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The FSK channel is used by the application program to send the FSK keying signal. FSK is used primarily for
RTTY. It is very important to understand the difference between FSK and AFSK.
FSK is a digital (On/Off) signal from the computer serial port (or an external modem). This signal is used in the
transceiver to generate a frequency shift. FSK must be supported by the transceiver (this mode is commonly
labeled RTTY or FSK).
AFSK is a analog (audio) signal generated by the computer sound card (or external modem) used in the
transceiver modulation circuits for operating digital modes as RTTY, PSK31, AMTOR etc. Computer sound card
generated AFSK or PSK does not require special transceiver support and can be used in the LSB, USB or FM
mode of the transceiver. Some radios have dedicated modes for AFSK (generally labeled PKT or DATA) with
special features.
It is very important to properly adjust the audio drive level of an AFSK system so as to not overdrive
the first transmit audio amplifier stage in the transceiver and produce a wide, distorted signal, full of
intermodulation products. It is important to appreciate that distortion generated at this point due to
overdrive CANNOT be reduced or eliminated by the reduction of the microphone gain control – it is
the signal level that must be adjusted to be about the same as would be expected from a
microphone. The microphone gain control then becomes a form of transmit power control.
An initial indication of proper audio drive level can be seen on the ALC meter of the radio. Provided
that there is NO audio processing in circuit and that the microphone gain control is in its normal
operating position, then, if the ALC does not show or just starts to indicate during transmission, the
signal is likely to be clean. It is also important is to turn off the microphone compressor, ANY transmit
audio equalizer, AND transmit DSP when AFSK is used.
modulation (sometimes called "Transmit DSP") with AFSK or PSK. Some transceivers bypass these
circuits automatically when signal is routed to the rear audio jack instead of the microphone jack, but
some do not (for example, the TS-850).
If your transceiver supports FSK, use FSK for RTTY whenever possible. It's the only sure way to get a
clean RTTY signal no matter the microphone gain or compressor (processor) settings on your radio.
When a COM port is assigned in Router but not in the application program (or the application is not running),
Router will indicate the channel is closed.
When an application opens the COM port, Router
will indicate the channel is open and display baud
rate, number of data bits, parity and number of stop
bits in use. For example, 45 5N1.5 means: 45
Baud, 5 data bits, no parity, 1.5 stop bits.
The second FSK port is useful when operating split with radios that have two receivers (e.g., FT-1000, FT-2000,
FT-9000, K3, Orion or IC-7800). The second instance of the RTTY program should specify "right channel" for its
audio source and should be configured to use the 2
Radios without a second receiver can use the 2
algorithm to provide diversity decoding and transmit from either program.
TIP: If you see a baud rate other than 45.5 baud (e.g., 4800 or 9600), the application is NOT configured
correctly for FSK RTTY operation.
PTT: The virtual port used for FSK can also support PTT (required by MMTTY). When you use MMTTY,
select the PTT box and RTS will used for PTT. Do not use the FSK port for any other function.
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nd
FSK & 2
FSK PORTS
nd
FSK port for its FSK output.
nd
FSK port for a second RTTY program with a different decoding
20
DO NOT use any form of digital
Edited by Geoff Anderson, G3NPA
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