jj@alice.UUCP (01/22/85)
>When I studied signal theory briefly about 12 years ago, there was >a theorem stating that it was *impossible* to push more information >through a signal than the bandwidth of the signal, e.g., one can't >send more than k bits per second through a k Hz bandlimnited channel. > >Telephone voice-grade channels are 2700 Hz limited, filtering to allow >signals only from 300 Hz to 3000 Hz. So how do 4800 and 9600 bps >modems work over dialup circuits? (The telco carriers, by the way, >are strict about bandlimiting their signals, since they frequency- >multiplex them onto higher-bandwidth channels.) > >The answer seems to be that the theory that generated that theorem >wasn't completely correct. Maybe the Nyquist theorem shouldn't be >regarded as gospel, either. > >-- >Ed Gould mt Xinu, 739 Allston Way, Berkeley, CA 94710 USA >{ucbvax,decvax}!mtxinu!ed +1 415 644 0146 Ed. When I studied the same thing, about the same length of time ago, I didn't hear of ANYTHING of the sort. Please start on page 297 of Wozencraft and Jacobs's "Principles of Communication Engineering", Wiley, NY 1965, and read section 5.4, which ends with the eqn 5.38. This book was written BEFORE 1965 (although it's still in use) and clearly meets your requirement of 12 years ago. The particular part of the theory, due to Claude Shannon, is quite a bit older. Give the book a read, and then let's hear from you. -- TEDDY BEARS PROTECT PENGUINS FROM WALRUSES "I wish I was home again, back home in my heart again, it's been such a time since my heart's home to me. ..." (allegra,harpo,ulysses)!alice!jj