Hal Communications CWR6850, Instruction Manual

Page: 16 / 64

CHAPTER 3 PAGE 16
Disadvantages:
1. The relatively high audio frequencies used in the high tone set may not fall with-
in the audio frequency response of the receiver or transmitter. In general, the
standard amateur shift, 170 Hz, will pass most current receivers and transmit-
ters (the Collins S-Line is an exception). However, few pieces of equipment will
pass the tones for both receiving and transmitting 425 or 850 shift with high
tones (the Drake TR-7 is an exception). The use of high tone demodulators for
HF RTTY is therefore restricted to transmission of just 170 shift and only re-
ceivers incorporating either a variable BFO or passband tuning will receive all
three shifts.
2. High tones are not the IARU standard and will not be compatible with VHF AFSK
in many countries of the world.
LOW TONES (Mark = 1275 Hz, Space = 1445, 1700 or 2125 Hz):
Advantages:
1. Low tones are the IARU international standard and their use assures compatibil-
ity with VHF AFSK operations in many areas of the world.
2. Low tones can be used with virtually all SSB receivers and transmitters for all
three standard shifts. Variable BFO or passband tuning features are not required
to assure good reception of 425 and 850 Hz shift stations.
Disadvantages:
1. Low tones are not compatible with existing U.S. VHF AFSK operations. Since
there is a great deal of high tone AFSK equipment already in use in the United
States and neighboring countries, it is unreasonable to expect that the U.S.
standard will shift to low tones.
2. When low tones are used with a LSB transmitter to generate F1 RTTY emission
(see Chapter 4), there is a strong probability that over-driving the transmitter
audio and modulator stages will result in spurious harmonics and the mixer
products that will be radiated. Of course, these problems will not occur if the
audio drive level is properly set.
Although you may develop your own personal preferences, we recommend that you consider the
following operating conditions as a starting point:
In the United States:
Use HIGH tones for all VHF AFSK amateur communications and for normal 170 Hz shift HF
operation; use LOW tones when receiving HF commercial RTTY stations unless you have a
receiver with a variable pitch BFO or with IF passband tuning. The exception applies to use
of the older Collins S-Line equipment – use LOW tones for all HF operations, amateur or
commercial.
In Europe (and all other areas where IARU standards apply):
Use LOW tones exclusively for both VHF AFSK and HF operations. The only exception would
be when you communicate with another VHF AFSK station who is using HIGH tones.