On Thu, 20 Aug 2009 21:44:58 -0700 "William \"Chops\" Westfield" wrote: > Or perhaps the LM567 tone decoder? Having been VERY interested in > making my own modem in the mid to late 1970s, I feel pretty confident > in saying that there was never a modem design based on the 555 timer. > (Note that the 555 was first sold in 1971, so it post-dates the really > early modems.) The 567 detected presence/absence of a tone, which is not FSK. I've been quite active with all kinds of modems (include vacuum tube, filter-type modems ;-) and vaguely, I seem to remember a modem which used two LM567's (one for each tone), with some logic to resolve the fact that the outputs can overlap when using two separate detectors. I'm quite sure this was a rarity though. Most popular modems where using active filters on HF, and PLL on VHF. Then the first integrated modems appeared with switched capacitor filter for 300/1200 bauds, like the 7810, which could, simultaneously, fry eggs. Then, some smaller versions appeared which did quite well, at least on VHF (Texas Instruments tml????). John -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist