📄 windows media audio compressor - the code project - managed c++.mht
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<P><IMG height=3D208 alt=3D"Sample Image - Audio Compress"=20
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jpg"=20
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<H2>Introduction</H2>
<P>As I mentioned in my previous article (<A=20
=
href=3D"http://www.codeproject.com/csharp/CSharpRipper.asp">C Sharp=20
MP3 Compressor</A>), I offer now a Windows Media Audio (WMA) =
compressor. This time I used managed C++ instead of C# in =
order to=20
avoid the translation of interfaces and structures of the =
Windows=20
Media Format SDK (WMF SDK). Microsoft has provided very =
limited=20
managed support for their Windows Media SDKs (e.g. for the =
Windows=20
Media Services SDK), but not for the WMF SDK. I think that =
Microsoft=20
will logically include managed support for their WMF SDK in =
future=20
versions, that's why I decided to avoid doing any =
translation. What=20
I did was to create managed C++ classes that use some =
unmanaged=20
classes, and have those unmanaged classes interact directly =
with the=20
various Windows Media Format objects and interfaces.</P>
<P>In this work, there is code from the article <A=20
href=3D"http://www.codeproject.com/cs/media/cswavplay.asp">A =
low level=20
audio player in C#</A> by <A=20
=
href=3D"http://www.codeproject.com/script/profile/whos_who.asp?id=3D90428=
">Ianier=20
Munoz</A>.</P>
<H2>Background</H2>
<P>If you have never looked into the WMF SDK before, it is =
advisable=20
that you take a look at the <A=20
=
href=3D"http://msdn.microsoft.com/windowsmedia/downloads/default.aspx"=20
target=3D_blank>Windows Media Format SDK</A> documentation =
for better=20
understanding this article.</P>
<P>The following diagram represents the steps to create an =
Advanced=20
Systems Format stream (ASF: the container format for Windows =
Media=20
Audio and Windows Media Video-based content) using the WMF =
SDK:</P>
<P><IMG height=3D323 alt=3D"Writing ASF Streams"=20
=
src=3D"http://www.codeproject.com/managedcpp/WmaCompressor/WmaWrite.gif" =
width=3D394></P>
<P><A name=3DFigure1>Figure 1</A>: Writing ASF Streams.</P>
<P>This diagram shows how to create ASF streams using a =
custom=20
"Writer Sink". Custom writer sinks are needed for writing =
the=20
resulting ASF stream to any kind of stream (in this case, =
any class=20
derived from <CODE>System.IO.Stream</CODE>). As the diagram =
shows,=20
the process is relatively simple. For more details about ASF =
creation, you can see <A=20
=
href=3D"http://msdn.microsoft.com/library/en-us/wmform/htm/writingasffile=
s.asp"=20
target=3D_blank>Writing ASF Files</A> and <A=20
=
href=3D"http://msdn.microsoft.com/library/default.asp?url=3D/library/en-u=
s/wmform/htm/usingcustomsinks.asp"=20
target=3D_blank>Using Custom Sinks</A> in the <A=20
=
href=3D"http://msdn.microsoft.com/library/default.asp?url=3D/library/en-u=
s/wmform/htm/introducingwindowsmediaformat.asp"=20
target=3D_blank>Windows Media Format SDK</A> =
documentation.</P>
<H2>The compressor</H2>
<P>The following figure is a simplified class diagram, which =
describes the managed version of the WMA compressor:</P>
<P><IMG height=3D305=20
alt=3D"Windows Media Audio Compressor: Managed class =
diagram"=20
=
src=3D"http://www.codeproject.com/managedcpp/WmaCompressor/ManClass.jpg" =
width=3D600></P>
<P><A name=3DFigure2>Figure 2</A>: Windows Media Audio =
Compressor:=20
Managed class diagram.</P>
<P>The main class is <CODE>WmaWriter</CODE>, which interacts =
with=20
the WMF SDK through unmanaged wrapper classes. It also acts =
as the=20
sink object so the buffers received by its <CODE>Write =
</CODE>method=20
are sent to the WMF writer object, which in turn sends back =
the=20
compressed buffers through the sink interface. Finally, =
those=20
buffers are written to the destination stream. =
<CODE>WmaWriter=20
</CODE>receives the profile information through the class=20
<CODE>WmaWriterProfile</CODE>, which represents the WMF=20
<CODE>IWMProfile</CODE>.</P>
<P><CODE>WmaWriterProfileManger </CODE>is a static class =
that=20
contains some methods to handle profile creation and =
information. It=20
also contains an array of <CODE>WmaWriterProfile</CODE> =
instances=20
whose values represent the WMF System Profiles (audio only). =
This=20
list of profiles should be enough for most purposes, but =
other=20
profiles could also be created.</P>
<P><CODE>ManBuffer</CODE> is a utility class for interfacing =
with=20
the WMF SDK. It is just a managed implementation of buffer =
objects=20
that implement <CODE>INSSBuffer</CODE>. Finally,=20
<CODE>ProfileManager</CODE> is a utility class that creates =
and=20
holds the WMF Profile Manager =
(<CODE>IWMProfileManager</CODE>).</P>
<P>If the WMF SDK was ported to managed code, this =
compressor could=20
be implemented as shown in <A=20
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