<s>
Digital	B-Device
cable	I-Device
is	O
the	O
distribution	O
of	O
cable	O
television	O
using	O
digital	B-General_Concept
data	I-General_Concept
and	O
video	O
compression	O
.	O
</s>
<s>
During	O
the	O
late	O
2000s	O
,	O
broadcast	O
television	O
converted	O
to	O
the	O
digital	O
HDTV	B-Device
standard	O
,	O
which	O
was	O
incompatible	O
with	O
existing	O
analog	O
cable	O
systems	O
.	O
</s>
<s>
In	O
addition	O
to	O
providing	O
high-definition	O
video	O
,	O
digital	B-Device
cable	I-Device
systems	O
provide	O
more	O
services	O
such	O
as	O
pay-per-view	O
programming	O
,	O
cable	B-Device
internet	I-Device
access	I-Device
and	O
cable	O
telephone	O
services	O
.	O
</s>
<s>
Most	O
digital	B-Device
cable	I-Device
signals	O
are	O
encrypted	O
,	O
which	O
reduced	O
the	O
incidence	O
of	O
cable	B-Application
television	I-Application
piracy	I-Application
which	O
occurred	O
in	O
analog	O
systems	O
.	O
</s>
<s>
In	O
1990	O
,	O
General	O
Instrument	O
(	O
acquired	O
by	O
Motorola	O
and	O
now	O
owned	O
by	O
ARRIS	O
Group	O
)	O
demonstrated	O
that	O
it	O
was	O
possible	O
to	O
use	O
digital	O
compression	O
to	O
deliver	O
high	O
quality	O
HDTV	B-Device
in	O
a	O
standard	O
6MHz	O
television	O
channel	O
.	O
</s>
<s>
Using	O
the	O
same	O
technology	O
General	O
Instrument	O
(	O
GI	O
)	O
demonstrated	O
the	O
digital	O
transmission	O
of	O
multiple	O
high	O
quality	O
standard	B-Device
definition	I-Device
programs	O
in	O
a	O
6MHz	O
cable	O
channel	O
.	O
</s>
<s>
In	O
the	O
1990s	O
,	O
cable	O
providers	O
began	O
to	O
invest	O
heavily	O
in	O
this	O
new	O
multi-channel	O
digital	B-Application
TV	I-Application
technology	O
to	O
expand	O
the	O
number	O
of	O
channels	O
and	O
services	O
available	O
to	O
subscribers	O
.	O
</s>
<s>
By	O
2000	O
,	O
most	O
cable	O
providers	O
in	O
the	O
US	O
were	O
offering	O
some	O
form	O
of	O
digital	B-Device
cable	I-Device
TV	O
to	O
their	O
customers	O
.	O
</s>
<s>
Digital	B-Device
cable	I-Device
technology	O
has	O
allowed	O
cable	O
providers	O
to	O
compress	O
video	O
channels	O
so	O
that	O
they	O
take	O
up	O
less	O
bandwidth	B-Algorithm
and	O
to	O
offer	O
two-way	O
communication	O
capabilities	O
.	O
</s>
<s>
This	O
has	O
enabled	O
providers	O
to	O
offer	O
more	O
channels	O
,	O
video-on-demand	B-Application
services	O
that	O
do	O
n't	O
require	O
a	O
separate	O
telephone	O
line	O
,	O
telephone	O
services	O
,	O
high	O
speed	O
internet	O
services	O
,	O
and	O
interactive	O
television	O
services	O
.	O
</s>
<s>
Digital	B-Device
cable	I-Device
implements	O
error	B-Error_Name
correction	I-Error_Name
to	O
ensure	O
the	O
integrity	O
of	O
the	O
received	O
signal	O
and	O
uses	O
a	O
secure	O
digital	O
distribution	O
system	O
(	O
i.e.	O
</s>
<s>
Most	O
digital	B-Device
cable	I-Device
providers	O
use	O
QAM	B-Device
for	O
video	O
services	O
and	O
DOCSIS	B-Protocol
standards	O
for	O
data	O
services	O
.	O
</s>
<s>
Digital	B-Device
cable	I-Device
technology	O
can	O
allow	O
many	O
TV	O
channels	O
to	O
occupy	O
the	O
frequency	O
space	O
that	O
would	O
normally	O
be	O
occupied	O
by	O
a	O
single	O
analog	O
cable	O
TV	O
channel	O
.	O
</s>
<s>
The	O
addition	O
of	O
this	O
capability	O
complicates	O
the	O
notion	O
of	O
a	O
"	O
channel	O
"	O
in	O
digital	B-Device
cable	I-Device
(	O
as	O
well	O
as	O
in	O
over-the-air	O
ATSC	B-Device
digital	I-Device
broadcasts	I-Device
)	O
.	O
</s>
<s>
The	O
formal	O
names	O
for	O
the	O
two	O
numbers	O
that	O
now	O
identify	O
a	O
channel	O
are	O
the	O
physical	O
channel	O
and	O
the	O
subchannel	B-Device
.	O
</s>
<s>
The	O
subchannel	B-Device
is	O
a	O
logical	O
channel	O
of	O
data	O
within	O
the	O
physical	O
channel	O
.	O
</s>
<s>
Technically	O
there	O
can	O
be	O
up	O
to	O
1024	O
subchannels	B-Device
in	O
a	O
physical	O
channel	O
,	O
though	O
in	O
practice	O
only	O
a	O
few	O
are	O
used	O
(	O
as	O
the	O
bandwidth	B-Algorithm
must	O
be	O
divided	O
among	O
all	O
the	O
subchannels	B-Device
)	O
.	O
</s>
<s>
The	O
first	O
,	O
accomplished	O
through	O
PSIP	B-Device
,	O
is	O
where	O
program	O
and	O
channel	O
information	O
is	O
broadcast	O
along	O
with	O
the	O
video	O
,	O
allowing	O
the	O
consumer	O
's	O
decoder	O
(	O
set-top	O
box	O
or	O
display	O
)	O
to	O
automatically	O
identify	O
the	O
many	O
channels	O
and	O
subchannels	B-Device
.	O
</s>
<s>
The	O
second	O
(	O
also	O
accomplished	O
through	O
PSIP	B-Device
)	O
is	O
where	O
,	O
in	O
an	O
effort	O
to	O
hide	O
subchannels	B-Device
entirely	O
,	O
many	O
cable	O
companies	O
map	O
virtual	B-Protocol
channel	I-Protocol
numbers	O
to	O
underlying	O
physical	O
and	O
sub-channels	O
.	O
</s>
<s>
In	O
such	O
arrangements	O
,	O
the	O
physical/sub	O
-channel	O
numbers	O
are	O
called	O
the	O
"	O
QAM	B-Device
channel	O
"	O
,	O
and	O
the	O
alternative	O
channel	O
designation	O
is	O
called	O
the	O
"	O
mapped	O
channel	O
"	O
,	O
"	O
virtual	B-Protocol
channel	I-Protocol
"	O
,	O
or	O
simply	O
"	O
channel	O
"	O
.	O
</s>
<s>
In	O
theory	O
,	O
a	O
set-top	O
box	O
can	O
decode	O
the	O
PSIP	B-Device
information	O
from	O
every	O
channel	O
it	O
receives	O
and	O
use	O
that	O
information	O
to	O
build	O
the	O
mapping	O
between	O
QAM	B-Device
channel	O
and	O
virtual	B-Protocol
channel	I-Protocol
.	O
</s>
<s>
However	O
,	O
cable	O
companies	O
do	O
not	O
always	O
reliably	O
transmit	O
PSIP	B-Device
information	O
.	O
</s>
<s>
Alternatively	O
,	O
CableCards	B-Application
receive	O
the	O
channel	O
mapping	O
and	O
can	O
communicate	O
that	O
to	O
the	O
set-top	O
box	O
.	O
</s>
<s>
The	O
standard	O
for	O
signal	O
transmission	O
over	O
digital	B-Device
cable	I-Device
television	I-Device
systems	O
in	O
the	O
United	O
States	O
is	O
now	O
fixed	O
as	O
both	O
64-QAM	B-Device
and	O
256-QAM	B-Device
(	O
quadrature	B-Device
amplitude	I-Device
modulation	I-Device
)	O
,	O
which	O
is	O
specified	O
in	O
SCTE	O
07	O
,	O
and	O
is	O
part	O
of	O
the	O
DVB	B-Application
standard	O
(	O
but	O
not	O
ATSC	B-Device
)	O
.	O
</s>
<s>
This	O
method	O
carries	O
38.47	O
Mbit/s	O
using	O
256-QAM	B-Device
on	O
a	O
6	O
MHz	O
channel	O
,	O
which	O
can	O
carry	O
nearly	O
two	O
full	O
ATSC	B-Device
19.39	O
Mbit/s	O
transport	O
streams	O
.	O
</s>
<s>
Each	O
6-MHz	O
channel	O
is	O
typically	O
used	O
to	O
carry	O
712	O
digital	O
SDTV	B-Device
channels	O
(	O
256-QAM	B-Device
,	O
MPEG2	B-Algorithm
MP/ML	O
streams	O
of	O
35	O
Mbit/s	O
)	O
.	O
</s>
<s>
On	O
many	O
boxes	O
with	O
QAM	B-Device
tuners	I-Device
(	O
most	O
notably	O
the	O
DVR	B-Application
boxes	O
)	O
,	O
high	O
definition	O
versions	O
of	O
local	O
channels	O
,	O
and	O
some	O
cable	O
channels	O
are	O
available	O
.	O
</s>
<s>
Digital	B-Device
cable	I-Device
allows	O
for	O
the	O
broadcast	O
of	O
EDTV	O
(	O
480p	O
)	O
as	O
well	O
as	O
HDTV	B-Device
(	O
720p	O
,	O
1080i	O
,	O
and	O
1080p	O
)	O
.	O
</s>
<s>
The	O
Advanced	B-Device
Television	I-Device
Systems	I-Device
Committee	I-Device
standards	I-Device
include	O
a	O
provision	O
for	O
16-VSB	B-Device
transmission	O
over	O
cable	O
at	O
38.4	O
Mbit/s	O
,	O
but	O
the	O
encoding	O
has	O
not	O
yet	O
gained	O
wide	O
acceptance	O
.	O
</s>
<s>
Some	O
SMATV	O
systems	O
may	O
carry	O
8-VSB	B-Device
and	O
QAM	B-Device
signals	O
,	O
mostly	O
in	O
apartment	O
buildings	O
and	O
similar	O
facilities	O
that	O
use	O
a	O
combination	O
of	O
terrestrial	O
antennas	O
and	O
cable	O
distribution	O
sources	O
(	O
such	O
as	O
HITS	O
or	O
"	O
Headend	O
in	O
the	O
Sky	O
"	O
,	O
a	O
unit	O
of	O
Comcast	O
that	O
delivers	O
digital	O
channels	O
by	O
satellite	O
to	O
small	O
cable	O
systems	O
)	O
.	O
</s>
<s>
Digital	B-Device
cable	I-Device
channels	O
typically	O
are	O
allocated	O
above	O
552MHz	O
,	O
the	O
upper	O
frequency	O
of	O
cable	O
channel	O
78	O
.	O
</s>
<s>
Between	O
552	O
and	O
750MHz	O
,	O
there	O
is	O
space	O
for	O
33	O
6-MHz	O
channels	O
(	O
231396	O
SDTV	B-Device
channels	O
)	O
;	O
when	O
going	O
all	O
the	O
way	O
to	O
864MHz	O
,	O
there	O
is	O
space	O
for	O
52	O
6-MHz	O
channels	O
(	O
364624	O
SDTV	B-Device
channels	O
)	O
.	O
</s>
<s>
In	O
the	O
U.S.	O
,	O
digital	B-Device
cable	I-Device
systems	O
with	O
750MHz	O
or	O
greater	O
activated	O
channel	O
capacity	O
are	O
required	O
to	O
comply	O
with	O
a	O
set	O
of	O
SCTE	O
and	O
CEA	O
standards	O
,	O
and	O
to	O
provide	O
CableCARDs	B-Application
to	O
customers	O
that	O
request	O
them	O
.	O
</s>
