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Last updated Jun. 2 1999 by Morten Rolland

The banner graphics

The banner above was designed using 'gimp' on a Linux workstation. During the hours spent on designing the graphics, I learned a lot about 'gimp', which basically was my motivation for spending so much time designing it. It is probably very naive, but it was fun making it.

Appart from the obvious fax-page on the left, there is a 'signal-space diagram' that shows the complex signal-points plotted in a Re/Im coordinate system. Basically, a point (marked with a cross) designates a given four-bit binary sequence. The distance of a point from the center indicates what amplitude the modulation carrier must have to transmit the given four-bit binary sequence, while the angle indicates how much the carrier has to be phase-shifted. With 16 signal-points, there is one point for each possible four-bit sequence, which makes it possible to transfer 4 bits in one "symbol-period" or "baud". In V.29 there are 2400 symbol periods per second, which gives a bit-rate of 4 x 2400 => 9600 bit/s.

The curve is what a given set of signal-points may translate into in terms of digital samples, which gradually gets smoothed as they turn analog somewhere along the line before they finaly reach the destination fax-machine.

The background is basically random sequences of 0/1, but a repeating pattern of '01111110' can be seen, which is the "flag-sequence" used in the synchronous HDLC link-level protocol used among other places, in fax transfers.