Question about Frequency Shift Keyed signals
 

There is an application that uses FSK. But the actual information is
not coded by the frequency shift but rather by the length of time one
of the pulses is turned on. The pulses are about 2 milliseconds long.
The information is coded on one of them. The other is always the
same length. The frequency shift is always a constant.

Now, my question. If you send this signal into a standard FM Rx it
will produce a tone. Is this tone simply the difference between the
two RF frequencys? Is the information coded on the one pulse lost in
such reception? Or is this audio tone something far more complex that
still contains encoded information?

TIA

In message , James
writes
There is an application that uses FSK. But the actual information is
not coded by the frequency shift but rather by the length of time one
of the pulses is turned on. The pulses are about 2 milliseconds long.
The information is coded on one of them. The other is always the
same length. The frequency shift is always a constant.

Now, my question. If you send this signal into a standard FM Rx it
will produce a tone.

Why? The two frequencies do not exist at the same time in the receiver.

Mike

In article ,
M. J. Powell wrote:
In message , James
writes

Now, my question. If you send this signal into a standard FM Rx it
will produce a tone.

Why? The two frequencies do not exist at the same time in the receiver.

But if the shifting, the pulse repetition, is taking place at a high
enough frequency it will be heard as a tone. And, yes, the information
is still there because the waveform of the tone should be that of the
modulating signal.

--

jhhaynes at earthlink dot net

"James" wrote in message
om...
There is an application that uses FSK. But the actual information is
not coded by the frequency shift but rather by the length of time one
of the pulses is turned on. The pulses are about 2 milliseconds long.
The information is coded on one of them. The other is always the
same length. The frequency shift is always a constant.

Now, my question. If you send this signal into a standard FM Rx it
will produce a tone. Is this tone simply the difference between the
two RF frequencys? Is the information coded on the one pulse lost in
such reception? Or is this audio tone something far more complex that
still contains encoded information?

You won't hear a "tone". The FM demod audio output will follow the rate of
change of the carrier frequency. For example, if the carrier frequency
changes at a constant rate, you'll hear a tone during the frequency
transition period. There will be no audio output during the period of time
that the carrier is not changing frequency. How long the carrier sits at
the frequency which contains the duration-encoded information will not be
detectable at the audio output because the carrier isn't changing frequency
at that time. If the frequency transitions are abrupt, you'll hear
something like a click between the mark and space frequencies.

..