<s>
In	O
signal	O
processing	O
,	O
sub-band	B-Algorithm
coding	I-Algorithm
(	O
SBC	O
)	O
is	O
any	O
form	O
of	O
transform	B-Algorithm
coding	I-Algorithm
that	O
breaks	O
a	O
signal	O
into	O
a	O
number	O
of	O
different	O
frequency	B-Algorithm
bands	I-Algorithm
,	O
typically	O
by	O
using	O
a	O
fast	O
Fourier	O
transform	O
,	O
and	O
encodes	O
each	O
one	O
independently	O
.	O
</s>
<s>
SBC	O
is	O
the	O
core	O
technique	O
used	O
in	O
many	O
popular	O
lossy	O
audio	O
compression	O
algorithms	O
including	O
MP3	B-Application
.	O
</s>
<s>
The	O
simplest	O
way	O
to	O
digitally	O
encode	O
audio	O
signals	O
is	O
pulse-code	B-Algorithm
modulation	I-Algorithm
(	O
PCM	B-Algorithm
)	O
,	O
which	O
is	O
used	O
on	O
audio	O
CDs	O
,	O
DAT	O
recordings	O
,	O
and	O
so	O
on	O
.	O
</s>
<s>
Digitization	O
transforms	O
continuous	O
signals	O
into	O
discrete	O
ones	O
by	O
sampling	O
a	O
signal	O
's	O
amplitude	O
at	O
uniform	O
intervals	O
and	O
rounding	B-Algorithm
to	O
the	O
nearest	O
value	O
representable	O
with	O
the	O
available	O
number	O
of	O
bits	O
.	O
</s>
<s>
This	O
process	O
is	O
fundamentally	O
inexact	O
,	O
and	O
involves	O
two	O
errors	O
:	O
discretization	B-Algorithm
error	I-Algorithm
,	O
from	O
sampling	O
at	O
intervals	O
,	O
and	O
quantization	O
error	O
,	O
from	O
rounding	B-Algorithm
.	O
</s>
<s>
With	O
PCM	B-Algorithm
,	O
the	O
audible	O
effects	O
of	O
these	O
errors	O
can	O
be	O
mitigated	O
with	O
dither	O
and	O
by	O
using	O
enough	O
bits	O
to	O
ensure	O
that	O
the	O
noise	O
is	O
low	O
enough	O
to	O
be	O
masked	O
either	O
by	O
the	O
signal	O
itself	O
or	O
by	O
other	O
sources	O
of	O
noise	O
.	O
</s>
<s>
In	O
effect	O
,	O
many	O
bits	O
are	O
wasted	O
in	O
encoding	O
masked	O
portions	O
of	O
the	O
signal	O
because	O
PCM	B-Algorithm
makes	O
no	O
assumptions	O
about	O
how	O
the	O
human	O
ear	O
hears	O
.	O
</s>
<s>
A	O
classic	O
method	O
is	O
nonlinear	O
PCM	B-Algorithm
,	O
such	O
as	O
the	O
μ-law	B-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
Sun	O
's	O
Au	O
file	O
format	O
for	O
sound	O
is	O
a	O
popular	O
example	O
of	O
mu-law	B-Algorithm
encoding	O
.	O
</s>
<s>
Using	O
8-bit	O
mu-law	B-Algorithm
encoding	O
would	O
cut	O
the	O
per-channel	O
bitrate	O
of	O
CD	O
audio	O
down	O
to	O
about	O
350	O
kbit/s	O
,	O
half	O
the	O
standard	O
rate	O
.	O
</s>
<s>
First	O
,	O
a	O
digital	O
filter	O
bank	O
divides	O
the	O
input	O
signal	O
spectrum	O
into	O
some	O
number	O
(	O
e.g.	O
,	O
32	O
)	O
of	O
subbands	B-Algorithm
.	O
</s>
<s>
The	O
psychoacoustic	O
model	O
looks	O
at	O
the	O
energy	O
in	O
each	O
of	O
these	O
subbands	B-Algorithm
,	O
as	O
well	O
as	O
in	O
the	O
original	O
signal	O
,	O
and	O
computes	O
masking	O
thresholds	O
using	O
psychoacoustic	O
information	O
.	O
</s>
<s>
Each	O
of	O
the	O
subband	B-Algorithm
samples	O
is	O
quantized	O
and	O
encoded	O
so	O
as	O
to	O
keep	O
the	O
quantization	O
noise	O
below	O
the	O
dynamically	O
computed	O
masking	O
threshold	O
.	O
</s>
<s>
The	O
frames	O
are	O
unpacked	O
,	O
subband	B-Algorithm
samples	O
are	O
decoded	O
,	O
and	O
a	O
frequency-time	O
mapping	O
reconstructs	O
an	O
output	O
audio	O
signal	O
.	O
</s>
<s>
Subband	B-Algorithm
coding	I-Algorithm
resides	O
at	O
the	O
heart	O
of	O
the	O
popular	O
MP3	B-Application
format	O
(	O
more	O
properly	O
known	O
as	O
MPEG-1	B-Application
Audio	I-Application
Layer	I-Application
III	I-Application
)	O
,	O
for	O
example	O
.	O
</s>
<s>
Sub-band	B-Algorithm
coding	I-Algorithm
is	O
used	O
in	O
the	O
G.722	B-Algorithm
codec	O
which	O
uses	O
sub-band	B-Algorithm
adaptive	B-Algorithm
differential	I-Algorithm
pulse	I-Algorithm
code	I-Algorithm
modulation	I-Algorithm
(	O
SB-ADPCM	O
)	O
within	O
a	O
bit	O
rate	O
of	O
64kbit/s	O
.	O
</s>
<s>
In	O
the	O
SB-ADPCM	O
technique	O
,	O
the	O
frequency	B-Algorithm
band	I-Algorithm
is	O
split	O
into	O
two	O
sub-bands	B-Algorithm
(	O
higher	O
and	O
lower	O
)	O
and	O
the	O
signals	O
in	O
each	O
sub-band	B-Algorithm
are	O
encoded	O
using	O
ADPCM	B-Algorithm
.	O
</s>
