Audio Engineering

Representation of Audio Signals 491

The encoder will need to have the matching inverse characteristic in order that the net
transfer characteristic is unity. The dynamic range of an 8-bit m or A-law converter is
around 62 dB and this can be compared to the 48 dB that a linear 8-bit converter can
provide. The use of companding (compressing and then expanding the coding range)
could be carried in the analogue domain prior to using a linear converter. The diffi culty
is then one of matching the analogue sections. This is an approach that has been taken in
some consumer video equipment.

Chord 0 Digital input signal X

(B1 through B7)

SB“0”SB“1”

A-law

Chord 5

Chord 6

Chord 4

Chord 3

Chord 1 Chord 2

Chord 5

Chord N

Chord N  1

Chord 6

μ-law

A-law μ-law

10.5μA

(for A-law)

Output analog signal Y (

μA)

Digital input

Analogue output

Step numberStep 1N^1

Step IN

Chord 3

Chord 4

640 μA

512 μA

384 μA

264 μA

256 μA

128 μA 55 μA

33 μA

8-bit

528 μA Linear DAC

132 μA

Figure 15.30 : The relationship between digital input word and analogue output current is

not linear. The sign bit is the MSB and the next three bits are used to set the chord slope.

The lower 4 bits set the output steps within each chord. The drawing shows the equivalent

output for a linear 8-bit DAC.