The
Start
How does they
work
How they are made
They future
History
The
diference between CD & DVD
What is aCD ROM
CD
ROM introduction
Index
Optical Disc
Technology
Introduction
Digital
Technology
From
Analogue to Digital
Scanning
The Disc
CIRC
Optical disc technology
is a growing force in home video which became popular with the introduction
of the audio Compact Disc. Today, various formats exist, which besides sound
can also carry data, graphics and full motion video
parts.
Digital
Technology
Optical disc technology makes use of digital signal processing, contrary to the analogue audio and video carriers, such as the gramophone record and the magnetic video tape. But why digital?
In analogue transmission
any imperfection during the registration, storage or reproduction phases
of recording will decrease the quality of the audio and/or video signal.
For example, a dirty record causes noise, a irregular revolution or winding
speed causes problems, a worn needle or a dirty head causes distortion. These
imperfections do not occur in digital registration. (For more detailed and
in-depth information, please refer to the basic principles of the Compact
Disc Technology).
From
Analogue to Digital
In optical disc
technology, analogue signals are converted into digital signals. During this
process, the analogue signal (audio and/or video) is measured in parts and
converted into a series of values, called sampling. One can envision
a waveform representing an audio or video signal, being measured with intervals.
The signal strength and polarity at these intervals can be expressed in decimal
numbers (1,2,3, etcetera) and express the signal strength and polarity (+
or -) from point to point. The frequency of how often the signal strength
is measured in a waveform, determines the accuracy of the registration of
the original waveform. These steps must therefore be very small. The measurements
are in the order of tenths of millivolts.
Figure 1:Conversion process of an analogue signal to digital and back to analogue.
An analogue-to-digital (A/D) converter translates these decimal values into binary notation (bits). Bits are made up of '1's and '0's only, and by combining these ones and zeros in different combinations, decimal numbers can be expressed in binary notation. Some examples of binary notation (in 3 bits) are:
Decimal Binary 1 001 2 010 3 011 4 100 5 101 6 110 7 111
Thus the analogue signal becomes a digital signal which
is now a series of pulses: pulses for the '1's, and non-pulses for the '0's.
For optical discs these pulse series are recorded on the surface of the disc
as microscopically small pits and lands, with the help of a fine laser beam.
Pits stand for '0's and lands stand for '1's.In most recordings every value
(44,100 per second) is converted into a string of 16 bits (instead of the
three bits example). This totals over one million bits per second. A 16-bit
number of '1's and '0's can indicate no less than 65,536 different values
(2 values possible for each bit = 216 = 65,536 possibilities).
Scanning The Disc
Figure 2: Cutaway view of the laser pickup. Depending on whether the laserbeam hits a pit or a land, the laserbeam is reflected and received by the photo-electrical cell.
Like gramophone records, the information on optical
discs is recorded on a spiral track. The laser starts 'reading' the disc
from the inside and ends at the outside. When played back, a laser beam shines
on the pits and lands. When the beam strikes a land, the beam is reflected
onto a photo-electrical cell. When it strikes a pit, the photo cell will
receive only a weak reflection. Thus the photo-electrical cell receives series
of light pulses corresponding to the pits and lands in the disc. A D/A-converter
(digital to analogue converter: DAC) converts the series of pulses back to
binary coding, and then to decimal values. Now the original analogue signal
can be rebuilt.
CIRC
Figure 3:Compact Disc player mechanism. The laser pickup reads the disc from below.
Thanks to this optical scanning system, there is no
friction between the laser beam and the disc. As a result, the discs do not
wear, however often they are played. However, they must to be treated carefully,
as scratches, grease stains and dust might intercept or diffract the light,
causing whole series of pulses to be skipped or distorted. This problem can
be solved, as during the recording the Cross Interleaved Reed Solomon Code
(CIRC) is added, which is an error correction system that automatically inserts
any lost or damaged information, by making a number of mathematical calculations.
Without this error correction system optical disc players would not have
existed, as even the slightest vibration of the floor would cause sound and
image distortions.