Date   : Fri, 26 Nov 2010 23:47:13 +0000
From   : pete@... (Pete Turnbull)
Subject: Electron Ferranti ULA reverse engineering progress

Rick Murray wrote:

> Speculation: Band-pass filter on the two freqs, presence of signal 
> triggers a positive state for either logic one or logic zero, which is 
> then clocked out as a stream of bits... That's my thought. It wouldn't 
> need to be terribly complicated as the input is fairly well defined. 
> Just a bit of leeway on the freqs and clocking in order to cope 
> gracefully with the minor variations in tape speed.

Yup, some modems used to do that.  Actually you only need one filter, 
and a fairly narrow one at that -- presence of a tone at that frequency 
is a logic zero ("space" state), anything else is a one ("mark" or idle 
state).  But some modems did have two filters, one for each tone, in 
order to provide a carrier detect (either filter passing signal = 
carrier present).

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York