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Digital TV stations try to cram as many subscription channels as possible
into their allotted frequencies. But if they use too much data compression, the
picture can judder and lose sharpness, causing viewers to complain. Tektronix of
Beaverton, Oregon, has patented a method of avoiding this (EP 869 685). Before
the compressed signal is transmitted, it is fed to a comparator along with the
original, perfect signal. The compressed signal is subtracted from the original
to produce a difference signal, which is checked by a detector. Too much
compression is shown by sharp edges in the difference signal.