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Let's talk about Tone, Tone Squelch, CTCSS, PL, Digital Squelch, DCS,
DPL or what ever you might have heard about.
CTCSS? DCS? Sub-channels? Yes,
Continuous Tone Coded Squelch System. Introduced by Motorola in the
early 1960s, as "Private Line" (also known as GE/Ericsson's "Channel
Guard", E.F. Johnson's "Call Guard", RCA's "Quiet Channel"), known
by many as simply "tone squelch". This is like a mains hum on the
modulation, a constant musical note... almost as if someone is
standing by you humming as you talk. The receiver can be set to only
open its squelch if this tone is received. Any interference would
not have the correct tone present, and so the radio would remain
quiet. CTCSS was developed into a system with numerous standard
tones to choose from, often 38. When CTCSS is used, the audio is
filtered out below about 300Hz, so that the only audio frequency
energy that ends up being transmitted between 0 and 300Hz is the
CTCSS tone itself (almost always any one tone between 67 and 254.1
Hz). The tone is then sometimes called 'sub audible' because it's
below the range of THEN audible voice frequencies - but it would be
audible if played on full range speakers without the filtering that
CTCSS radio receivers also use on receive (we can hear down to 20Hz
Some Amateur Radio Repeaters are
configured, that the tone may not be transmitted only received, this
allows the security of the the encode ability of your transceiver.
so depending on your manufactures software it may be labeled Tone,
or Encode. If the repeater is configured to transmit the same or
different tone your setting may be labeled Tone Squelch, TSquelch,
or Encode/Decode. The most common tone for or FRC District #5 is
103.5 CTCSS and 411DPL. This helps us from hearing other systems on
the same frequency from other Florida Distracts when conditions are
rolling. These stations are not "Bleeding Over" that is something
completely different. They have the same rights on the frequency
that you have.
DCS (Digital Coded Squelch) is digital
data or code word that is transmitted with the voice audio. This
data is sub-audible with most of it's energy below 300Hz. However is
does have a wide bandwidth from 2 to 300 Hz. Unlike CTCSS
(Continuous Tone Coded Squelch System) which uses continuous tones
below 300 Hz., 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 radio transmission and about 1/2
second before the transmitter un-keys, the radio will encode a 134
Hz tone that serves as a turn off code. The FM deviation level of
DCS data should be in the range of 500 to 800 Hz.
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.
DCS is operated at a low baud rate (134.4 bits per second) and
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 lease 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.
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 a 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. With DCS, error
correction is necessary. 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. A sub audio filter that removes this low frequency
energy before the audio is re-transmitted is necessary for reliable
Now there are a few systems that use
both the DCS and CTCSS on one side or the other and even using both.
CTCSS Encode DCS Decode - DCS Encode
CCTCSS Decode - CTCSS/DCS ENCODE CTCSS/DCS Decode.
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