ARCHIVE June 2010
Tech Talk
By Jim Purvis WA7HRG
CTCSS (PL) Tones
In the early days of Land Mobile Radio, when usable VHF frequencies were at a premium, and it was very likely to have more than one user licensed on a channel, it became cluttered very quickly. Commercial repeaters were scarce and a coveted resource. It would not be uncommon to have several companies with several mobile radios each on the same repeater or channel. Let’s say that Joe’s Plumbing had 5 service trucks, Dan’s Delivery had 3 vans and Universal Concrete had 7 trucks. That would be 15 mobile radios all on the same frequency. It became quit annoying for Joe’s plumbers to have to listen to Dan’s and Universal’s communications all day. And visa versa. Yes, they all still had to share the frequency but there needed to be a way that each user only heard their own calls.
In the early 1950s, the geniuses in Motorola R&D developed a system that added a low-frequency tone to the existing transmitter audio that was present as long as the transmitter was keyed. The receiver was modified by adding a tone detector circuit to the receiver and then the receiver squelch was modified so that it wouldn’t open unless the tone was detected. They also modified the receiver audio circuits to strip the audio tone out of the speaker audio. The legal folks patented the idea and the technique then trademarked the term "Private Line" (PL for born). Sales literature of the time compared the system to the receiver having a front door with a lock on it, and the transmitter having a key ring full of keys, and only the signal with the correct "key" could open the lock. Note that the entire system consisted of adding a new tone modulation circuit in the transmitter, and a new tone decoding circuit in the receiver - nothing that would actually reduce or eliminate RF interference. But in our example above, as long as Joe’s and Dan’s and Universal all used a different PL tone then they would only hear their own stations (silence is golden). This didn’t mean that the other stations were not there and that they didn’t have to share the frequency but at least while monitoring they did not hear all the other traffic. When the mic was lifted from the hang up bracket it disabled the decode circuit so that again all traffic could be heard. It is an FCC rule that the channel has to be monitored before transmitting to minimize interference to other users on the channel.
So why are the tone frequencies so exact, down to the tenths of a Hertz and so random looking?
The tones were picked, not at random as it may look, but because there is NO harmonic or simple integer relationship between any of the tones. There is no way to multiply or divide say 110.9 Hz by any simple relationship that would arrive at any of the other tone frequencies. This is done to prevent false activation of the tone decoder. The tones are actually 103.5% away from each other. If you multiply 110.9 by 103.5% you will arrive at the next tone; 114.7815 Hz. It was rounded to 114.8 Hz because, again in 1950, the frequency accuracy of the tone equipment was not all that accurate anyway.
In fact the early tone decoders and encoders used a vibrating mechanical reed device, much like a tuning fork. More exactly, like a tuning fork with a coil of wire around it. Receive audio was filtered and passed through the coil. If the audio frequency in the coil was the same as what the mechanical tuning fork was cut for it would vibrate. These vibrations would induce a small voltage in another coil. This voltage was used to control the receiver squelch. In the transmitter a similar device generated the tone. The tuning fork in this device however vibrated when a voltage was applied to it. The dimensions of the fork determined the frequency of the tone. The Q of the mechanical devices and the manufacturing tolerances dictated the spacing. Any more or less than the 103.5% would increase the likelihood of false activation and waste audio spectrum. Remember these tones had to be carried on the normal voice audio path and had to be filtered out before it went to the speaker. Too broad of a tone range would mean cutting into the voice spectrum and effecting fidelity.
This was cutting edge - state of the art stuff back then. It was not long before the other radio manufactures caught on and similar circuitry was introduced with names like Channel Guard (CG), Quiet Channel (QC), and finally just the industry standard name of Continuous Tone Coded Squelch System(CTCSS)
style='font-family:"Times New Roman",serif'>In recent years the Amateur community has adopted CTCSS as a way to control access to repeaters. It was first used as a way to “close” a repeater. By keeping the CTCSS frequency unknown to the general Ham and only giving it to a select few, say a club or select group, the repeater would not be used unless you were “authorized”. Then with more and more repeaters showing up around the country it became, as in the above example, a way to limit hearing some of the co-channel interference. And as linking repeaters became popular it is also a way to steer or route traffic through selected repeaters. It is common to use one CTCSS tone to activate the local repeater and then on the same frequency use a different tone to activate a link to a second distant repeater. There have been all sorts of signaling and controlling schemes devised using CTCSS tones.
style='font-family:"Times New Roman",serif'>Also common is to encode CTCSS on the repeater transmitter so that a decoder is needed on the users portable or mobile receiver. In a standard carrier squelch system, especially if the squelch is set pretty loose, the receiver is quit susceptible to local interference such as near by computers or engine noise. By enabling CTCSS decode there will be no noise from the speaker unless there is a valid signal at the receiver input.
style='font-family:"Times New Roman",serif'>
style='font-family:"Times New Roman",serif'>There is one other piece of magic with tone coded systems that came along shortly after its introduction. That was “reverse burst” or “squelch tail elimination”. In our above example with Joes’, Don’s, and Universal each company did not hear the other company’s traffic. However, when the desired traffic was being received, say a Dispatcher is talking to one of Don’s vans, there would be a short burst of white noise or “static crash” as it is sometimes called at the end of the transmission. This was caused by the timing delay between when the repeater transmitter shut off and the mobile receiver squelch closing again. The CTCSS tone would keep the squelch open during the transmission but there would be a short time between when the tone reed quit vibrating and the squelch would shut.
style='font-family:"Times New Roman",serif;mso-bidi-font-style:italic'>Reverse burst is a process that uses a change of the phase of the PL tone encoder for a short period of time after the user un-keys the PTT button. The term "reverse burst" is used to describe the deliberate phase change for a specific amount of time while the transmitter carrier stays on - about 150 to 200ms - with the phase of the PL encoder offset by from 120 to 180 degrees (180, naturally is a complete reversal). During the reverse burst time period the reverse phase stops the PL decode reed dead in its tracks - which slams the receiver squelch closed right now. By the time the transmitter actually drops off the air the RX squelch is already closed - which results in no burst of squelch noise being heard.
style='font-family:"Times New Roman",serif;mso-bidi-font-style:italic'>Most Amateur radios do not have reverse burst or squelch tail elimination circuitry. But with the high volume of commercial radios being pressed in to Amateur service it becomes a handy feature and its nice to know how it works.
style='font-family:"Times New Roman",serif;mso-bidi-font-style:italic'> style='font-family:"Times New Roman",serif'>
TABLE OF COMMON PL TONES (in Hz)
67.0 94.8 131.8 171.3 203.5
69.3 97.4 136.5 173.8 206.5
71.9 100.0 141.3 177.3 210.7
74.4 103.5 146.2 179.9 218.1
77.0 107.2 151.4 183.5 225.7
79.7 110.9 156.7 186.2 229.1
82.5 114.8 159.8 189.9 233.6
85.4 118.8 162.2 192.8 241.8
88.5 123.0 165.5 196.6 250.3
91.5 127.3 167.9 199.5 254.1
73’s Jim
Reference;
http://www.dra73.org/pl.html
Tech Talk
By Jim Purvis WA7HRG
CTCSS (PL) Tones
In the early days of Land Mobile Radio, when usable VHF frequencies were at a premium, and it was very likely to have more than one user licensed on a channel, it became cluttered very quickly. Commercial repeaters were scarce and a coveted resource. It would not be uncommon to have several companies with several mobile radios each on the same repeater or channel. Let’s say that Joe’s Plumbing had 5 service trucks, Dan’s Delivery had 3 vans and Universal Concrete had 7 trucks. That would be 15 mobile radios all on the same frequency. It became quit annoying for Joe’s plumbers to have to listen to Dan’s and Universal’s communications all day. And visa versa. Yes, they all still had to share the frequency but there needed to be a way that each user only heard their own calls.
In the early 1950s, the geniuses in Motorola R&D developed a system that added a low-frequency tone to the existing transmitter audio that was present as long as the transmitter was keyed. The receiver was modified by adding a tone detector circuit to the receiver and then the receiver squelch was modified so that it wouldn’t open unless the tone was detected. They also modified the receiver audio circuits to strip the audio tone out of the speaker audio. The legal folks patented the idea and the technique then trademarked the term "Private Line" (PL for born). Sales literature of the time compared the system to the receiver having a front door with a lock on it, and the transmitter having a key ring full of keys, and only the signal with the correct "key" could open the lock. Note that the entire system consisted of adding a new tone modulation circuit in the transmitter, and a new tone decoding circuit in the receiver - nothing that would actually reduce or eliminate RF interference. But in our example above, as long as Joe’s and Dan’s and Universal all used a different PL tone then they would only hear their own stations (silence is golden). This didn’t mean that the other stations were not there and that they didn’t have to share the frequency but at least while monitoring they did not hear all the other traffic. When the mic was lifted from the hang up bracket it disabled the decode circuit so that again all traffic could be heard. It is an FCC rule that the channel has to be monitored before transmitting to minimize interference to other users on the channel.
So why are the tone frequencies so exact, down to the tenths of a Hertz and so random looking?
The tones were picked, not at random as it may look, but because there is NO harmonic or simple integer relationship between any of the tones. There is no way to multiply or divide say 110.9 Hz by any simple relationship that would arrive at any of the other tone frequencies. This is done to prevent false activation of the tone decoder. The tones are actually 103.5% away from each other. If you multiply 110.9 by 103.5% you will arrive at the next tone; 114.7815 Hz. It was rounded to 114.8 Hz because, again in 1950, the frequency accuracy of the tone equipment was not all that accurate anyway.
In fact the early tone decoders and encoders used a vibrating mechanical reed device, much like a tuning fork. More exactly, like a tuning fork with a coil of wire around it. Receive audio was filtered and passed through the coil. If the audio frequency in the coil was the same as what the mechanical tuning fork was cut for it would vibrate. These vibrations would induce a small voltage in another coil. This voltage was used to control the receiver squelch. In the transmitter a similar device generated the tone. The tuning fork in this device however vibrated when a voltage was applied to it. The dimensions of the fork determined the frequency of the tone. The Q of the mechanical devices and the manufacturing tolerances dictated the spacing. Any more or less than the 103.5% would increase the likelihood of false activation and waste audio spectrum. Remember these tones had to be carried on the normal voice audio path and had to be filtered out before it went to the speaker. Too broad of a tone range would mean cutting into the voice spectrum and effecting fidelity.
This was cutting edge - state of the art stuff back then. It was not long before the other radio manufactures caught on and similar circuitry was introduced with names like Channel Guard (CG), Quiet Channel (QC), and finally just the industry standard name of Continuous Tone Coded Squelch System(CTCSS)
style='font-family:"Times New Roman",serif'>In recent years the Amateur community has adopted CTCSS as a way to control access to repeaters. It was first used as a way to “close” a repeater. By keeping the CTCSS frequency unknown to the general Ham and only giving it to a select few, say a club or select group, the repeater would not be used unless you were “authorized”. Then with more and more repeaters showing up around the country it became, as in the above example, a way to limit hearing some of the co-channel interference. And as linking repeaters became popular it is also a way to steer or route traffic through selected repeaters. It is common to use one CTCSS tone to activate the local repeater and then on the same frequency use a different tone to activate a link to a second distant repeater. There have been all sorts of signaling and controlling schemes devised using CTCSS tones.
style='font-family:"Times New Roman",serif'>Also common is to encode CTCSS on the repeater transmitter so that a decoder is needed on the users portable or mobile receiver. In a standard carrier squelch system, especially if the squelch is set pretty loose, the receiver is quit susceptible to local interference such as near by computers or engine noise. By enabling CTCSS decode there will be no noise from the speaker unless there is a valid signal at the receiver input.
style='font-family:"Times New Roman",serif'>
style='font-family:"Times New Roman",serif'>There is one other piece of magic with tone coded systems that came along shortly after its introduction. That was “reverse burst” or “squelch tail elimination”. In our above example with Joes’, Don’s, and Universal each company did not hear the other company’s traffic. However, when the desired traffic was being received, say a Dispatcher is talking to one of Don’s vans, there would be a short burst of white noise or “static crash” as it is sometimes called at the end of the transmission. This was caused by the timing delay between when the repeater transmitter shut off and the mobile receiver squelch closing again. The CTCSS tone would keep the squelch open during the transmission but there would be a short time between when the tone reed quit vibrating and the squelch would shut.
style='font-family:"Times New Roman",serif;mso-bidi-font-style:italic'>Reverse burst is a process that uses a change of the phase of the PL tone encoder for a short period of time after the user un-keys the PTT button. The term "reverse burst" is used to describe the deliberate phase change for a specific amount of time while the transmitter carrier stays on - about 150 to 200ms - with the phase of the PL encoder offset by from 120 to 180 degrees (180, naturally is a complete reversal). During the reverse burst time period the reverse phase stops the PL decode reed dead in its tracks - which slams the receiver squelch closed right now. By the time the transmitter actually drops off the air the RX squelch is already closed - which results in no burst of squelch noise being heard.
style='font-family:"Times New Roman",serif;mso-bidi-font-style:italic'>Most Amateur radios do not have reverse burst or squelch tail elimination circuitry. But with the high volume of commercial radios being pressed in to Amateur service it becomes a handy feature and its nice to know how it works.
style='font-family:"Times New Roman",serif;mso-bidi-font-style:italic'> style='font-family:"Times New Roman",serif'>
TABLE OF COMMON PL TONES (in Hz)
67.0 94.8 131.8 171.3 203.5
69.3 97.4 136.5 173.8 206.5
71.9 100.0 141.3 177.3 210.7
74.4 103.5 146.2 179.9 218.1
77.0 107.2 151.4 183.5 225.7
79.7 110.9 156.7 186.2 229.1
82.5 114.8 159.8 189.9 233.6
85.4 118.8 162.2 192.8 241.8
88.5 123.0 165.5 196.6 250.3
91.5 127.3 167.9 199.5 254.1
73’s Jim
Reference;
http://www.dra73.org/pl.html