Handbook for Sound Engineers

Optical Disc Formats for Audio Reproduction and Recording 1137

pictures but is generally used to display simple text

information.

30.2.4 EFM Encoding

After the audio, parity, and subcode data is assembled,
the bit stream is modulated using EFM (eight-to-four-
teen modulation). Blocks of 8 data bits are translated
into blocks of 14 channel bits, assigning an arbitrary
and unambiguous word of 14-bits to each 8-bit word.
By choosing select 14-bit words with a low number
(and known rate) of 1/0 transitions, greater data density
can be achieved. It would be inefficient to store the 8-bit
symbols directly on the disc; the large number of 1/0
transitions would demand many pits. In addition, 8-bit
symbols have many similar patterns. With 14-bit words,
more unique patterns can be selected. EFM thus expe-
dites error correction.
Blocks of 14-bits are linked by three merging bits;
two merging bits (always 0s) are required to prevent the
possibility of successive 1s between serial words (a
violation of the EFM coding scheme). The additional
merging bit (either a 1 or a 0, depending on the
preceding and succeeding patterns) is added to each
code pattern to aid in clock synchronization and to
suppress the signal’s low-frequency component. The
latter is accomplished by selecting merging bits that
maintain the signal’s average digital sum value at zero.
The ratio of bits before and after modulation is 8:17.
During demodulation, only 14-bits will be processed,
the 3 merging bits are discarded.
The 8 data bits require 2^8 or 256 different code
patterns. However the 14-bit channel word can offer
16,384 combinations. To achieve pits of controlled
length, only those combinations are selected in which
more than two but less than ten 0s appear continuously.
In addition, unique patterns are sought. Only 267
combinations satisfy these criteria. Because only 256
patterns are needed, 11 of the 267 patterns are discarded
(two of them are used for subcode synchronization
words).

The resultant channel stream produces pits and lands

that comprise at least two (3T) but no more than ten

(11T) successive 0s in length. It is the combination of

these varying dimensions that physically encodes the

data. The selection of EFM bit patterns defines the

physical relationship of the pit dimensions. The pits and

intervening reflective land on the CD surface do not

directly designate 1s and 0s. Rather, each pit edge

whether leading or trailing, is a 1 and all increments in

between, whether inside or outside a pit, are 0s, as

shown in Fig. 30-4.

With EFM there are more bits to accommodate, but

with modulation the highest frequency in the output

signal is decreased. Therefore a lower track velocity can

be utilized and longer playing time is achieved. This is

an efficient encoding method because the number of

bits transmitted divided by the number of transitions

needed on the medium to convey them is high.

30.3 CD Player Design

A CD player hardware architecture may be considered

as five functional elements each working in concert with

the other: Optical readout, servo system, spindle motor,

control and display, and decoding circuits. The data path

directs the modulated light from the pickup through a

series of processing circuits, ultimately yielding a stereo

analog signal. The data path typically consists of ele-

ments such as data separator, deinterleaving RAM, error

detection, correction and concealment circuits, oversam-

pling filters, D/A converters, and analog output filters.

The servo, control, and display system must direct

mechanical operation of the disc, including spindle

drive, auto-tracking, and auto-focusing, and handle user

interface with the player’s controls and displays. A

block diagram of the data path is shown in Fig. 30-5.

30.3.1 Optical Pickup

The CD optical pickup must focus, track, and read the

data spiral. The entire lens assembly, a combination of

the laser source and the reader, must be small enough to

Figure 30-4. Pit/land edges represent logical 1 data.

4T 10T 4T 4T 4T 7T 5T 4T

0 1 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 1 0