanded section (bubble) is scattered and therefore has a lower intensity compared to the ray reflected from the flat surface of the layer;
• The method of multiple recording, in which the phase state or the color of the recording layer portion for the data bit changes with the help of a laser beam. The refractive index or the absorption coefficient depends on the phase state of the recording layer material.
The data on the CD are located along the track in the form of a waveform, which can be represented as a set of depressions and areas. The shape of the signalogram, or the configuration of the labels, depends on the way the logical levels are coded. As an example, Fig. 6.11 shows the signalograms for two ways of encoding the logical levels with the same source data code 110101:
• In Fig. 6.11, and the logical unit is encoded by the presence of an impulse, a logical zero is the absence of an impulse;
• In Fig. 6.11, b the logical unit is encoded by changing the signal level, the logical zero is the absence of a level change.
Fig. 6.11. Signalograms for encoding a unit by the presence of a pulse, zero by the absence of momentum (a) and by encoding a unit by changing the level signal, zero - saving the current level ( b )
The difference in waveform shapes is due to the fact that the way of encoding the logical levels 0 and 1 of the source data code affects the shape of the recording signal.
CD Creator, Easy CD, CD Publisher, Direct CD, etc. are used to write data to the CD-R and CD-RW CD. These programs support various ways of organizing, or modes, records.
Consider some ways to organize a record.
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