Re: [RTTY] Fun, but

["Junior" <charlesw_anderso@bellsouth.net>]

Chen,
 I can always hear a slightly higher tone in a AFSK generated signal.
Listening this morning to two different software packages generating AFSK
one does generate a higher tone. It could be my big ears. Most likely the
way the different software generates the signal ?
Back in my Navy days I was stationed down in VK6 land up in the
outback/bush. We keyed ( 2kw nominal ) the sub broadcast at certain times
from some 1300ft towers out into the Indian ocean. A young sailor who
transferred in complained to the Doc he could hear strange noises every once
in a while. I recall the human can hear tones up to 2000cyles ? If my memory
is correct we xmitted at 2300cycles to the subs. Rumor was they wired the
young sailor up to one of those hearing devices and found out he was hearing
the vlf signal as he could hear things on the machine no one else could.
I sure was glad to hear ( pardon the pun ) that as I thought I was mistuning
the vlf xmitter and creating spurs/splatter :(
73
Charles/kk5oq



-----Original Message-----
From: rtty-bounces@contesting.com [mailto:rtty-bounces@contesting.com] On
Behalf Of Kok Chen
Sent: Sunday, March 01, 2009 4:55 PM
To: RTTY Contesting
Subject: Re: [RTTY] Fun, but


On Mar 2, 2009, at 12:24 AM, Junior wrote:

> If you listen close enough you can tell the difference by ear.
> Find one of each on the air which is strong and no noise present.

Charles, you are wrong.  You can generate an AFSK signal to look on  
the spectrum precisely like an RF generated FSK signal.

Except that you can do better with AFSK than you can with FSK since it  
is much easier for you and I to implement proper wave shaping in AFSK  
than we can do in FSK -- in most cases, you have zero control over how  
the manufacturer has implemented FSK.

AFSK just means that you are generating a frequency shifted signal at  
a frequency that is in the "audio" range and then shifting  
(modulating) it through a balanced modulator.  In the modern world, a  
quadrature base band signal is shifted using using a quadrature  
sampling exciter -- in fact, the "A" in AFSK can be anywhere from DC  
to a couple of hundred kiloHertz.

73
Chen, W7AY

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