Compact Disc 505fl at glass plate, rather than a spiral groove in a metal one, and that the width of the spiral
track is very much smaller (about 1/60th) than that of the vinyl groove. (Detail of the CD
groove pattern is, for example, too fi ne to be resolved by a standard optical microscope.)
When the master disc is made, “ mother ” and “ daughter ” discs are then made preparatory
to the production of the stampers, which are used to press out the track pattern on a thin
(1.4 mm) plastics sheet, prior to the metallization of the pit pattern for optical readout in
the fi nal disc.
16.3 The Replay System ...............................................................................................
16.3.1 Physical Characteristics
For the reasons shown earlier, the minimum bandwidth required to store the original
20-Hz to 20-kHz stereo signal in digitally encoded form has now been increased
215-fold, to some 4.3 MHz. It is, therefore, no longer feasible to use a record/replay
system based on an undulating groove formed on the surface of a vinyl disc because
the excursions in the groove would be impracticably close together unless the rotational
speed of the disc were to be enormously increased, which would lead to other problems,
such as audible replay noise, pick-up tracking diffi culties, and rapid surface wear.
The technique adopted by Philips/Sony in the design of the CD replay system is therefore
based on an optical pick-up mechanism, in which the binary coded ‘ 0’s and ‘ 1’s are
read from a spiral sequence of bumps on an internal refl ecting layer within a rapidly
rotating (approximately 400 rpm) transparent plastic disc. Because the replay system is
noncontacting, this also offers the advantage that there is no specifi c disc wear incurred in
the replay of the records and they have, in principle, if handled carefully, an indefi nitely
long service life.
16.3.1.1. CD Performance and Disc Statistics
Bandwidth 20 Hz to 20 kHz, 0.5 dB
Dynamic range 90 dB
S/N ratio 90 dB
Playing time (max.) 74 min
Sampling frequency 44.1 kHz