Pacific Research Solutions RI-200 User Manual
Page 21
Pacific Research Solutions
RI-200 User Manual
Page 20
Depending on your repeater, you may have to change the polarity of the DCS TX/RX data. Once you have confirmed that
your radio audio frequency response is correct, you can enable a DCS code for the repeater decode (S-Command 20). When
you send that code to the repeater, check if the CTCSS/DCS LED goes on. If the controller does not decode DCS data,
change the RX polarity (S-Command 25). If your radio will not decode the DCS data from the repeater, change the TX
polarity (S-Command 25).
CTCSS (Continuous Tone Coded Squelch System) uses continuous tones below 300 Hz. A single tone is assigned for a group
of users. When this tone is decoded by the repeater, the repeater will pass the users audio. Sometimes two tones next to each
other can become confused with each other. In contrast, DCS uses digital data or code words. Each code word is unique and
all code words may be used on the same channel without interference. At the end of the repeater transmission and 1/2 second
before the transmitter un-keys, the controller will encode a 134 Hz tone that serves as a turn off code. For correct operation,
the DCS data should have a FM deviation of 500 to 800 Hz.
Your transmitter and receiver waveforms
should closely resemble the waveform diagram,
figure 1. If this is the condition, then the low
frequency response of your repeater should be
adequate. If your waveform resembles figure 2,
your modulator or discriminator does not have
enough low frequency response. Figure 2 was
created with a high pass filter with a corner
frequency at about 5 Hz. Even with a cutoff
this low, the filter strips vital data from the
digital code word.
Because DCS may have extended periods of all ones and zeros, almost all components in the transmitter and receiver chain
must be coupled down to at least 2 Hz or lower. This requirement means that certain transmitters and receivers must be
modified before they are capable of DCS operation. Phase modulators, in particular, need special consideration because they
theoretically are incapable of being directly modulated by DC, unlike direct FM modulation methods. Low frequency
response is the primary requirement for DCS systems.
Unlike CTCSS, DCS signal spectrum occupies considerable more bandwidth. A poor low frequency response in the
transmitter or receiver may not seriously distort a single frequency tone signal but may seriously degrade a wide band signal
containing multiple frequency components. The distortion risk is especially high if the frequency response delays the wide
band frequency components.
The RI-200 does include data error correction. But if too many errors occur, you may experience some blocking out of the
decoder. Errors can occur because of unwanted low frequency energy. The DCS decoders can be effected by voice energy
that falls below 300 Hz. Some radios do not remove this energy before transmission and can cause voice blocking of the
decoder. The RI-200 has a sub audio filter that removes this low frequency energy before the audio is re-transmitted. This
feature will increase the performance of the receiving radios. You may need to confirm that the radios used to operate the
repeater, have some type of sub audio filtering.
You will find that it is extremely important for the receiver and transmitter to be on frequency to achieve maximum
performance of the DCS function. Errors in the transmitter and receiver frequencies show up at the discriminator output as a
step function. Because of the long time constant required for the low frequency response, a step function can block the
decoder momentarily.
Before you start modifying your radio to operate DCS, make sure your service monitor is DCS capable. Some older monitors
require modifications to obtain the low frequency audio response needed for DCS operation. If in doubt, contact the
manufacturer.
Because there are so many radios that may need modification for DCS operation, we may not be familiar with your radio. For
this reason, we may not be able to provide technical support for some radio modifications. We want to hear constructive and
useful feedback. We appreciate having details about your successful modification of a radio for DCS operation.
Figure 1
Figure 2