pretty much OT: acoustic coupling

We will get several comments after this post... I hope y'all will correct me. The acoustic coupler on the old teletype was 1200 baud, 10 characters per sec. to match the machine. It was FSK (Frequency Shift Keying) with two tones 1200hz and 2400hz for 0 and one. Depending on the # of start, stop bits in the serial stream and parity baud .NE. bytes/sec. Phone lines have a 2500hz bandwidth, and today's modems use QAM (Quadrature Amplitude Modulation). You can visualize this if you think of a signal as a rotating vector on an xy grid - say a constant sine at constant amplitude would represent a circle. put 4 points uniformly on the circle and XY grid, and you have 4 QAM. 16 points (rectangular array, not on the unit circle), 16-QAM etc. Each point represents 2 bits in 4-QAM (dibits), and 4 bits in 16-QAM. this is called the signal constellation. To further encode the signal and reduce errors, they use techniques called viterbi and trellis encoding, these limit the accepted signal transition In addition, modern modems train the channel. That's the white noise you hear at the beginning of the dialup - both ends are doing equalization for this connection. If errors occur, they fall back to lower data rates and retrain. Our 56K modems never get there here in the US, it is an FCC limit, and the reasons I am not sure. These basic principles above also apply to cell/radio transmission. There are other goodies they can throw into the algorithms to reduce multipath reflection (ghosts, as in TV) due to buildings objects, aircraft. to conclude, Ill bet with a homemade cell phone acoustic coupler to -> RJ-45 jack interface you could get same performance as dialup, near 56K. Needed: Sponge/muffs to adapt the cell handset electrostatic mic/speaker some op amp stuff to get the signals leveled resolve the dial-in and dial-out issues to fool the PC for dial tone and auto answer (and interface to the cell keypad to answer and send) Randy _________________________________________________________________ 5.5%* 30 year fixed mortgage rate. Good credit refinance. Up to 5 free quotes - *Terms https://www2.nextag.com/goto.jsp?product=100000035&url=%2fst.jsp&tm…