Audio Engineering

504 Chapter 16

blemishes, such as dust, scratches, or thumb prints on the surface of the disc, and it is
necessary to include additional information in the data code to allow any erroneous data
to be corrected. A number of techniques have been evolved for this purpose, but the one
used in the CD is known as the “ cross-interleave Reed–Solomon code ” (CIRC). This is
a very powerful error correction method and allows complete correction of faulty data
arising from quite large disc surface blemishes.

Because all possible ‘ 0 ’ or ‘ 1 ’ combinations may occur in the 8-bit encoded words, and
some of these would offer bit sequences rich in consecutive ‘ 0’s or ‘ 1’s, which could
embarrass the disc speed or spot and track location servo-mechanisms, or, by inconvenient
juxtaposition, make it more diffi cult to read the pit sequence recorded on the disc surface,
a bit-pattern transformation stage known as the “ eight to fourteen modulation ” (EFM)
converter is interposed between the output of the error correction (CIRC) block and the
fi nal recording. This expands the recorded bit sequence into the form shown in Figure 16.7
to facilitate the operation of the recording and replay process. The functions and method of
operation of all these various stages are explained in more detail later in this chapter.

16.2.2 Disc Recording

This follows a process similar to that used in the manufacture of vinyl EP and LP records,
except that the recording head is caused to generate a spiral pattern of pits in an optically

8-bit word

Input

Joining bits 14-bit symbol Joining bits 14-bit symbol

8-bit word

Disc indentations

Disc indentations (with EFM)

(if EFM was not employed)

8/14 bit

ROM

Shift register

Output

1

0 0 111 0 0 00 00000100010001100 00

10001111111110 0 1 0

Figure 16.7 : The EFM process.