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The Digital Cliff The "digital cliff" is just another misunderstood and even incorrect term. We have been told that the cliff is steep and quick. In truth, the "cliff" isn't a cliff at all, but rather a waterfall. "Represented graphically, the general error-performance characteristics of most digital communication systems have a waterfall-shaped appearance. System performance improves (i.e., bit-error rate decreases) as the signal-to-noise ratio increases. The two curves shown below compare the performance of a typical system with and without forward error-correction coding." This graph is from "How Forward Error Correction works": http://www.aero.org/publications/crosslink/winter2002/04_sidebar1.html
If we were to orient this graph to represent the waterfall, we would turn it so that it reads, from left to right, "nothing" or zero, increasing to "all".
In this orientation, we can see the "waterfall'. The water is flowing from the top-right (good performance), to the bottom left (poor performance). This particular graph, and most that I've found are incomplete. This graph does depict the emergence of usable signal (the nothing point, bottom left) but it does not show the supposed "All" point. IF the digital performance was actually "All-or-nothing", we would be viewing a straight line graph. A graph of unchanging values is rendered as a straight line. The following graph has the fictional "all-or-nothing" graph added.
This graph, surprisingly enough, does have what I view as correct orientation for our use. the blue line is the Shannon bound or the upper limit of performance. I have added the fictional 'All-or-nothing" representation (in black) at the top, or "All" point. As we can plainly see, and is described in the link above, we have a graph that represents CHANGE in performance. There is no "all-or-nothing" and there is no "cliff".
BER vs. signal http://happy.emu.id.au/lab/rep/rep/9801/9801_317.htm#s3p17 "Performance Comparison of ATSC 8-VSB and DVB-T COFDM Transmission Systems for Digital Television Terrestrial Broadcasting" http://paginas.fe.up.pt/~mandrade/tvd/2003/docs/DVB/ATSC/ICCE99paper.pdf"Lab Report 98/01 Section 1: Australian Laboratory Testing of Modulation Systems" http://happy.emu.id.au/lab/rep/rep/9801/9801_001.htm#index Note: The Dish Network 622 and 722, among others, have built in 8VSB and 8PSK technology. |
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