Hal Communications CWR6850, Instruction Manual

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CHAPTER 3 PAGE 15
If you have a general coverage receiver, you may now wish to try receiving short-wave press RTTY
signals. Commercial press RTTY stations can often be found on frequencies around: 5.2 MHz, 5.4
MHz, 5.8 MHz, 6.8 MHz, 7.5 MHz, 7.8 MHz, 8.0 MHz, 9.0 MHz, 9.4 MHz, 9.8 MHz, 10.2 MHz, 10.5
MHz, 10.8 MHz, 11.1 MHz, 11.5 MHz, 12.2 MHz, 13.5 MHz, 14.5 MHz, 14.9 MHz, 15.5 MHz, 15.9
MHz, 16,2 MHz, 16.4 MHz, 17.3 MHz, 17.5 MHz, 18.2 MHz, 18.4 MHz, 18.7 MHz and 19.0 to 20.5
MHz (plus others!). Commercial RTTY stations will operate with either 850 or 425 Hz shift and may
have speeds of 45 (60 wpm), 50 (67 wpm), 57 (75 wpm) or 74 (100 wpm) baud Baudot code. The
signals may be of either signal polarity, so try both NORM and REV conditions. There may be a few
commercial press stations operating at 110 baud ASCII also. Tuning these commercial stations will
require some patience due to the wide variety of shifts, speeds and polarities used.
The receive non-overprint feature will automatically place characters on the next line of the screen
if more than 32 characters are received between line feed characters. To further prevent overprint,
the receive section ignores all received carriage return (CR) characters and always executes a car-
riage return and line feed (LF) whenever a LF character is received. A space may be displayed
when the CR character only is received.
3.3 Low tones or high tones?
So far we have only used one of the two possible CWR6850 demodulator combinations. You may
wish to switch to the RTTY "high-tone" option – push the HIGH / LOW button in. The "high tones"
(for higher-frequency audio tones) are really the traditional standard U.S. RTTY tones, used since
the early days of amateur RTTY. The "low tones" are the IARU international standard and are used
extensively in most other countries of the world. When receiving (or transmitting) on the HF bands
(3–30 MHz), either set of tones will work since you tune the receiver to produce the desired beat
note frequency. However, when AFSK modulation is added to an FM or AM signal, you must be
prepared to receive the same tone frequencies as those used by the transmitting station (the AM
or FM receiver does not use a BFO to produce the audio tone). In the United States, the long-
standing VHF AFSK tone standard has been to use the "high tones" (2125 Hz mark and 2295,
2550 or 2975 Hz space); you must use a high-tone demodulator to be compatible! In Europe, in
particular, the reverse standard is developing – the IARU "low tone" (1275 Hz mark and 1445,
1700 or 2125 Hz space) is the standard to be observed. The two system are basically incompatible
for VHF AFSK operation! Due to low-pass filter parameters, use of data rates greater than 110
BAUD is not recommended when "high" or "low" tone demodulator combinations are used; an ex-
ternal modem should be used for transmissions at 300 baud.
Each tone set has its advantages and disadvantages – the CWR6850 lets you choose the optimum
combination for your station. Some of the considerations for each tone set are as follows:
HIGH TONES (Mark = 2125 Hz, Space = 2295, 2550 or 2975 Hz):
Advantages:
1. High tones are the U.S. VHF AFSK standard – their use is required for compatibi-
lity when operating VHF AFSK in the U.S. A high tone demodulator may be used
for both VHF and HF use in the United States.
2. When high tones are used on HF, using tones into a LSB transmitter audio input
(microphone or phone patch input – see Chapter 4), there may be fewer prob-
lems with spurious signals (usually due to overdriving the transmitter audio
stages). Since the tone frequencies are high, the harmonics and most distortion
products occur at audio frequencies beyond the audio passband of the transmit-
ter and should therefore not be transmitted.