<s>
Harmonic	B-Algorithm
Vector	I-Algorithm
Excitation	I-Algorithm
Coding	I-Algorithm
,	O
abbreviated	O
as	O
HVXC	B-Algorithm
is	O
a	O
speech	B-Application
coding	I-Application
algorithm	O
specified	O
in	O
MPEG-4	B-Algorithm
Part	I-Algorithm
3	I-Algorithm
(	O
MPEG-4	B-Algorithm
Audio	I-Algorithm
)	O
standard	O
for	O
very	O
low	O
bit	O
rate	O
speech	B-Application
coding	I-Application
.	O
</s>
<s>
HVXC	B-Algorithm
supports	O
bit	O
rates	O
of	O
2	O
and	O
4	O
kbit/s	O
in	O
the	O
fixed	O
and	O
variable	B-General_Concept
bit	I-General_Concept
rate	I-General_Concept
mode	O
and	O
sampling	B-Algorithm
frequency	I-Algorithm
8kHz	O
.	O
</s>
<s>
It	O
also	O
operates	O
at	O
lower	O
bitrates	O
,	O
such	O
as	O
1.2	O
-	O
1.7	O
kbit/s	O
,	O
using	O
a	O
variable	B-General_Concept
bit	I-General_Concept
rate	I-General_Concept
technique	O
.	O
</s>
<s>
It	O
was	O
published	O
as	O
subpart	O
2	O
of	O
ISO/IEC	O
14496-3:1999	O
(	O
MPEG-4	B-Algorithm
Audio	I-Algorithm
)	O
in	O
1999	O
.	O
</s>
<s>
An	O
extended	O
version	O
of	O
HVXC	B-Algorithm
was	O
published	O
in	O
MPEG-4	B-Algorithm
Audio	I-Algorithm
Version	O
2	O
(	O
ISO/IEC	O
14496-3:1999/Amd	O
1:2000	O
)	O
.	O
</s>
<s>
MPEG-4	O
Natural	O
Speech	B-Application
Coding	I-Application
Tool	O
Set	O
uses	O
two	O
algorithms	O
:	O
HVXC	B-Algorithm
and	O
CELP	B-Algorithm
(	O
Code	B-Algorithm
Excited	I-Algorithm
Linear	I-Algorithm
Prediction	I-Algorithm
)	O
.	O
</s>
<s>
HVXC	B-Algorithm
is	O
used	O
at	O
a	O
low	O
bit	O
rate	O
of	O
2	O
or	O
4	O
kbit/s	O
.	O
</s>
<s>
Higher	O
bitrates	O
than	O
4	O
kbit/s	O
in	O
addition	O
to	O
3.85	O
kbit/s	O
are	O
covered	O
by	O
CELP	B-Algorithm
.	O
</s>
<s>
HVXC	B-Algorithm
uses	O
Linear	B-Algorithm
predictive	I-Algorithm
coding	I-Algorithm
(	O
LPC	O
)	O
with	O
block-wise	O
adaptation	O
every	O
20ms	O
.	O
</s>
<s>
The	O
LPC	O
parameters	O
are	O
transformed	B-Architecture
to	O
Line	B-Algorithm
spectral	I-Algorithm
pair	I-Algorithm
(	O
LSP	O
)	O
coefficients	O
,	O
which	O
are	O
jointly	O
quantized	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
voiced	O
speech	O
,	O
the	O
residual	O
is	O
coded	O
in	O
a	O
parametric	O
representation	O
(	O
operating	O
as	O
a	O
vocoder	B-Application
)	O
,	O
while	O
in	O
the	O
case	O
of	O
unvoiced	O
speech	O
,	O
the	O
residual	O
waveform	O
is	O
quantized	O
(	O
thus	O
operating	O
as	O
hybrid	O
speech	B-Application
codec	I-Application
)	O
.	O
</s>
<s>
To	O
extract	O
these	O
values	O
,	O
the	O
LPC	O
residual	O
signal	O
is	O
transformed	B-Architecture
into	O
the	O
DFT-domain	O
.	O
</s>
<s>
The	O
DFT-spectrum	O
is	O
segmented	O
into	O
bands	O
,	O
one	O
band	O
per	O
harmonic	O
.	O
</s>
<s>
The	O
frequency	O
band	O
for	O
the	O
m-th	O
harmonic	O
consists	O
of	O
the	O
DFT-coefficients	O
from	O
(	O
m-1/2	O
)	O
ω0	O
to	O
(	O
m+	O
1/2	O
)	O
ω0	O
,	O
ω0	O
being	O
the	O
pitch	O
frequency	O
.	O
</s>
<s>
The	O
amplitude	O
value	O
for	O
the	O
m-th	O
harmonic	O
is	O
chosen	O
to	O
optimally	O
represent	O
these	O
DFT-coefficients	O
.	O
</s>
<s>
The	O
spectral	O
envelope	O
is	O
then	O
coded	O
using	O
variable-dimension	O
weighted	O
vector	B-Algorithm
quantization	I-Algorithm
.	O
</s>
<s>
Unvoiced	O
segments	O
are	O
encoded	O
according	O
to	O
the	O
CELP	B-Algorithm
scheme	O
,	O
which	O
is	O
also	O
referred	O
to	O
as	O
vector	O
excitation	O
coding	O
(	O
VXC	O
)	O
.	O
</s>
<s>
The	O
CELP	B-Algorithm
coding	O
in	O
HVXQ	O
is	O
performed	O
using	O
only	O
a	O
stochastic	O
codebook	O
.	O
</s>
<s>
In	O
other	O
CELP	B-Algorithm
codecs	O
,	O
a	O
dynamic	O
codebook	O
is	O
used	O
additionally	O
to	O
perform	O
long-term	O
prediction	O
of	O
voiced	O
segments	O
.	O
</s>
<s>
However	O
,	O
since	O
HVXC	B-Algorithm
does	O
not	O
use	O
CELP	B-Algorithm
for	O
voiced	O
segments	O
,	O
the	O
dynamic	O
codebook	O
is	O
omitted	O
from	O
the	O
design	O
.	O
</s>
