87 lines
3.6 KiB
HTML
87 lines
3.6 KiB
HTML
<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<meta name="HISTORY" content="19990602: First web-announce of i4lfax Software Fax OSS project">
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<meta name="KeyWords" content="V.21, V.29, V.17, fax, modem, DSP, modulation, demodulation">
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<title>i4lfax - Software fax solution</title>
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</head>
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<body text="#FFFFFF" bgcolor="#000000" link="#00EE00" vlink="#00EE00" alink="#00EE00">
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<table border=0 cellspacing=0 cellpadding=0>
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<tr>
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<td colspan=3><a href="banner.html"><img src="i4lfaxbanner.gif" height=110 width=760 border=0></a></td>
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<tr>
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<td valign=top>
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<table width=200 cellpadding=3>
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<tr><td valign=top> </td></tr>
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<tr><td valign=top><h2><a href="index.html">Back to<br>main page</a></h2></td></tr>
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<tr><td valign=top> </td></tr>
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<tr><td valign=top>Last updated Jun. 2 1999 by <a href="mailto:Morten.Rolland@asker.mail.telia.com">Morten Rolland</a></td></tr>
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</table>
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</td>
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<td valign=top>
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<table width=560>
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<td>
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<h1>How does it work?</h1>
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<p>Audio can be digitally sampled like on a music-CD or sound-card.
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A computer can run software algorithms on such digital samples
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that mimic the inner workings of a standard modem that may itself be designed
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purely from amplifiers, filters, signal-generators and what-not.</p>
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<p>Actually, a software algorithm can also do stuff that is
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practically impossible with standard electronic circuits, so all modern
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modems uses one or several software algorithms to perform signal analysis
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and generation. New 56k modems would be impossible where it not for
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such software algorithms. A special breed of processors called
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<i>"Digital Signal Processors"</i> (DSPs) are mostly used for this kind of
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task, as they are designed to cope with the enormous amounts of
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multiplications, additions and fast/strange indexing that are
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usually needed by the software algorithms.</p>
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<P>In modems that uses software algorithms, the circuits needed to connect
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to the phone-line resembles that of a sound-card; they can turn the
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incomming signal into digital samples, and they can convert the outgoing
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digital samples into an electrical signal suitable for the phone-line.
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Most of what happens inside a modern modem is thus the result of a
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computer program. There is basically no reason why this computer program
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can't be run on the main processor of the computer instead of inside the
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modem, except from:</p>
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<ol>
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<li>The algorithms has to be developed/known, or licensed from a company
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with experience from modem design</LI>
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<li>Running the software algorithms of a modem may be too demanding for
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the main processor of the computer.</li>
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</ol>
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<p>The second point is no longer very relevant, as computers keep
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getting faster all the time, with the occational big leap like
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the SIMD extensions of the Pentium. The first point however, is the reason
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why there is no such software for Linux. We hope to change this.</p>
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<p>By using the (soft-) realtime scheduler in the standard Linux kernel
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and disabeling paging for the modem software, the problems with running
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a timing-critical task on a multiuser system can be solved.</p>
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<p>By using an ISDN-system, the audio-samples can be sent directly
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towards a remote fax-machine without any other equipment than the ISDN-card.
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There will probably be support for Win-modems some time in the future if
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specifications can be obtained.</p>
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</td>
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</table>
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</td>
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</tr>
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</table>
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</body>
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</html>
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