13
TOR. Unfortunately, some data converters use 200 Hz. shift for RTTY, which adds to the
problem of optimally filtering data signals.
Center Frequency
The center frequency of a FSK signal is independent of the frequency shift or the baud rate. In
the audio spectrum, either before an AFSK signal modulates a RF signal or after the RF FSK
signal is demodulated, there are several common center frequencies. In the North America,
2210 Hz. is the standard center frequency for RTTY, PACTOR, AMTOR and G-TOR, while
both 1700 Hz. and 2210 Hz. share the standard for HF packet. In Europe and some other parts
of the world, lower center frequencies of 1300, 1360 and 1530 Hz. are more common.
Baud Rate
Baud rates vary from 45.5 baud for RTTY to 300 baud for HF packet.
For a more complete discussion of data modes, see the latest edition of the ARRL Handbook.
.
RTTY, AMTOR, PACTOR, G-TOR, HF Packet
Normal operation for RTTY, AMTOR, PACTOR and G-TOR uses a 2210 Hz. center frequency
filter (marked 2.21k in red letters by the
Center Freq
control). Use the
Bandwidth
control to
select the appropriate bandwidth for each mode (RTTY - 250 Hz., AMTOR - 350 Hz., PACTOR
- 450 Hz., G-TOR - 550 Hz., HF Packet - 550 Hz.).
The Option position of the
Center Freq
control selects a bandpass filter center frequency which
has been preset by the internal Back Panel Jumpers. There are five optional data center
frequencies - 1700 Hz. (factory setting), 1300 Hz., 1360 Hz., 1530 Hz., and 2125 Hz. There is
only one Option position on the
Center Freq
control, so only one preset filter is available from
the entire option set (data and CW filters).
RTTY Remodulator
To select the RTTY remodulator, first chose a RTTY filter by setting the
Center Freq
control
to 2.21k (red letters) or Option (red letters) to select a RTTY filter center frequency and setting
the
Bandwidth
control to 250 Hz.. Then, depress the
Tone
(or
Marker
in some units) push-
button to enable the remodulator. To switch the remodulator off, release the
Tone
push-button.
The selected filter will remain on. The remodulator mode is easily recognized by a lack of any
receiver background noise - only the pure audio RTTY tones are audible when the remodulator
is on and a RTTY signal is present. The DSP-59+ mutes the audio output when it doesn’t detect
a RTTY signal. Normally 250 Hz. is the recommended bandwidth for RTTY; however, other
bandwidths from 200 Hz. to 350 Hz. may improve copy under some band conditions.
HF Packet
HF packet signals are usually centered at 1700 Hz. or 2210 Hz., depending upon the modem
manufacturer. The frequency shift is 200 Hz. in both cases. If you are using 2210 center
frequency (2110-2310 Hz. mark-space frequencies), select 2.21k (red letters -
Center Freq
control) and 550 (red letters -
Bandwidth
control). If you are using 1700 Hz. center frequency
(1600-1800 Hz. mark-space frequencies), select the DSP-59+ Option filter (red letters -
Center
Freq
control) of 1700 Hz. (factory setting) and 550 (red letters -
Bandwidth
control).
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