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A Beginners Guide to Scanning

IC-R20 operators manual

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 2006
 
 

 

 

 

 

 

 

 

                                 
                                 
                                 
                                 
       
                                 
    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. The early devices used to use a mechanical vibrating reed to both encode and decode CTCSS.

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.

   
                                 
   

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 identify a user almost instantly when they share 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 is the older method which identified users with one of around 50 simple tones. More recently we are slowly beginning to see more and more use of DTCS or Digital Tone Coded Squelch, also known as DCS. This is similar in that it works at subaudiable frequencies but instead of a simple constant sine wave tone being transmitted, it transmits a constant stream of low bitrate digital data. It is this which identifies the user as belonging to a given group.    
                                 
    If you plug a pair of headphones or a speaker into your scanner, it's quite likely you will hear these tones in use. They are very common with PBR just about anywhere above 30MHz but you won't hear them on legacy systems such as aviation and VHF maritime. Where you are likely to hear them in use is with say a taxi firm or shopwatch scheme. You might notice a steady low hum all through the length of their transmissions. That will be CTCSS that you are hearing and you can use tone scan on the R20 to find the subtone frequency being used.    
    At other times, you may hear a more raspy, less smooth sounding tone, and then, right at the end of each transmission, a more pronounced tone at about 300Hz. This is DTCS that you are hearing and you can decode this with the DTCS decode function.    
                                 
                                 
 
 
                                 
  Changing the current CTCSS subtone  
     
   

Push and hold [8 SET] for one second to enter  SET MODE .

If you haven't done so already, use  SET EXPAND  to show the full range of parameters in the  SET MODE  menu.

Rotate the frequency dial to select  >TONE FREQ  with the pointer chevron.

    
                                 
                                 

 
                     
         

IC-R20

          
                       
   

*** SET MODE ***

   
   

   
   

OFFSET FREQ

   
   

DUPLEX

   
   

>

TONE FREQ

   
   

DTCS CODE

   
   

DTCS POLARITY

   
   

BANK LINK

   
                       
     

COMMUNICATIONS RECEIVER

      
                     
                                 
   

Press briefly [8 SET] and the dialogue  TONE FREQ  will appear. Below it will be the current CTCSS frequency in Hertz.

    
   

Rotate the frequency dial to select your required value.

    

 
                     
         

IC-R20

          
                       
   

TONE FREQ

   
   

   
   

88.5

   
   

   
   

   
   

   
   

   
   

   
                       
     

COMMUNICATIONS RECEIVER

      
                     
                                 
    You can either return to the SET MODE menu with [8 SET] or to the main screen by pressing  [DUALWATCH]  .    
                                 
Any changes you make on the fly are enacted as you make them and so are effective immediately.  You are not required to confirm your changes at any stage. However, you may wish to re-save your newly changed settings to a previously saved memory channel in order to permanently update its parameters .
                                 
  Changing the current DTCS code              
                                 
   

Push and hold [8 SET] for one second until it bleeps to enter  *** SET MODE *** .

If you haven't done so already, use  SET EXPAND  to show the full range of parameters in the  SET MODE  menu.

Rotate the frequency dial to select  >DTCS CODE  with the pointer chevron.

    
                                 
                                 

 
                     
   

IC-R20

    
                       
   

*** SET MODE ***

   
   

   
   

DUPLEX

   
   

TONE FREQ

   
   

>

DTCS CODE

   
   

DTCS POLARITY

   
   

BANK LINK

   
   

LCD CONTRAST

   
                       
   

COMMUNICATIONS RECEIVER

    
                     

Press briefly [8 SET] and the dialogue  DTCS CODE  will appear. Below it will be the current DTCS code value.

Rotate the frequency dial to select your required value.

 
                     
   

IC-R20

    
                       
   

DTCS CODE

   
   

   
   

023

   
   

   
   

   
   

   
   

   
   

   
                       
   

COMMUNICATIONS RECEIVER

    
                     
You can either return to the SET MODE menu with [8 SET] or to the main screen by pressing [DUALWATCH] .
   

Any changes you make on the fly are enacted as you make them and so are effective immediately.  You are not required to confirm your changes at any stage. However, you may wish to re-save your newly changed settings to a previously saved memory channel in order to permanently update its parameters .

   
                                 
   
   
                                 
                                 
  Changing the current DTCS polarity              
         
    It's extremely unlikely that you will ever need to use anything other than normal polarity. The reverse codes actually work out to be the same 104 codes as the normal DTCS codes but just with differing numbers. It is useful for engineering use when there is a phase inversion in the telecommunication system but not something that most R20 operators will be concerned about.    
                                 
                                 
   

Push and hold [8 SET] for one second until it bleeps to enter  *** SET MODE *** .

If you haven't done so already, use  SET EXPAND  to show the full range of parameters in the  SET MODE  menu.

Rotate the frequency dial to select >DTCS POLARITY with the pointer chevron.

    
                                 
                                 

 
                     
   

IC-R20

    
                       
   

*** SET MODE ***

   
   

   
   

TONE FREQ

   
   

DTCS CODE

   
   

>

DTCS POLARITY

   
   

BANK LINK

   
   

LCD CONTRAST

   
   

CI-V ADDRESS

   
                       
   

COMMUNICATIONS RECEIVER

    
                     

Press briefly [8 SET] and the title dialogue  DTCS POLARITY  will appear. Below it will be the current DTCS polarity.

 
                     
   

IC-R20

    
                       
   

DTCS POLARITY

   
   

   
   

>

NORMAL

   
   

REVERSE

   
   

   
   

   
   

   
   

   
                       
   

COMMUNICATIONS RECEIVER

    
                     

Rotate the frequency dial to select either >NORMAL or >REVERSE with the pointer chevron .
You can either return to the SET MODE menu with [8 SET] or to the main screen by pressing [DUALWATCH] .
 

Any changes you make on the fly are enacted as you make them and so are effective immediately.  You are not required to confirm your changes at any stage. However, you may wish to re-save your newly changed settings to a previously saved memory channel in order to permanently update its parameters .

 
                                 
                                 
   

If you need to store this new value to update the current memory channel, hold down [MR S.MW] for just one second until the channel menu appears. Check that you have the right memory channel. Press [MR S.MW] again until you hear two bleeps. The screen will return to the default memory channel mode screen and the value has been saved. You can check this by flipping to another channel and back again. You should find that your changes have been successfully updated. 

   
                                 
                                 
 

You may also from time to time hear a rapid melodic succession of tones being transmitted over the air. This is probably SelCall or possibly DTMF.

 
                                 
                                 
                                 
       
 

 
       
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