[Boatanchors] FW: Feb 1964 CQ?
mlangner at swcp.com
Wed Feb 16 16:53:17 EST 2011
Inverted sideband with carrier was routinely used by A. T. & T. for HF
telephone circuits to minimize eavesdropping.
If you could notch out the carrier and insert your own BFO at the right
frequency, the voice signals could be copied loud and clear.
I cannot tell you how many hours I spent as a kid listening to "This is a
test transmission from. . . the American Telephone and Telegraph Company.
This transmission is for technical adjustment purposes only" or words to
It's been over 50 years since I tuned these things during the "magical
golden age of shortwave," so I apologize that I can't remember the exact
language! Maybe someone can fill in the blanks!
929 Alameda Road NW
Albuquerque, NM 87114-1901
(505) 898-3212 home/home office
(505) 238-8810 cell
mlangner at swcp.com
From: boatanchors-bounces at puck.nether.net
[mailto:boatanchors-bounces at puck.nether.net] On Behalf Of doc at kd4e.com
Sent: Wednesday, February 16, 2011 2:33 PM
Subject: Re: [Boatanchors] Feb 1964 CQ?
I received this message privately so I will not add the sender's
name/call/E-mail address, but this was his expressed concern:
Inverted DSB was used as a form of encryption to prevent someone with a BFO
from copying the signals.
Later they went with independent DSB and inverted the audio and split part
of it in each side band. This required a very special demodulator. They also
had the sidebands offset so that they could also put in normal ISB signals
and a RTTY signal. So in one transmission there was an encrypted voice, one
RTTY, maybe 2 and two other voice channels.
I am wondering if the same descriptive label was applied to two somewhat
different modulation schemes and thus the confusion.
What do you think of the circuit and is there any reason to believe that a
6JH8 would not work as well as a 7360 in that circuit?
> Kludge wrote:
> -----Original Message----- From: boatanchors-bounces at puck.nether.net
> [mailto:boatanchors-bounces at puck.nether.net] On Behalf Of doc at kd4e.com
>> I have been challenged to consider if modern SSB receivers, and older
>> AM receivers, can handle the signal produced by this circuit.
> What? You mean I'm not the only one who does that to you? :-)
> Of course they can. AM receivers just need a BFO to re-inject a
> "carrier" locally and modern SSB receivers will only hear one sideband
> but one's all you need.
>> In theory it can be handled but may seem somewhat "odd".
> The only thing odd about it is having a second sideband to play with.
> An adequately tight passband on an AM Rx can reject one sideband with
> a little finagling while a SSB Rx can simply switch sidebands if one
> is less QRM troubled than the other. Of course, that means that the
> op on the other end even knows you have two of them out there. :-)
>> Someone else questioned if this modulation is permitted, or if any
>> DSB is permitted.
> Good grief! "Someone" needs to read part 97. Saying DSB isn't
> permitted is like saying USB isn't permitted on 80& 40, LSB isn't
> permitted on the upper bands and WBFM isn't permitted on the upper
> part of 10m. "Isn't commonly used" is not "isn't permitted." Big
>> As best I understand things DSB is AM with suppressed carrier just as
>> SSB is AM with suppressed carrier and one sideband suppressed as
>> well. (Oversimplified.)
> Simplified but accurate and, to head off any folks who want to pick
> nits, the key word is "simplified."
> Best regards,
> Michael, WH7HG BL01xh
> http://wh7hg.blogspot.com/ http://kludges-other-blog.blogspot.com
> Hiki Nô!
Thanks! & 73, KD4E
David Colburn http://kd4e.com
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