On Jul 22, 2011, at 7:56 AM, Tom Osborne wrote:
> It can't be the setting on the radio or AFSK would be SSB, also--the function
> is the same.
Direct FSK is also just an FM mode.
Direct FSK and AFSK are just two different ways of generating a frequency
shifted signal on the RF spectrum. The Direct FSK uses a brute force shifting
of the carriers, while the AFSK approach is more mathematical.
AFSK makes use of what is called the Convolution Theorem in mathematics
(http://en.wikipedia.org/wiki/Convolution_theorem).
When you multiply two signals in the time domain, in the frequency domain they
look like the convolutions (http://en.wikipedia.org/wiki/Convolution) of the
Fourier transforms of the two signals. In this context, you can visualize
"Fourier Transform" as "spectrum."
Now, if one of the signals is a pure sine wave, then the convolution theorem
simplifies to a case, there the second signal is simply shifted up in the
spectrum by the frequency of this oscillator.
One way this "multiplying in the time domain" is done is through the use of a
mixer.
A mixer is also called a product detector, a term that is less confused with
the "mixers" use in the audio recoding world -- I am sure you had stumbled into
that in the Windows world when playing with sound cards. The audio mixer is
not a multiplying device, but simply produces the sum of two or more signals at
different gains.
The multiplication process actually produces two "sidebands" at the output, one
being a "mirror" of the other. The sine wave oscillator itself does not appear
at the output. We call this "mirrored" signal the double sideband suppressed
carrier (DSB) signal. If you now also filter away the lower portion of the
mirror, you end up with an upper sideband suppressed carrier (USB) signal. If
you filter away the upper component, you end up with a lower sideband
suppressed carrier (LSB) signal.
Engineers realized early on (through the convolution theorem, actually) that to
transmit human voice (or music) on the RF spectrum, you do not need the carrier
(if you design an appropriate demodulator). Indeed, you do not even need to
transmit both the sidebands, and therefor get further improvement in SNR while
reducing spectrum usage.
So, voice started to be passed through an SSB modulator.
In essence, and this is all important, so I will say it slowly -- SSB_does_not
mean_voice_transmission! SSB is just one way that people use to modulate a
voice signal into the RF spectrum efficiently.
Similarly, you can use SSB to take an audio FSK signal ("AFSK") and shift it
into the RF spectrum with the use of an SSB transmitter.
The pure beauty of a SSB modulator is that the input can be as complex as you
want. So, you can use SSB to send voice, you can send CW (J2A emission mode),
you can use it to send SSTV,you can send it an audio PSK signal, you can send
MFSK, and you can send it an audio FSK signal. You can even send a windowed
AFSK signal, which reduces co-channel interferences between two RTTY signals.
What a SSB transmitter produces is a function of what you feed it. Feed it
voice and you get voice in the RF spectrum. Feed it an audio SSTV signal and
you get SSTV in the RF spectrum, feed it audio FSK (AFSK) that uses 5 bit
Baudot character encoding and you get RTTY.
73
Chen, W7AY
_______________________________________________
RTTY mailing list
RTTY@contesting.com
http://lists.contesting.com/mailman/listinfo/rtty
|