On Aug 20, 2009, at 6:30 PM, John Coppens wrote:

>> Does anyone have a link to a schematic of the first modems based
>> apparently on LM555s ? Or anything similar ? Old RTTY modems,  
>> including
>> valved ones, are not quite in this class although they come close.
>
> I'm guessing you mean the NE565 (there may be an LM565 version). That
> chip is a PLL which can be used to decode FSK signals.

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.)

You might find these articles on the 1200Hz/2400Hz "Kansas City  
Standard" Tape format to be interesting:  http://www.swtpc.com/mholley/AC30/AC30_Index.htm
http://www.hartetechnologies.com/manuals/SD%20Sales/SD%20Cassette%20Interface%20Manual.pdf

BillW

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