<s>
A	O
software	B-Device
modem	I-Device
,	O
commonly	O
referred	O
to	O
as	O
a	O
softmodem	B-Device
,	O
is	O
a	O
modem	B-Application
with	O
minimal	O
hardware	O
that	O
uses	O
software	O
running	O
on	O
the	O
host	O
computer	O
,	O
and	O
the	O
computer	O
's	O
resources	O
(	O
especially	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
,	O
random	B-Architecture
access	I-Architecture
memory	I-Architecture
,	O
and	O
sometimes	O
audio	B-Device
processing	I-Device
)	O
,	O
in	O
place	O
of	O
the	O
hardware	O
in	O
a	O
conventional	O
modem	B-Application
.	O
</s>
<s>
Softmodems	B-Device
are	O
also	O
sometimes	O
called	O
winmodems	B-Device
due	O
to	O
limited	O
support	O
for	O
platforms	O
other	O
than	O
Windows	B-Application
.	O
</s>
<s>
By	O
analogy	O
,	O
a	O
linmodem	B-Device
is	O
a	O
softmodem	B-Device
that	O
can	O
run	O
on	O
Linux	B-Application
.	O
</s>
<s>
Softmodems	B-Device
are	O
sometimes	O
used	O
as	O
an	O
example	O
of	O
a	O
hard	O
real-time	O
system	O
.	O
</s>
<s>
The	O
audio	O
signals	O
to	O
be	O
transmitted	O
must	O
be	O
computed	O
on	O
a	O
tight	O
interval	O
(	O
on	O
the	O
order	O
of	O
every	O
5	O
or	O
10	O
milliseconds	O
)	O
;	O
they	O
cannot	O
be	O
computed	O
in	O
advance	O
,	O
and	O
they	O
cannot	O
be	O
late	O
or	O
the	O
receiving	O
modem	B-Application
will	O
lose	O
synchronization	O
.	O
</s>
<s>
The	O
first	O
generations	O
of	O
hardware	O
modems	B-Application
(	O
including	O
acoustic	B-Device
couplers	I-Device
)	O
and	O
their	O
protocols	O
used	O
relatively	O
simple	O
modulation	B-Application
techniques	O
such	O
as	O
FSK	O
or	O
ASK	O
at	O
low	O
speeds	O
.	O
</s>
<s>
Under	O
these	O
conditions	O
,	O
modems	B-Application
could	O
be	O
built	O
with	O
the	O
analog	O
discrete	O
component	O
technology	O
used	O
during	O
the	O
late	O
70s	O
and	O
early	O
80s	O
.	O
</s>
<s>
New	O
modulation	B-Application
required	O
mixing	O
analog	O
and	O
digital	O
components	O
,	O
and	O
eventually	O
incorporating	O
multiple	O
integrated	O
circuits	O
(	O
ICs	O
)	O
such	O
as	O
logical	O
gates	O
,	O
PLLs	O
and	O
microcontrollers	B-Architecture
.	O
</s>
<s>
Later	O
techniques	O
used	O
in	O
modern	O
V.34	O
,	O
V.90	O
and	O
V.92	O
protocols	O
(	O
such	O
as	O
a	O
1664-point	O
QAM	B-Device
constellation	O
)	O
are	O
so	O
complex	O
that	O
implementing	O
them	O
with	O
discrete	O
components	O
or	O
general	O
purpose	O
ICs	O
became	O
impractical	O
.	O
</s>
<s>
Furthermore	O
,	O
improved	O
compression	O
and	O
error	O
correction	O
schemes	O
were	O
introduced	O
in	O
the	O
newest	O
protocols	O
,	O
requiring	O
extra	O
processing	O
power	O
in	O
the	O
modem	B-Application
itself	O
.	O
</s>
<s>
This	O
made	O
the	O
construction	O
of	O
a	O
mainly	O
analog/discrete	O
component	O
modem	B-Application
impossible	O
.	O
</s>
<s>
Finally	O
,	O
compatibility	O
with	O
older	O
protocols	O
using	O
completely	O
different	O
modulation	B-Application
schemes	O
would	O
have	O
required	O
a	O
modem	B-Application
made	O
with	O
discrete	O
electronics	O
to	O
contain	O
multiple	O
complete	O
implementations	O
.	O
</s>
<s>
Initially	O
the	O
solution	O
was	O
to	O
use	O
LSI	O
ASICs	O
which	O
shrank	O
the	O
various	O
implementations	O
into	O
a	O
small	O
number	O
of	O
components	O
,	O
but	O
since	O
standards	O
continued	O
to	O
change	O
,	O
there	O
was	O
a	O
desire	O
to	O
create	O
modems	B-Application
that	O
could	O
be	O
upgraded	O
.	O
</s>
<s>
In	O
1993	O
,	O
Digicom	O
marketed	O
the	O
Connection	B-Device
96	I-Device
Plus	I-Device
,	O
a	O
modem	B-Application
based	O
around	O
a	O
DSP	B-Architecture
which	O
was	O
programmed	O
by	O
an	O
application	O
on	O
startup	O
.	O
</s>
<s>
Because	O
the	O
program	O
was	O
replaceable	O
,	O
the	O
modem	B-Application
could	O
be	O
upgraded	O
as	O
standards	O
improved	O
.	O
</s>
<s>
Digicom	O
branded	O
this	O
technology	O
SoftModem	B-Device
,	O
perhaps	O
originating	O
the	O
term	O
.	O
</s>
<s>
Likewise	O
,	O
the	O
term	O
"	O
Winmodem	B-Device
"	O
may	O
have	O
originated	O
with	O
USRobotics	O
 '	O
Sportster	B-Device
Winmodem	I-Device
,	O
a	O
similarly	O
upgradable	O
DSP-based	O
design	O
.	O
</s>
<s>
In	O
1996	O
,	O
two	O
types	O
of	O
modem	B-Application
began	O
to	O
reach	O
the	O
market	O
:	O
host-based	B-Device
modems	I-Device
,	O
which	O
offloaded	O
some	O
work	O
onto	O
the	O
host	O
CPU	B-General_Concept
,	O
and	O
software-only	B-Device
modems	I-Device
which	O
transferred	O
all	O
work	O
onto	O
the	O
host	O
system	O
's	O
CPU	B-General_Concept
.	O
</s>
<s>
In	O
1997	O
,	O
the	O
AC'97	O
standard	O
for	O
computer	O
audio	O
would	O
introduce	O
channels	O
for	O
modem	B-Application
use	O
,	O
making	O
software	B-Device
modem	I-Device
technology	O
common	O
in	O
PCs	O
.	O
</s>
<s>
Since	O
then	O
,	O
some	O
softmodems	B-Device
have	O
been	O
created	O
as	O
standalone	O
software	O
projects	O
utilizing	O
standard	O
sound	B-Device
card	I-Device
interfaces	O
,	O
such	O
as	O
an	O
experimental	O
open-source	O
96	O
kbit/s	O
leased-line	O
softmodem	B-Device
called	O
from	O
1999	O
,	O
and	O
the	O
project	O
which	O
implements	O
several	O
FSK	O
modem	B-Application
standards	O
.	O
</s>
<s>
Softmodems	B-Device
can	O
be	O
separated	O
into	O
two	O
classes	O
:	O
controllerless	B-Device
modems	I-Device
and	O
pure	B-Device
software	I-Device
modems	I-Device
.	O
</s>
<s>
Controllerless	B-Device
modems	I-Device
utilize	O
a	O
DSP	B-Architecture
on	O
the	O
modem	B-Application
itself	O
to	O
perform	O
modulation	B-Application
,	O
demodulation	B-Device
and	O
other	O
tasks	O
.	O
</s>
<s>
Some	O
,	O
known	O
as	O
"	O
host-based	O
"	O
modems	B-Application
,	O
may	O
still	O
use	O
some	O
amount	O
of	O
the	O
host	O
's	O
CPU	B-General_Concept
power	O
for	O
some	O
tasks	O
.	O
</s>
<s>
Pure	B-Device
software	I-Device
modems	I-Device
perform	O
all	O
modem	B-Application
tasks	O
on	O
the	O
host	O
PC	O
's	O
CPU	B-General_Concept
,	O
while	O
the	O
hardware	O
provides	O
only	O
analog-digital	O
conversion	O
and	O
connection	O
to	O
the	O
telephone	O
network	O
.	O
</s>
<s>
The	O
original	O
stated	O
purpose	O
of	O
the	O
DSP-based	O
softmodem	B-Device
was	O
to	O
provide	O
for	O
upgradeability	O
,	O
a	O
concern	O
in	O
an	O
era	O
when	O
modem	B-Application
standards	O
were	O
changing	O
rapidly	O
.	O
</s>
<s>
Both	O
DSP	B-Architecture
and	O
pure	B-Device
software	I-Device
modems	I-Device
offer	O
this	O
feature	O
.	O
</s>
<s>
A	O
downside	O
of	O
either	O
type	O
of	O
softmodem	B-Device
is	O
that	O
drivers	O
must	O
be	O
provided	O
,	O
and	O
the	O
terms	O
"	O
softmodem	B-Device
"	O
and	O
"	O
winmodem	B-Device
"	O
have	O
gained	O
negative	O
connotations	O
,	O
particularly	O
within	O
the	O
open-source	O
community	O
,	O
due	O
to	O
drivers	O
for	O
Linux	B-Application
often	O
being	O
omitted	O
or	O
provided	O
only	O
as	O
unmaintainable	O
binaries	O
.	O
</s>
<s>
While	O
DSP-based	O
softmodems	B-Device
usually	O
only	O
require	O
host	O
attention	O
during	O
startup	O
,	O
pure	B-Device
software	I-Device
modems	I-Device
consume	O
some	O
CPU	B-General_Concept
cycles	O
on	O
the	O
host	O
,	O
which	O
can	O
conceivably	O
slow	O
down	O
application	B-Application
software	I-Application
on	O
older	O
computers	O
.	O
</s>
<s>
Although	O
"	O
softmodem	B-Device
"	O
typically	O
applies	O
to	O
PSTN	O
modems	B-Application
,	O
there	O
are	O
some	O
software-based	O
DSL	O
modems	B-Application
or	O
even	O
routers	B-Protocol
,	O
which	O
work	O
on	O
the	O
same	O
principles	O
but	O
at	O
higher	O
bandwidth	O
and	O
with	O
more	O
complex	O
encoding	O
schemes	O
.	O
</s>
<s>
One	O
of	O
the	O
first	O
software	O
based	O
DSL	O
modem	B-Application
chipsets	O
was	O
Motorola	O
's	O
.	O
</s>
<s>
DSL	O
softmodems	B-Device
generally	O
require	O
the	O
same	O
interfaces	O
as	O
PSTN	O
softmodems	B-Device
,	O
such	O
as	O
USB	B-Protocol
or	O
PCI	B-Protocol
.	O
</s>
