221 lines
4.9 KiB
HTML
221 lines
4.9 KiB
HTML
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<HTML
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><HEAD
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><TITLE
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>Sound Card Technology</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.63
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"><LINK
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REL="HOME"
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TITLE="The Linux Sound HOWTO"
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HREF="index.html"><LINK
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REL="PREVIOUS"
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TITLE="Introduction"
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HREF="x24.html"><LINK
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REL="NEXT"
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TITLE="Supported Hardware"
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HREF="x96.html"></HEAD
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><BODY
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CLASS="SECT1"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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WIDTH="100%"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>The Linux Sound HOWTO</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="x24.html"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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></TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="x96.html"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="AEN71"
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>2. Sound Card Technology</A
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></H1
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><P
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>This section gives a <EM
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>very</EM
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> cursory overview of
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computer audio technology, in order to help you understand
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the concepts used later in the document. You should consult a book on
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digital audio or digital signal processing in order to learn more.</P
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><P
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>Sound is an <EM
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>analog</EM
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> property; it can take on any value over a
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continuous range. Computers are <EM
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>digital</EM
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>; they like to work
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with discrete values. Sound cards use a device known as an <EM
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>Analog
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to Digital Converter</EM
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> (A/D or ADC) to convert voltages
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corresponding to analog sound waves into digital or numeric values
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which can be stored in memory. Similarly, a <EM
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>Digital to Analog
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Converter</EM
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> (D/A or DAC) converts numeric values back to an analog
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voltage which can in turn drive a loudspeaker, producing sound.</P
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><P
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>The process of analog to digital conversion, known as
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sampling, introduces some error. Two factors are key in determining
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how well the sampled signal represents the original. <EM
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>Sampling
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rate</EM
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> is the number of samples made per unit of time (usually
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expresses as samples per second or Hertz). A low sampling
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rate will provide a less accurate representation of the analog
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signal. Sample size is the range of values used to represent each
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sample, usually expressed in bits. The larger the sample size, the
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more accurate the digitized signal will be.</P
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><P
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>Sound cards commonly use 8 or 16 bit samples at sampling rates from
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about 4000 to 44,000 samples per second. The samples may also be
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contain one channel (mono) or two (stereo).</P
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><P
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><EM
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>FM Synthesis</EM
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> is an older technique for producing
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sound. It is based on combining different waveforms (e.g. sine,
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triangle, square). FM synthesis is simpler to implement in
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hardware that D/A conversion, but is more difficult to program and
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less flexible. Many sound cards provide FM synthesis for backward
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compatibility with older cards and software. Several independent sound
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generators or <EM
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>voices</EM
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> are usually provided.</P
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><P
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><EM
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>Wavetable Synthesis</EM
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> combines the flexibility of
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D/A conversion with the multiple channel capability of FM
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synthesis. With this scheme digitized voices can be downloaded into
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dedicated memory, and then played, combined, and modified with little
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CPU overhead. State of the art sound cards all support wavetable
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synthesis.</P
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><P
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>Most sound cards provide the capability of <EM
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>mixing</EM
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>, combining
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signals from different input sources and controlling gain levels.</P
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><P
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><EM
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>MIDI</EM
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> stands for Musical Instrument Digital
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Interface, and is a standard hardware and software protocol for
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allowing musical instruments to communicate with each other. The
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events sent over a MIDI bus can also be stored as MIDI files for later
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editing and playback. Many sound cards provide a MIDI interface. Those
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that do not can still play MIDI files using the on-board capabilities
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of the sound card.</P
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><P
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><EM
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>MOD files</EM
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> are a common format for computer
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generated songs. As well as information about the musical notes to be
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played, the files contain digitized samples for the instruments (or
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voices). MOD files originated on the Amiga computer, but can be played
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on other systems, including Linux, with suitable software.</P
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><P
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><EM
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>MP3 files</EM
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> are a popular format for distributing
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computer music and speech. MP3 uses a sophisticated encoding scheme
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(MPEG layer 3) to compress audio by roughly a factor of 10 with little
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reduction in quality as compared to CD audio.</P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="x24.html"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="index.html"
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>Home</A
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></TD
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|
><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="x96.html"
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>Next</A
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></TD
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|
></TR
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|
><TR
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|
><TD
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||
|
WIDTH="33%"
|
||
|
ALIGN="left"
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VALIGN="top"
|
||
|
>Introduction</TD
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|
><TD
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||
|
WIDTH="34%"
|
||
|
ALIGN="center"
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VALIGN="top"
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> </TD
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><TD
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|
WIDTH="33%"
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||
|
ALIGN="right"
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|
VALIGN="top"
|
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|
>Supported Hardware</TD
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|
></TR
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|
></TABLE
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></DIV
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></BODY
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></HTML
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>
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