Compact Disc 511spot focused on the disc and that use errors found in the bit repetition frequency, derived
from the recovered sequence of ‘ 1’s and ‘ 0’s, to correct inaccuracies in the disc rotation
speed. All these problems would be worsened in the presence of mechanical vibration.
The method chosen to solve this problem is to translate the 256-bit sequences possible
with an 8-bit encoded signal into an alternative series of 256-bit sequences found in a
14-bit code, which are then reassembled into a sequence of symbols as shown graphically
in Figure 16.7. The requirements for the alternative code are that a minimum of two
‘ 0’s shall separate each ‘ 1 ’ and that no more than ten ‘ 0’s shall occur in sequence. In the
14-bit code, there are 267 values that satisfy this criterion, of which 256 have been chosen
and stored in a ROM-based “ look-up ” table. As a result of the EFM process, there are
only nine different pit lengths that are cut into the disc surface during recording, varying
from 3 to 11 clock periods in length.
Because the numerical magnitude of the output (EFM) digital sequence is no longer
directly related to that of the incoming 8-bit word, the term “ symbol ” is used to describe
this or other similar groups of bits.
Since the EFM encoding process cannot by itself ensure that the junction between
consecutive symbols does not violate the requirements noted earlier, an “ interface ” or
“ coupling ” group of three bits is also added, at this stage, from the EFM ROM store, at
the junction between each of these symbols. This coupling group will take the form of a
‘ 000 ’ , ‘ 100 ’ , ‘ 010 ’ , or ‘ 001 ’ sequence, depending on the position of the ‘ 0’s or ‘ 1’s at the
end of the EFM symbol. As shown in Figure 16.6 , this process increases the bit rate from
1.882 to 4.123 MB/s, and the further addition of uniquely styled 24-bit synchronizing
words to hold the system in coherence, and to mark the beginnings of each bit sequence,
increases the fi nal signal rate at the output of the recording chain to 4.322 MB/s. These
additional joining and synchronizing bits are stripped from the signal when the bit stream
is decoded during the replay process.
16.3.2.2 Digital-to-Analogue Conversion
The transformation of the input analogue signal into, and back from, a digitally encoded
bit sequence presents a number of problems. These stem from the limited time (22.7 μ s)
available for the conversion of each signal sample into its digitally encoded equivalent
and from the very high precision needed in allocating numerical values to each sample.
For example, in a 16-bit encoded system the magnitude of the MSB will be 32,768 times
as large as the LSB. Therefore, to preserve the signifi cance of a ‘ 0 ’ to ‘ 1 ’ transition in the