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Old 2nd Apr 2020, 10:28 am   #1
G6Tanuki
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Default CTCSS frequencies - why the odd numbers?

I've often pondered the 'strange' frequencies used for CTCSS: things like 94.8 and 118.8Hz; I'd have expected the frequencies to be some nice multiple of something-easy-to-generate (in the same way as you successively divide a 32.768KHz oscillator by 2 to get 1Hz clock-pulses) but can't see *what* the oscillator origin-frequency of the CTCSS tones is.

Did they have to do something strange, like deliberately avoiding anything that might be harmonics/sub-harmonics of 50 or 60Hz? No doubt someone within Motorola-US made a decision based on logic back in the early-1950s.

The Military sub-audible tone used for rebroadcast etc has always been 150Hz.
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Old 2nd Apr 2020, 12:04 pm   #2
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Default Re: CTCSS frequencies - why the odd numbers?

What about the musical notes perhaps? They did generate/decode with reeds in the early days. Just an idea - not checked it.
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Old 2nd Apr 2020, 12:25 pm   #3
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Default Re: CTCSS frequencies - why the odd numbers?

When I used to work with Motorola radio gear the CTCSS tone was always referred to as a "PL" tone, as in private line. This might be worth remembering if you happen to do any searching on the web for info. The tones are the same as those used in the UK but I am not sure if our friends across the water were the first to use the system.
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Old 2nd Apr 2020, 12:28 pm   #4
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Default Re: CTCSS frequencies - why the odd numbers?

The oddness is to limit simple harmonics, quite ingenoius.
 
Old 2nd Apr 2020, 1:00 pm   #5
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Default Re: CTCSS frequencies - why the odd numbers?

Although it doesn't say why the frequencies were selected, there is a lot of info here
http://www.repeater-builder.com/tech...-overview.html

Amongst the info is the fact that the original system was based on reeds as mentioned above.
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Old 2nd Apr 2020, 2:32 pm   #6
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Default Re: CTCSS frequencies - why the odd numbers?

Fascinating.... I'm sure that somewhere there's an obscure US patent-document that goes into nice detail about the maths used to calculate the optimum tones.

I was aware that the first versions used resonant reeds for the tones (a similar approach was used in some early radio-controlled-model systems). Never came across reed-decoders in service myself; all the 'second generation' ones I saw were either based around tuned ferrite pot-cores or IC-based PLLs (NE567?).

I do recall a really-complex system based around a mix of TTL and CMOS which could transmit up to *four* tones simultaneously from the base-station, to give 'group call' functions.
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Old 3rd Apr 2020, 10:21 am   #7
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Default Re: CTCSS frequencies - why the odd numbers?

Pye Telecom called the system "Tonelock". When it first started being used in my area, there was a preferred frequency of 118.8Hz that the sales people liked to use. The reasoning was if it was a common frequency, that the Plug-in filters used earlier on in mobiles but in a lot of base stations would be in stock. These early larger plug in facility modules were called "TL1". Defeated the whole object of it really when on shared vhf channels. Anyone else remember those black filters about 3/4" cube? 118.8Hz had suffix /12.
Mobiles like the Europa and Olympic had a space about 3" wide, 1" high and 5" deep for this style of plug in facility module.
Later on, with advent of a smaller mobile, the Reporter, the larger TL1 module was replaced with a smaller preset pot tunable module, but still had I think 4 different daughter boards depending on the band of frequencies.
There was also a facility module housing that had a cable and 15 way D plug to interface with the later smaller mobiles. The facility modules all had the same size and family of interface connections. eg +ve, -ve, ptt, inband audio, subaudio, ptt, mute/unmute etc. The 5 tone modules were used a great deal by Securicor with their "Help" message handling service.
I used to have a 2 tone encoder/decoder fitted within my Pye transport scheme mobile, my callsign number was 224. Upon receipt of a suitable (slow) tone sequence on 85.85MHz it would unmute my rx audio path and a few milliseconds later transmit a reply sequence of 224 on 72.35MHz back to control. They knew my vehicle was in range. I had a pager on 72.35MHz which responded to this mobile tx reply code.
At home, if the tv was on, it wasn't really needed as 30W of 80% mod AM wiped the tv out and blasted through the speaker!
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Old 3rd Apr 2020, 11:20 am   #8
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Default Re: CTCSS frequencies - why the odd numbers?

I've never looked too deeply into it, but I've always assumed the oddness was to avoid harmonic responses in the reed-type detectors. A 150 Hz reed will probably respond to a 50 Hz tone and raise a false alarm. I would imagine it'll be mainly 'odd' harmonics that will cause the problems.
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Old 4th Apr 2020, 11:10 am   #9
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Default Re: CTCSS frequencies - why the odd numbers?

Most Selcall was 5 tones using the CCIR or ZVEI tone sets. CCIR rattled through the 5 tones much faster.
I seem to remember that a German company called Dersen were a major force behind it. I also remember the two tone sequential version that came before.
On Pye gear the module fitted in the same facility slot as the Tonelock module.

I had a reed based decoder from a PF2 once upon a time. Probably no longer.
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Old 4th Apr 2020, 11:24 am   #10
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Default Re: CTCSS frequencies - why the odd numbers?

Jon, I still have the FOGA5 test set for encode and decode of 5 or 6 tones. Very expensive back in the 80s. Brilliant piece of kit. We had one at the depot and I eventually got one for my van as I had a large local authority sewage outstation pump alarm system to maintain that was on highband am. I thought about selling it a while back, but kept it as a "might come in handy" one day. It still sits there!
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Old 4th Apr 2020, 11:56 am   #11
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Default Re: CTCSS frequencies - why the odd numbers?

Very nice Rob. I still have some modules.

I think I built one from scratch too a long time ago. The tapped inductor was saved from a scrap module and I seem to remember it clocked around the tones using a 4017 to switch the taps. It made an LC tuned circuit with a large polystyrene capacitor and an inductor wound on a ferrite pot core. Hi-tech!
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Old 4th Apr 2020, 12:21 pm   #12
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Default Re: CTCSS frequencies - why the odd numbers?

Closer to the original topic. If the CTCSS tone frequencies are put in excel and analysed then there is a frequency difference between most adjacent tones that is very close to 3.5%.

Presumably that was chosen to suit the Q of likely decoder circuits of the time.

What is strange is occasionally a 2% or even a 1.5% difference shows up.
Often if you take the tones on either side then it comes out at 3.5% again.
So these tones were added later maybe?

206.5 229.1 are two cases. 97.4 and 100.0 are strange too.

100.0 was supposed to be avoided in UK due to being the 2nd harmonic of mains hum.

Last edited by Jon_G4MDC; 4th Apr 2020 at 12:45 pm.
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Old 4th Apr 2020, 1:56 pm   #13
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Default Re: CTCSS frequencies - why the odd numbers?

I seem to remember our fleet of Pye MX290s having a TED6 module in them which was quite a big board full of ICs. I think it may have been configurable for CTCSS but we just used them for 5/6 tone EEA signalling for the transmission of vehicle crew status back to control, and of course selective calling of the vehicle. When the vehicle was called it made this horrendous distorted squawk through the speaker then opened the squelch for voice. These were legacy radios hanging on from a previous system which eventually gave way to the new Motorola network around 1989.
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Old 4th Apr 2020, 4:15 pm   #14
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Default Re: CTCSS frequencies - why the odd numbers?

TED6 could do both. I'm glad I never had to work on them!
TEDX was the processor controlled version - 8085 based I think.
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Old 5th Apr 2020, 9:58 am   #15
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Default Re: CTCSS frequencies - why the odd numbers?

TED6 had a lot of options, but most required the additional components to be fitted. CTCSS was one option and left quite an unpopulated area if not included. Then a fixed 5 tone, 5 tone including last digit variable and another with last 2 digits variable. Various jumpers were provided and also some longer wire links as well. You neded the handbook to work out your operational reqirements. The 5 tone digit sequence was programmed by very thin wires across a 28 pin (i think) plug in ic header. One set for encode and one for decode. There was lots of ferrite beads on all the links! CTCSS used several ic and lots of discrete components. I think there were just 3 "custom" chips that were smd. QQ2254 and QQ2255 come to mind.

The TEDX was all programmed in software and used a CML fx*** series custom ctcss chip.
The CML chips became popular here for CTCSS use. I used to buy (from Intercall, I think) encoder decoders that were dip switch programmable and about 5cm x 4cm and fitted directly into any M290 series radio front panel and mated with the facility connector of the radio when the front was screwed back on . These I found to be very reliable.
A few monthxs ago I think someone asked a question about a test box that was the original Pye TL1 equipment supplied to engineers. This had maybe just 24 tones on it.
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Old 5th Apr 2020, 10:18 am   #16
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Default Re: CTCSS frequencies - why the odd numbers?

Quote:
Originally Posted by Jon_G4MDC View Post
TEDX was the processor controlled version - 8085 based I think.
8051 microprocessor family, the actual chip being the ROMless 8031 or 8032 I think - I did a firmware rewrite which made the TEDX function as an amateur friendly CTCSS / 1750Hz toneburst sender. Someone else (A G3) took it further and also added channel selection through the front panel buttons rather than the usual rotary switch.

There was an 8085 based MX control head PCB (The CX290) but that was for the E31 trunk signalling system rather than for CTCSS.
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Old 5th Apr 2020, 8:54 pm   #17
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Default Re: CTCSS frequencies - why the odd numbers?

I avoided TED6 like the plague. Horrid.
Anyway that drifts away from the CTCSS discussion of the OP.

I would suggest that the scheme was started with tones spaced by 3.5% rounded to the nearest 0.1 HZ and with 67.0Hz as the first tone.

Other views are very welcome.
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Old 6th Apr 2020, 6:15 pm   #18
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Default Re: CTCSS frequencies - why the odd numbers?

5-tone was rather fun [at one time I had my personal 5-tone on a rather large UK-wide net as the tone-sequence from "Close Encounters of the Third Kind" !]

Reed-based tone calling was also used in the Pye "Firecall" pager network - used to summon 'retained' volunteer firefighters in rural areas. [The SR1 paging-receiver could easily be repurposed as a monitor-receiver for your local 2-Metre repeater....]

The reason for my initial query is that - in these virus-times - I'm developing a renewed interest in 2-Metre operation. Local geography means I can only hear a couple of repeaters - but I'm thinking that if I retrofitted CTCSS I could re-activate the Pye MX290 I mothballed a decade and a half ago when I moved here.
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Old 6th Apr 2020, 6:57 pm   #19
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Default Re: CTCSS frequencies - why the odd numbers?

Find yourself an old TED X control unit / front panel - one with an LCD display - and I'll dig out the EPROM code which makes it into an amateur specific unit (Options: No Tone / 1750Hz momentary / CTCSS continuous tones A through to J, all indicated on the LCD display).

It only sends tones though, does not implement CTCSS on receive although the hardware is capable of that. I didn't want to give amateurs one more reason to ignore each other than they already had

MX293s were very commonly found with TED Xs on them, for some reason. I sometimes bought 'M' band ones (hardly worth the trouble of conversion) just to harvest their control boards and other parts common to all of the MX series.

Alternatively there are the small PCBs (mentioned earlier) with little more than a CML IC on, which plugged into the connector near the front of the radio - however these usually only have a fixed CTCSS code set by a DIP switch on the PCB, which is fine as long as you stay in one CTCSS area.
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Old 6th Apr 2020, 7:46 pm   #20
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Default Re: CTCSS frequencies - why the odd numbers?

Alas, we're not living in the 1990s any more: the days of seeing heaps of MX290s with TED-boards [and a few with the nice LED displays/rotary-channel-select] at rallies are long gone.

The FX465 chip seems a lot more attractive.
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