[GreenKeys] FSK on WABC

[sdaitch at kuw.ibb.gov]

The April, 1970, issue of BM/E (Broadcast Management/
Engineering) had an article written by Barry Research's
Larry Peden and John W. McMains titled "Multiplexing
Teletype Signals at an AM Station."

The transmitter unit was the BR Teleprinter Convertor
TCT-2 and phase modulator TPM-2.  No mention of a 
matching BR demodulator unit.

The article discusses TTY speeds up to 100 WPM.  "At
a 60 WPM data rate, phase deviations in this system
are 25 degrees P-P or plus or minus 10 Hz, well within
the 20-Hz AM tolerance."

The article also says VOA was using the system on the
shortwave transmitters, no doubt Greenville, for 
the wireless file.  When I came to Greenville in 1979, the
wireless file was sent as RTTY AFSK on one of the ISB 
transmitters, and the BR equipment was not being used.
In fact, I am not even sure if any of it was in the
racks at the transmitter sites.  I just don't remember
seeing them, or at this stage of my life, remember them
at all.

The article says the VOA tests were successful.  

Upon reflection of the operation of the VOA HF relay 
station network, I'd have to say the test's success might
have to be qualified.

In the late 1960s, VOA was accepting large groups of 
RCA SSB-R3 ISB receivers at the relay stations overseas.
The receivers were used to pick up programming from the
domestic VOA transmitters for retransmission on the 
overseas transmitters.  These receivers had an AFC
circuit which used either the carrier on the AM transmitters
or the pilot carrier, roughly 20 dB down from PEP on the
ISB transmitters to keep the receivers locked on frequency.

This was important as the demodulator in these receivers
used synchronous detection and any offset in RF frequency 
would change the audio frequencies by the same amount as
the frequency error.  Think tuning in an SSB signal and
not being exactly on frequency.

The RCA had a quite sharp carrier filter, which passed 
the carrier signal to the AFC circuit.

The AFC circuit variable capacitor was a motor driven cap,
the circuit very similar to the AFC in the CV-157 convertor.
Because of this arrangment, I can't imagine the AFC would
track the 60 WPM FSK signal, keeping the RCA receiver on
frequency, and I think you'd have to be deaf not to hear
the shift on the audio, as it would be warble at the FSK
keying rate.

73
Sheldon
WA4MZZ
















----- Original Message -----
From: Larry Tighe <larryradio at worldnet.att.net>
Date: Tuesday, December 1, 2009 8:50 pm
Subject: Re: [GreenKeys] FSK on WABC

> What with George saying all those Clear Channel 1A stations doing 
> FSK, there 
> must've been quite a dependable commo network...one 
> direction....around the 
> USA.  All those are on their frequency exclusively ...not sharing 
> the 
> channel anywhere and devoid of AM nite skip interference...ever.  
> Cuba did, 
> for a while, run some really high power interfering with WOR...a 
> 1B shared 
> channel...I don't know of any others.  My late friend, John 
> Serafin..W2KQT, 
> later W2QD, was a VP of engineering and told me of the FSK.
> 
> lar
> ----- Original Message ----- 
> From: "Richard Dillman" <ddillman at igc.org>
> To: "Larry Tighe" <larryradio at worldnet.att.net>; "Nick England" 
> <navy.radio at gmail.com>; <greenkeys at mailman.qth.net>
> Sent: Tuesday, December 01, 2009 12:44 PM
> Subject: FSK on WABC
> 
> 
> >
> >>The military had contracted with WABC  770 KHz in NYC to operate 
> FSK, 
> >>very,
> >>very narrow, in the 70's.  I was unable to find out much about 
> it in those
> >>days...it was imperceptible on the AM radio.
> >
> > I read your post with great interest.  A friend was a studio 
> engineer at 
> > WABC at the time and once told me how he happened to notice that 
> the 
> > needle on the station's frequency monitor was bouncing all over 
> the scale. 
> > He was told "you don't need to worry about that" but later found 
> out about 
> > the program you mention.  I was never able to find anything more 
> about it 
> > so I was glad to see your post as confirmation that the program 
> did indeed 
> > exist.
> >
> > One assumes it was for submarine communications but I wonder if 
> anyone has 
> > any further details.
> >
> > Regards,
> >
> > RD
> >
> 
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