B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

6.2 Pulse Code Modulation (PCM) 277
Fi g ure 6. 18
Ratio of signal to
quantization
noise in PCM
with and without
compression.
50
40
dB
No
20
IO
IO 20 30 40 50 60
NWW
Relative signal power m 2 (t), dB ➔
Fi g ure 6. 19
Piecewise linear
compressor
characteristic.
t
I
Output
Coder input
Though a true µ = 255 compressor working with a µ= 255 expander will be superior to similar
piecewise linear devices, a digital terminal device exhibiting the true characteristic in
today's network must work end-to-end against other network elements that use the piecewise
linear approximation. Such a combination of differing characteristics is inferior to either of
the characteristics obtained when the compressor and the expander operate using the same
compression law.
In the standard audio file format used by Sun, Unix and Java, the audio in "au"
files can be pulse-code-modulated or compressed with the ITU-T G.711 standard through
either the µ-law or the A-law. 6 The µ-law compressor (µ = 255) converts 14-bit
signed linear PCM samples to logarithmic 8-bit samples, leading to storage saving. The
A-law compressor (A = 87.6) converts 13-bit signed linear PCM samples to logarithmic
8-bit samples. In both cases, sampling at the rate of 8000 Hz, a G. 77 encoder thus creates from
audio signals bit streams at 64 kilobits per second (kbit/s). Since the A-law and the µ-law are
mutually compatible, audio recoded into "au" files can be decoded in either format. It should
be noted that the Microsoft WAY audio format also has compression options that use µ-law
and A-law.