<s>
RealAudio	B-Algorithm
,	O
or	O
also	O
spelled	O
as	O
Real	B-Algorithm
Audio	I-Algorithm
is	O
a	O
proprietary	O
audio	O
format	O
developed	O
by	O
RealNetworks	O
and	O
first	O
released	O
in	O
April	O
1995	O
.	O
</s>
<s>
It	O
uses	O
a	O
variety	O
of	O
audio	B-Algorithm
codecs	I-Algorithm
,	O
ranging	O
from	O
low-bitrate	O
formats	O
that	O
can	O
be	O
used	O
over	O
dialup	O
modems	O
,	O
to	O
high-fidelity	O
formats	O
for	O
music	O
.	O
</s>
<s>
In	O
the	O
past	O
,	O
many	O
internet	O
radio	O
stations	O
used	O
RealAudio	B-Algorithm
to	O
stream	O
their	O
programming	O
over	O
the	O
internet	O
in	O
real	O
time	O
.	O
</s>
<s>
RealAudio	B-Algorithm
was	O
heavily	O
used	O
by	O
the	O
BBC	O
websites	O
until	O
2009	O
,	O
though	O
it	O
was	O
discontinued	O
due	O
to	O
its	O
declining	O
use	O
.	O
</s>
<s>
BBC	O
World	O
Service	O
,	O
the	O
last	O
of	O
the	O
BBC	O
websites	O
to	O
use	O
RealAudio	B-Algorithm
,	O
discontinued	O
its	O
use	O
in	O
March	O
2011	O
.	O
</s>
<s>
RealAudio	B-Algorithm
files	O
were	O
originally	O
identified	O
by	O
a	O
filename	O
extension	O
of	O
.ra	B-Algorithm
(	O
for	O
Real	B-Algorithm
Audio	I-Algorithm
)	O
.	O
</s>
<s>
However	O
,	O
the	O
latest	O
version	O
of	O
RealProducer	O
,	O
Real	O
's	O
flagship	O
encoder	O
,	O
reverted	O
to	O
using	O
.ra	B-Algorithm
for	O
audio-only	O
files	O
,	O
and	O
began	O
using	O
.rv	O
for	O
video	O
files	O
(	O
with	O
or	O
without	O
audio	O
)	O
,	O
and	O
.rmvb	O
for	O
VBR	O
video	O
files	O
.	O
</s>
<s>
The	O
.ram	B-Algorithm
(	O
Real	B-Algorithm
Audio	I-Algorithm
Metadata	O
)	O
and	O
.smil	B-Language
(	O
Synchronized	B-Language
Multimedia	I-Language
Integration	I-Language
Language	I-Language
)	O
file	O
formats	O
are	O
sometimes	O
encountered	O
as	O
links	O
from	O
web	O
pages	O
(	O
see	O
.Streaming	O
Audio	O
section	O
below	O
)	O
.	O
</s>
<s>
The	O
official	O
player	O
for	O
RealMedia	O
content	O
is	O
RealNetworks	O
 '	O
RealPlayer	B-Application
SP	I-Application
,	O
currently	O
at	O
version	O
16	O
,	O
and	O
is	O
available	O
for	O
various	O
platforms	O
in	O
binary	O
form	O
.	O
</s>
<s>
However	O
,	O
in	O
recent	O
years	O
,	O
RealNetworks	O
has	O
made	O
efforts	O
to	O
be	O
somewhat	O
more	O
open	O
,	O
and	O
has	O
founded	O
the	O
Helix	B-Application
Community	I-Application
,	O
a	O
collaborative	O
open	B-License
source	I-License
project	O
,	O
to	O
extend	O
their	O
media	O
framework	O
.	O
</s>
<s>
When	O
RealAudio	B-Algorithm
was	O
introduced	O
,	O
RealNetworks	O
disclosed	O
no	O
technical	O
details	O
about	O
the	O
audio	O
format	O
or	O
how	O
it	O
was	O
encoded	O
,	O
but	O
it	O
was	O
soon	O
noticed	O
that	O
some	O
of	O
the	O
audio	B-Algorithm
codecs	I-Algorithm
used	O
in	O
RealAudio	B-Algorithm
were	O
identical	O
to	O
those	O
used	O
in	O
cellular	O
telephones	O
and	O
digital	B-Application
television	I-Application
.	O
</s>
<s>
As	O
these	O
formats	O
had	O
been	O
described	O
in	O
detail	O
in	O
various	O
technical	O
papers	O
and	O
standards	O
documents	O
,	O
it	O
was	O
possible	O
to	O
write	O
software	O
capable	O
of	O
playing	O
RealAudio	B-Algorithm
based	O
on	O
this	O
information	O
.	O
</s>
<s>
A	O
variety	O
of	O
unofficial	O
players	O
now	O
exist	O
,	O
including	O
MPlayer	B-Application
,	O
and	O
Real	O
Alternative	O
.	O
</s>
<s>
However	O
,	O
Real	O
Alternative	O
does	O
not	O
decode	O
the	O
audio	O
data	O
by	O
itself	O
,	O
but	O
relies	O
on	O
the	O
dynamically	O
linked	O
libraries	O
(	O
DLLs	O
)	O
from	O
the	O
official	O
RealPlayer	B-Application
.	O
</s>
<s>
Thus	O
Real	O
Alternative	O
requires	O
RealPlayer	B-Application
to	O
be	O
installed	O
(	O
or	O
at	O
least	O
its	O
DLLs	O
)	O
in	O
order	O
to	O
function	O
.	O
</s>
<s>
Most	O
other	O
players	O
are	O
based	O
on	O
FFmpeg	B-Library
,	O
which	O
has	O
its	O
own	O
audio	B-Algorithm
codec	I-Algorithm
library	O
.	O
</s>
<s>
The	O
audio	B-Algorithm
codecs	I-Algorithm
in	O
ffmpeg	B-Library
were	O
written	O
based	O
on	O
the	O
publicly	O
available	O
information	O
about	O
the	O
formats	O
,	O
and	O
do	O
not	O
use	O
the	O
RealPlayer	B-Application
or	O
Helix	B-Application
software	O
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
obtain	O
codecs	B-General_Concept
which	O
allow	O
Windows	B-Device
Media	I-Device
Player	I-Device
to	O
play	O
some	O
versions	O
of	O
RealAudio	B-Algorithm
.	O
</s>
<s>
Although	O
RealNetworks	O
has	O
made	O
the	O
Helix	B-Application
player	I-Application
available	O
as	O
an	O
open	B-License
source	I-License
project	O
,	O
they	O
have	O
kept	O
some	O
of	O
the	O
audio	B-Algorithm
codecs	I-Algorithm
proprietary	O
,	O
and	O
the	O
Helix	B-Application
player	I-Application
can	O
not	O
play	O
all	O
RealAudio	B-Algorithm
files	O
.	O
</s>
<s>
RealAudio	B-Algorithm
was	O
developed	O
as	O
a	O
streaming	O
media	O
format	O
,	O
meaning	O
that	O
it	O
can	O
be	O
played	O
while	O
it	O
is	O
downloaded	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
stream	O
RealAudio	B-Algorithm
using	O
HTTP	B-Protocol
.	I-Protocol
</s>
<s>
In	O
this	O
case	O
,	O
the	O
RealAudio	B-Algorithm
file	O
is	O
retrieved	O
similarly	O
to	O
a	O
normal	O
web	O
page	O
,	O
but	O
playback	O
begins	O
as	O
soon	O
as	O
the	O
first	O
part	O
is	O
received	O
and	O
continues	O
while	O
the	O
rest	O
of	O
the	O
file	O
is	O
downloaded	O
.	O
</s>
<s>
Using	O
HTTP	B-Protocol
streaming	O
works	O
best	O
with	O
pre-recorded	O
files	O
so	O
some	O
alternative	O
protocols	O
have	O
been	O
developed	O
which	O
work	O
better	O
for	O
live	O
broadcasts	O
.	O
</s>
<s>
The	O
first	O
version	O
of	O
RealAudio	B-Algorithm
used	O
a	O
proprietary	O
protocol	O
called	O
PNA	O
or	O
PNM	O
to	O
send	O
streaming	O
audio	O
data	O
.	O
</s>
<s>
RealNetworks	O
later	O
switched	O
to	O
the	O
IETF	O
standardized	O
Real	B-Protocol
Time	I-Protocol
Streaming	I-Protocol
Protocol	I-Protocol
(	O
RTSP	B-Protocol
)	O
but	O
they	O
use	O
RTSP	B-Protocol
only	O
to	O
manage	O
the	O
connection	O
.	O
</s>
<s>
The	O
actual	O
audio	O
data	O
is	O
sent	O
with	O
their	O
own	O
proprietary	O
RDT	B-Protocol
protocol	O
,	O
which	O
they	O
initially	O
kept	O
secret	O
.	O
</s>
<s>
Recently	O
,	O
some	O
specifications	O
for	O
the	O
RDT	B-Protocol
protocol	O
have	O
been	O
made	O
public	O
through	O
the	O
Helix	B-Application
Community	I-Application
project	O
.	O
</s>
<s>
By	O
around	O
2004	O
the	O
open-source	B-License
MPlayer	B-Application
project	O
developed	O
a	O
means	O
of	O
playing	O
the	O
RDT	B-Protocol
streams	O
.	O
</s>
<s>
In	O
many	O
cases	O
,	O
web	O
pages	O
do	O
not	O
link	O
directly	O
to	O
a	O
RealAudio	B-Algorithm
file	O
.	O
</s>
<s>
Instead	O
,	O
they	O
link	O
to	O
a	O
.ram	B-Algorithm
(	O
Real	B-Algorithm
Audio	I-Algorithm
Metadata	O
)	O
or	O
SMIL	B-Language
file	O
.	O
</s>
<s>
This	O
is	O
a	O
small	O
text	B-General_Concept
file	I-General_Concept
containing	O
a	O
link	O
to	O
the	O
audio	O
stream	O
.	O
</s>
<s>
When	O
a	O
user	O
clicks	O
on	O
such	O
a	O
link	O
,	O
the	O
user	O
's	O
web	O
browser	O
downloads	O
the	O
.ram	B-Algorithm
or	O
.smil	B-Language
file	O
and	O
launches	O
the	O
user	O
's	O
media	B-Device
player	I-Device
.	O
</s>
<s>
The	O
media	B-Device
player	I-Device
reads	O
the	O
PNM	O
or	O
RTSP	B-Protocol
URL	O
from	O
the	O
file	O
and	O
then	O
plays	O
the	O
stream	O
.	O
</s>
<s>
With	O
RealPlayer	B-Application
SP	I-Application
,	O
it	O
is	O
now	O
possible	O
to	O
save	O
an	O
audio	O
stream	O
to	O
a	O
file	O
.	O
</s>
<s>
Other	O
programs	O
,	O
including	O
MPlayer	B-Application
,	O
RM	O
Downloader	O
,	O
VLC	B-Application
media	I-Application
player	I-Application
,	O
StreamBox	O
VCR	O
,	O
HiDownload	O
and	O
Real7ime	O
Converter	O
can	O
also	O
save	O
streams	O
to	O
a	O
file	O
.	O
</s>
<s>
RealAudio	B-Algorithm
files	O
are	O
compressed	O
using	O
several	O
audio	B-Algorithm
codecs	I-Algorithm
.	O
</s>
<s>
Each	O
codec	B-General_Concept
is	O
identified	O
by	O
a	O
four	B-Application
character	I-Application
code	I-Application
.	O
</s>
<s>
Below	O
is	O
a	O
list	O
of	O
the	O
codecs	B-General_Concept
and	O
the	O
version	O
in	O
which	O
each	O
was	O
introduced	O
:	O
</s>
<s>
While	O
the	O
newest	O
version	O
of	O
RealPlayer	B-Application
should	O
be	O
able	O
to	O
play	O
any	O
RealAudio	B-Algorithm
file	O
,	O
other	O
programs	O
may	O
not	O
support	O
all	O
codecs	B-General_Concept
.	O
</s>
