Subtone Signalling
DCS and CTCSS fall into a group of techniques called Subtone Signalling. Here's what they are and how they came about
Back in the late 1970's, commercial radios used to have a receiver which would open the squelch in the presence of any signal
received on the frequency, both wanted signals and man made interference, as well as, under enhanced signal conditions, other user groups further away who
were allocated to the same carrier frequency.
This wasn't so much of a problem back then as the airwaves were much less densely populated and there
were less computers and point of sale machines causing interference.
As the demands on the airwaves grew, Motorola developed the idea of injecting a subaudiable tone (often heard as a hum) into
each user groups' transmission.
In return, each receiver would only open its squelch when both the squelch was opened by a carrier and the expected tone was received.
This meant that commercial users were much less prone to interference from machines and other users.
The early devices used to use a mechanical vibrating reed to both encode and
decode the constant tone. Later radios used solid state circuitry.
It's in the interest of radio users to keep their tones mutually exclusive from other users in the area. Ofcom often issue a subtone as
well as a frequency when they issue a PBR licence, having, presumably, checked that there are no other users with that tone for a good distance. With other licences and licence exempt equipment, it is left to the user to self select their own tone frequency.
This is hugely useful to scanner listeners, particularly in built up areas as it gives us a way to
tell users apart even when they co-use channels with others. Some frequencies such as PMR446, UK General, Short term hire and the like, tend to become soup channels, particularly
at large events. Subtones are an excellent way of sorting out the wheat from the
chaff.
CTCSS or Constant Tone Coded Squelch System was the original system which identified users with one of around 50 simple tones
and is still widely in use but now we are
slowly beginning to see more and more use of DCS or
Digital Coded Squelch. This works in a similar way at subaudiable frequencies
but instead of a simple constant tone being transmitted, it transmits a constant
stream of low bitrate digital data. This contains a 23bit code which is repeated
many times per second. It is this which identifies the user as
belonging to a given group.
Motorola call CTCSS and DCS by the names PL (Private Line) and DPL (Digital Private Line) respectively. This is frankly stupid because firstly it is by no means private and secondly because there is no line. Icom use the term DTCS (Digital Tone Coded Squelch) which many may feel is unnecessarily long winded. It is also sometimes referred to as CDCSS
Kenwood refer to CTCSS and DCS as 'Quiet Tone' which again is not a very enlightening description and again throws another confusing pair of acronyms into the pot in the form of QT and DQT. Several Chinese manufacturers follow this terminology too as they frequently clone Kenwood products.
The use of subaudiable signalling is very common with PBR just about anywhere above 30MHz but you won't find it used with legacy systems such as aviation and VHF maritime. You will frequently encounter subtone signalling in use by taxi or shopwatch scheme users. If you plug a pair of hi-fi headphones or a speaker into your scanner, you may well hear these tones in the form of a steady low hum all through the length of their transmissions. If the tone is smooth and regular, it will be CTCSS that you are hearing.
With other users, you may hear a more raspy, less regular sounding tone, and then, right at the end of each transmission, a more pronounced tone at about 300Hz. This is characteristic of DCS.
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