65 lines
2.7 KiB
HTML
65 lines
2.7 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>The banner graphics</h1>
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<p>The banner above was designed using 'gimp' on a Linux workstation.
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During the hours spent on designing the graphics, I learned a lot
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about 'gimp', which basically was my motivation for spending so much
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time designing it. It is probably very naive, but it was fun making it.</p>
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<p>Appart from the obvious fax-page on the left, there is a 'signal-space
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diagram' that shows the complex signal-points plotted in a Re/Im coordinate
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system. Basically, a point (marked with a cross) designates a given
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four-bit binary sequence. The distance of a point from the center indicates
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what amplitude the modulation carrier must have to transmit the given four-bit
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binary sequence, while the angle indicates how much the carrier has to be
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phase-shifted. With 16 signal-points, there is one point for each possible
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four-bit sequence, which makes it possible to transfer 4 bits in one
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"symbol-period" or "baud". In V.29 there are 2400 symbol periods per second,
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which gives a bit-rate of 4 x 2400 => 9600 bit/s.</p>
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<p>The curve is what a given set of signal-points may translate
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into in terms of digital samples, which gradually gets smoothed as they
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turn analog somewhere along the line before they finaly reach the destination
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fax-machine.</p>
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<p>The background is basically random sequences of 0/1, but a repeating
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pattern of '01111110' can be seen, which is the "flag-sequence" used
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in the synchronous HDLC link-level protocol used among other places, in fax
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transfers.</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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