[GreenKeys] AP Model 15 information

[Dave Emery]

On Mon, Feb 23, 2004 at 12:08:22PM -0500, Dan Beach wrote:
> 
> I have recently acquired a Model 15 which was used by the Associated Press.
> It appears to have a straight 120vac power connection (although the plug is
> the 3-pin  connector in Y-layout, usually associated with DC), along with a
> red/green pair which went to the phone line connection. In addition, there
> is a panel in the rear of the machine I've never seen before which has a
> transformer, capacitor and various  diodes and ic's.  Does anyone know if I
> am safe assuming that this is the loop power supply? And if so, what would
> have been the signal provided thru the phone line to trigger the relays?
> Just polarity reversal?
> 

	In the mid 60s most wire service (AP, UPI, Reuters etc) networks
were converted from DC +-20 ma telegraph circuits to VFT multiplex audio
transmission of the tty data.   These consisted of up to 24 channels of
low speed FSK signals FDM multiplexed together in a 3.2 khz one way 
audio channel (VF data circuit).

	It rather sounds like you have one of the modems for that.

	The standard used by the AP was 120 hz between channels and
around 60 hz FSK shift with the lowest channel starting at 420 hz.

	This technology was in use at least into the mid 80s and AFAIK
may have seen service even after that in some areas.

	Prior to this conversion, wire service circuits were distributed
by more antique Telco based VFT equipment to local private line centers
and then distributed by DC loops to the customers.    And it turns out
that if there were multiple customers served by one CO they put the
loops in series.    I discovered this back in the 60s when we tried to
move our college radio station wire service machine and disconnected the
loop and got panic calls from the local TV station that their UPI
service  was dead...

	In addition to avoiding that problem, the VFT mux system allowed
multiple circuits to be installed at low cost - for newspapers for
example that might have a sports wire, a national A news wire and and a
local area news wire.

	Later on in the 70s the wire services took to transmitting
material via 1200 baud 202 FSK modems (the basic modem standard ham AFSK
packet uses) to newspapers with early computerized text processing.   
Then in the 80s and 90s much of this went to satellite distribution and
now a lot is via the Internet (VPNs and so forth).

-- 
   Dave Emery N1PRE,  die@dieconsulting.com  DIE Consulting, Weston, Mass 02493