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last updated 2 Feb 2004
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. The military saw how useful this was and adopted 150Hz as their tone. For business use however, a further advantage was that a channel could be shared by many different groups each with their own tone, and each group would not be affected by the others. Transmit with a different tone and another set's receiver will stay quiet unless you change its tone setting to match.

CTCSS was developed into a system with numerous standard tones to choose from, often 38. Some manufacturers call these "sub-channels" and make it seem that over 300 real channels are available. It may seem that way, but problems can occur if two different people transmit on the same frequency at once, with different tones - a "channel busy" indicator (sometimes even preventing transmission) helps here. If there are two signals you'll only hear what you want to hear if the wanted signal is stronger than the unwanted one in terms of received power levels - i.e. if your intended contact is closer. It should be fairly obvious if this is happening, if someone is cutting out - with a bit of luck your receiver may have a monitor button to allow you to hear exactly what's going on i.e. bypass the CTCSS circuit momentarily.

EXAMPLE with 6 radio users on same channel :

A0 & B0 : no CTCSS, or tone 00 (switched off)
C1 & D1 : CTCSS, tone 1
E2 & F2 : CTCSS, tone 2

A0 and B0 can hear all stations 
          but can only talk to each other
C1 and D1 can only hear each other
E2 and F2 can only hear each other

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 or so).

For outside broadcasting use (mics and programme links), this sounds thin and weedy, with no bass, and CTCSS isn't needed anyway - so they don't use it. Their signals have a response that goes all the way down to bass frequencies (less than 100Hz.. down to 20Hz or less). A receiver with CTCSS scan may well detect SOME Audio Frequency energy within a relevant passband below 254Hz that momentarily activates the CTCSS decode circuit. This doesn't mean that they are using CTCSS. The human voice has dominant frequencies in this range (vocal chord resonance etc), in fact most voice energy is not only below 500Hz but actually not very important in terms of understanding what's being said. 'Communications bandwidth' is around 300 to 3000 Hz.

DCS

DCS is similar to CTCSS in practice, but uses a digital code instead of continuous analogue tones. DCS is a series of 23 bit Golay encoded words sent without sync bits at 134.4 baud, providing 104 different octal values between 000 and 777 (only 52 are allocated by the UK RA due to the other 52 being the 'complement' (i.e. the inverse) of the first 52.
The three digit numbers correspond to the octal weight of the code, not including the other required overhead bits for the Golay code. DCS sends a fixed octal digit 4 as the first digit, followed by the three octal digits, where each digit is represented by 3 bits. 023 would be 100,000,010,011, thus representing 12 bits of the total 23 (?? MPT1381 says bits 1-9 are the code, with 10-12 as 001). The remaining 11 bits are a CRC (checksum). The code is repeated constantly at 137 bps, so decode time is approx 23/137 of a second, or under 200ms, about the same as regular CTCSS. "Although it looks like there should be 512 codes available, there are only 83 possibilities. This is to prevent a codeword that is misaligned as it is serially shifted into the decoder, matching one of the other codes."

(table of DCS codes appears towards the end of this page)

Privacy

Using CTCSS does not give you privacy from being heard, it simply gives you the privacy of not hearing ANY OTHER activity on the channel (if they're using no tones, or different ones).

Anyone using tones is simply adding an extra signal to their otherwise normal FM modulation. This gives them no privacy as such (anyone can listen in if they want to), but merely means that they are not bothered by any other signals unless those signals also carry the same tone. By using tone 00 (which means "no tone" - if your PMR446 set provides this) YOU are able to hear any FM modulation regardless of any extra CTCSS or DCS tones THEY may be using. So tone 00 (like the monitor button) allows you to listen to ANY other conversations - except any scrambled ones.

Tone Sets

I would like to maintain a table here of which CTCSS tones are which. Please pass on any info along the lines of : brand/model, tone 1/freq ... tone 38/freq etc. Thanks. Please use the guestbook, you can be anonymous if you like, or I'll credit you if you want. I'd also like to hear of other models than those listed here, new ones, relevant URL links, etc., thanks.

Since I first started this page, there have been many more new 446 radios launched. A standard appears to have emerged, on my Models page this is : 38 CTCSS (67,71.9-250.3), which I may start to abbreviate to 38 CTCSS (std). If there is +off it means that CTCSS can be turned off.

So, what's all the (67,71.9-250.3) number stuff? Read on...
Many amateur sets feature this set of 39 tones : 67 69.3 71.9 74.4 77 79.7 82.5 85.4 88.5 91.5 94.8 97.4 100 103.5 107.2 110.9 114.8 118.8 123 127.3 131.8 136.5 141.3 146.2 151.4 156.7 162.2 167.9 173.8 179.9 186.2 192.8 203.5 210.7 218.1 225.7 233.6 241.8 250.3 - these sets would be described on my Equipment page as 39 CTCSS (67,69.3-250.3). The EIA (Electronic Industry Association) standardised 33 of these tones into two groups A and B, with tones 74.4 79.7 85.4 91.5 also available as group C. Tones 69.3/4 and 97.4 are non-standard. The UK drew up a Land mobile radio specification MPT 1306 (http://www.radio.gov.uk/publication/mpt/mpt_pdf/mpt1306.pdf) allowing only 32 of the group A and B tones, missing 100 as this is twice the 50Hz UK mains electricty frequency. MPT1306 applies to business radio and amateur repeaters, but whether it actually applies to PMR446 is doubtful. MPT1306 is a UK standard, but any Euro-approved PMR446 sets may be used in the UK.

The following 11 tones (non-standard for commercial use) are also sometimes used, making a pool of 50 in all : 159.8, 165.5, 171.3, 177.3, 183.5, 189.9, 196.6, 199.5, 206.5, 229.1, 254.1

69.3 is sometimes 69.4 (the 0.5Hz tolerance of CTCSS circuits makes these two frequencies effectively the same), or otherwise missed out altogether leaving just 38 tones of the main set - 38 CTCSS (67,71.9-250.3), some radios miss 67 or 97.4 and may also feature 254.1 instead. Some sets feature 47 tones, with 165.5, 171.3 and 177.3 missed out - 47 CTCSS (67,69.3-254.1). Some 38 tone sets feature all possible values above, but only up to 192.8 : 38 CTCSS (67,69.3-192.8) - beware!
(I have even seen 64 tone charts, with the extra 14 being 33 35.4 36.6 37.9 39.6 44.4 47.5 49.2 51.2 53 54.9 56.8 58.8 63 fitting in below 67 - http://www.wiscointl.com/CSpecialists/ctcss.htm)

The tones are arranged so that most standard tones are at least 3.5% higher than the one below - most tones are almost exactly 1.035 times the frequency of the previous tone. Plot these on a graph and you'll see where there are a few small adjustments shifting things along a bit to avoid possible problems with harmonics of the lower tones. Quite a clever arrangement, really.

CTCSS

       38: 39: 47: 38: 32: EIA Al.  Moto.
 67     1   1   1   1   1  A   A00  XZ/L1
 69.3       2   2   2        x C00  WZ
 71.9   2   3   3   3   2   B  B00  XA/L2
 74.4   3   4   4   4        C C01  WA
 77     4   5   5   5   3  A   A01  XB/L3
 79.7   5   6   6   6        C C02  WB/SP
 82.5   6   7   7   7   4   B  B01  YZ/L4
 85.4   7   8   8   8        C C03  YA
 88.5   8   9   9   9   5  A   A02  YB/L4A
 91.5   9  10  10  10        C C04  ZZ
 94.8  10  11  11  11   6   B  B02  ZA/L5
 97.4  11  12  12  12        x C05  ZB
100    12  13  13  13      A   A03  1Z
103.5  13  14  14  14   7   B  B03  1A
107.2  14  15  15  15   8  A   A04  1B
110.9  15  16  16  16   9   B  B04  2Z
114.8  16  17  17  17  10  A   A05  2A
118.8  17  18  18  18  11   B  B05  2B
123    18  19  19  19  12  A   A06  3Z
127.3  19  20  20  20  13   B  B06  3A
131.8  20  21  21  21  14  A   A07  3B
136.5  21  22  22  22  15   B  B07  4Z
141.3  22  23  23  23  16  A   A08  4A
146.2  23  24  24  24  17   B  B08  4B
151.4  24  25  25  25  18  A   A09  5Z
156.7  25  26  26  26  19   B  B09  5A
159.8          27  27
162.2  26  27  28  28  20  A   A10  5B
165.5              29
167.9  27  28  29  30  21   B  B10  6Z
171.3              31
173.8  28  29  30  32  22  A   A11  6A
177.3              33
179.9  29  30  31  34  23   B  B11  6B
183.5          32  35
186.2  30  31  33  36  24  A   A12  7Z
189.9          34  37
192.8  31  32  35  38  25   B  B12  7A
196.6          36
199.5          37
203.5  32  33  38      26  A   A13  M1
206.5          39                   8Z
210.7  33  34  40      27   B  B13  M2
218.1  34  35  41      28  A   A14  M3
225.7  35  36  42      29   B  B14  M4
229.1          43                   9Z
233.6  36  37  44      30  A   A15  M5
241.8  37  38  45      31   B  B15  M6
250.3  38  39  46      32  A   A16  M7
254.1          47                   0Z

Column  A   B   C   D   E   F   G   H

A 38 CTCSS (67,71.9-250.3)    or (std)
  Standard 38 tones

B 39 CTCSS (67,69.3-250.3)
  Standard + 69.3

C 47 CTCSS (67,69.3-254.1)
  Standard + 69.3 + 8 more

D 38 CTCSS (67,69.3-192.8)
  First 38 of all tones
  Misses 7 standard tones
  but includes 3 unique ones

E 32 CTCSS (67,71.9-250.3)
  UK MPT1306 standard,
  misses 6 standard tones

F EIA groups A/B/C x=non-standard

G Alinco tone codes - 39

H Motorola tone codes

Which tones are best to use? Any tone that isn't being used on the channel. Or close by on another channel. One USA page I saw suggested avoiding tones below 7 or above 30 due to problems between different brand radios ( http://wildsnow.com/tips/ham_radio.htm ). Another page says to avoid tone 136.5 because Motorola's "Digital Private Line" (DCS) has a 200ms turn off 'burst' that comes in at an audio frequency near 136Hz (134.4), so you may end up with your radio clicking for no apparent reason." ( http://www.genave.com/ctcss.htm ).


I like PL tones in the middle of the range because I've found they work the best and aren't objectionable to listen to.

I avoid the low tones below 100.0 Hz because:
* They take a little longer to decode, regardless of the encoded tone level.
* Noise from interference sources often falls in the range of these tones, causing them to false decode on the noise.
* The lower tones tend to false decode adjacent tones. For example, a PL decoder set to decode 77.0Hz will often false decode 74.4Hz and 79.7Hz. The higher tones don't suffer from this because they are spaced farther apart in frequency.
* Some radios have trouble encoding all PL tones at the proper level across the entire tone range. The lower tones are often lower in level as the radio is adjusted to prevent the higher tones form being over-deviated (too loud on the air). The lower tones end up being too low in level to be decoded reliably by other radios.

I avoid PL tones above 192.8Hz because:
* You definitely can hear them "sing" and can be annoying.
* The higher tones are more likely to false decode on voice frequency components in the received signal's audio. Sigals recieved that don't have the proper PL tone may cause the decoder to "false" on some of the voice components in the audio.

I avoid any PL tone that is close to a harmonic of 60Hz. A transmitting radio that has a lot of AC power supply hum in the audio can cause a 118.8Hz PL decoder to false decode on the AC hum. The same can also happen with 179.9Hz as it is very close to 180.0Hz, the third harmonic of 60Hz. I know of a repeater that was plagued by intermod problems that had TV sync buzz in the intefering signal's audio. The 120Hz buzz sound was falsely decoded by the repeater's 118.8Hz PL decoder and kept the repeater on the air. The intermod problems eventually got solved and that PL tone was shut off in the repeater.

I avoid using 136.5Hz because of the DPL "kerchunk" problem. A radio transmitting Digital PL (DPL) transmits a 134.4Hz turn-off code for squelch tail elimination for 180 milliseconds or so when the radio un-keys. This is close enough to 136.5Hz to cause a 136.5Hz PL decoder to falsely decode the turnoff code and respond with a "kerchunk" squelch tail. I found this out from firsthand experience. I also found that 131.8Hz can be affected in the same way on some radios.

I also recommend avoiding the use of the "oddball" tones that some radios have, like 159.8, 171.3, 183.5, 199.5, 206.5, etc. These tones are in addition to the standard 38 tones. Not every radio will have these tones plus they are closely spaced in between the standard 38 tones.

So what tones are left that are good? My list of known "good" tones are:
100.0 103.5 107.2 110.9
114.8 123.0 127.3 141.3
146.2 151.4 156.7 162.2
167.9 173.8 186.2 192.8

Out of these 16 remaining tones, my favorites are:

141.3 146.2 151.4 156.7 162.2 167.9

These tones work well and are in the middle of the tone range. They decode fast enough plus they don't "sing" too much. They work well and are a good compromise, all things considered.

I use 141.3 as my default and because it's the defacto GMRS "travel tone". For more information on the "travel tone" usage, click on the Travel Tone 141.3 link at the top of the page.

Lately, I've been using DPL instead of PL to avoid being bothered by the unlicensed bubblepack riff-raff. A friend of mine once told me DPL also stands for "Definitely Prevents Losers". It definitely applies to bubblepack pirates!

Just my $0.02 worth. Hope this information helps.

~ n1das


If you want the potential to contact any other users (or monitor only them) then you'll want as many tones and DCS codes as possible when you're choosing which radio to buy. On the other hand, a set that has unique tones (i.e. has the first 38 of all possible tones including 3 that even 47-tone radios don't have) allows you to have a private channel of your own where hardly anyone else can bother you.

As all 8 channels are evenly spaced, in theory you could find intermodulation problems IF TWO OR MORE other sets are transmitting VERY strong signals - i.e. quite close by. If you hear odd snippets of what seems to be two people talking at once then you're suffering intermod. For example channel 3 and channel 5 will combine in an overloaded (or otherwise non-linear) "front end" (the first part of the radio reception circuit) to produce a signal on channel 7. This is because the signals mix together - the difference is two channels worth (25kHz) which will mix AGAIN onto the channel 5 signal to produce an image on channels 7. Likewise channel 1 will be affected via channel 3.

If you have a problem like this, from two other signals, it's because one of their tones is the same as the one you're using - change channel to avoid the images. Either keep the same tone so that you'll know if the problem is there again, or change it and not be bothered again. There's a small chance that you might just pick the OTHER tone!

Intermod problems happen this way on CB too, with it's regularly spaced channels. It's possible to annoy someone close by who you know monitors channel 35 - if there's a strong signal on 25 just key up on 15! Or try a half-way point on 30. Oh what fun! (Note the CEPT channels are not all in order, and there are gaps which makes it more tricky!)

It takes time for the CTCSS circuitry to recognize a valid tone. You might loose the first part of the first word of someone's message. I'd be interested to know if the higher tones respond better i.e. if it takes a certain number of waveforms before the circuit locks on - then the lower frequency tones would take longer to open the squelch?

If you're monitoring on a normal scanner, the audio filtering may remove CTCSS tones or it may not. It may let through the higher tones audibly but not the lower ones. If you want to consider people listening in (or try and discourage them) choose a tone with this in mind. A low rumbling hum on an extension speaker can be very annoying! Sometimes these tones are called 'sub audible' - this doesn't mean the frequencies used are below the range of human hearing (20Hz) but rather that they are not heard on the proper communication radios due to the filtering process that removes them from the speech.

Joe : "I think most people will agree that PL (CTCSS) works better than DPL (DCS) for several reasons. PL is capable of decoding way into the mud, where you can't even understand voice - DPL requires about a 15dB SNR to properly work. PL will work with virtually any type of radio, DPL takes a true FM modulator and extremely good low frequency response. DPL can be tricky in operation, especially between radios of different makes. The real advantage of DPL is that it offers more codes than PL, hence more people can share a channel. Its decode time is about the same as PL, maybe slightly longer on average. PL will give you less problems in the long run. Use DPL only if you really need it." (Thanks Joe)

DCS

Code/inv  104  83  52 Yae
006 /
007 /
015 /
017 /
021 /
023 /047   *   *   6  A
025 /244   *   *   6  B
026 /464   *   *   6  C
031 /627   *   *   6
032 /051   *   *   6  D
036 /172   *       6  E
043 /445   *   *   5  F
047 /023   *   *
050 /
051 /032   *   *
053 /452   *       5
054 /413   *   *   5
065 /271   *   *   5
071 /306   *   *   5
072 /245   *   *   5
073 /506   *   *   5
074 /174   *   *   5
114 /712   *   *   4
115 /152   *   *   4
116 /754   *   *   5
122 /225   *   *   4
125 /365   *   *   4
131 /364   *   *   4
132 /546   *   *   4
134 /223   *   *   4
141 /
143 /412   *   *   4
145 /274   *       4
152 /115   *   *
155 /731   *   *   4
156 /265   *   *   4
162 /503   *   *   4
165 /251   *   *   4
172 /036   *   *
174 /074   *   *
205 /263   *   *   4
212 /356   *       3
214 /
223 /134   *   *
225 /122   *
226 /411   *   *   4
243 /351   *   *   3
244 /025   *   *
245 /072   *   *
246 /523   *       3
251 /165   *   *
252 /462   *       3
255 /446   *       3
261 /732   *   *   4
263 /205   *   *
265 /156   *   *
266 /454   *       3
271 /065   *   *
274 /145   *
306 /071   *   *
311 /664   *   *   3
315 /423   *   *   3
325 /526   *       3
331 /465   *   *   5
332 /455   *       3
343 /532   *   *   4
346 /612   *   *   3
351 /243   *   *
356 /      *
364 /131   *   *
365 /125   *   *
371 /734   *   *   5
411 /226   *   *
412 /143   *   *
413 /054   *   *
423 /315   *   *
431 /723   *   *   3
432 /516   *   *   5
445 /043   *   *
446 /255   *   *
452 /053   *
454 /266   *
455 /332   *
462 /252   *
464 /026   *   *
465 /331   *   *
466 /662   *   *   4
503 /162   *   *
506 /073   *   *
516 /432   *   *
523 /246   *   *
526 /325   *   *
532 /343   *   *
546 /132   *   *
565 /703   *   *   4
606 /631   *   *   4
612 /346   *   *
624 /632   *   *   5
627 /031   *   *
631 /606   *   *
632 /624   *   *
654 /743   *   *   4
662 /466   *   *
664 /311   *   *
703 /565   *   *
712 /114   *   *
723 /431   *   *
731 /155   *   *
732 /261   *   *
734 /371   *   *
743 /654   *   *
754 /116   *   *


52: UK - MPT1381
 (# of contiguous bits)