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Author Topic: PL and CTSS, What and How it is  (Read 8744 times)
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Posts: 58

« on: June 28, 2014, 07:26:05 AM »

Taken off a web page for benefit of other hams.


PL / CTCSS Introduction:

This section has background information that applies to two similar analog coded squelch systems. One squelch system is used by Motorola and the other squelch system is used by almost every major radio manufacturer. The name PL is a Motorola trade mark and Continuous Tone-Coded Squelch System (CTCSS) is a generic name. The difference between PL and CTCSS is in how the optional reverse burst is implemented. See the DPL / DCS page for information on a digital coded squelch system (unlike the analog PL/CTCSS squech systems, the 23 bit DCS/DPL squelch system is the same for all manufacturers).

The Syntor X has specific information on it's PL/DPL.
PL / CTCSS is an analog squelch scheme in which an audio tone is sent by a transmitter to control the squelch opening of a receiver. PL / CTCSS is designed to work in the "so called" sub-audible portion of the communications audio band below 300 Hz. The individual PL or CTCSS tone operates below 300 Hz while the voice modulation operates from 300 Hz to 3000 Hz. These tones are not really sub-audible, except that the radio receiver filters out anything below 300 Hz. Encoded tones are sent by the transmitter and decoded tones are used by the receiver. When used, the receiver must receive the programmed tone or it will mute the audio output (i.e. the operator will not hear anything from the radio unless the programmed tone is present). There are 42 standard Electronic Industries Association CTCSS tones from 67 Hz to 254.1 Hz. Execpt for the original Syntor, these radios can be programmed for all standard tones and non standard tones. However, there are cutoff filters that make it difficult to go much above 254.1 Hz and frequencies around or below 60 Hz are not very practical. For example, some MARS operators may need a tone of 150 Hz which is not an EIA standard tone.

Carrier Squelch (CSQ) is used when PL / CTCSS coded receive is not used. Even if any PL / CTCSS tone is present in the received signal, the receiver just ignores it and the squelch opens strictly when any received signal is strong enough to overcome the squelch level setting.
Because a receiver that uses PL / CTCSS will not open its squelch without a matching tone and in commercial use many different users share the same frequencies in the exact same geographic area, there is a secondary way to turn off the PL/CTCSS tone decoding. It is called the hang-up box that the microphone rests on. The original versions were actually small boxes, sometimes with a slide switch which is where the name hang-up box comes from. Modern radios might use a simple grounded clip, where the radio detects if the button on the back of the microphone is grounded or not. Some microphones also include a switch built into the back of the microphone itself. While the microphone is in the hang-up condition the receive squelch only opens when the programmed PL / CTCSS tone is present. However, when the microphone is removed from the hang-up state the receive PL / CTCSS decoding is disabled and the radio operates in CSQ. When sharing a frequency the PL / CTCSS decoding keeps you from having to listen to radio transmissions that are not of any interest to you, however when you want to transmit this same PL / CTCSS decoding prevents you from hearing if the frequency is already in use or not in use and clear to transmit on. The hang-up function solves this problem. When you pick up the microphone to trasmit, the PL / CTCSS decoding is disabled and you can hear anyone on the frequency which allows you to avoid interfeering with anyone else already transmitting. In the Spectra the hang-up function can optionally be programmed to stop the scan.
PL / CTCSS usage:

The basic premise behind PL / CTCSS is: stations that share the same frequency, but use different PL / CTCSS tones do not have to listen to each other. HAM radio operators do not have much use for this application of PL / CTCSS. PL / CTCSS tones are also used to protect repeater input frequencies among other things (HAMs do have a use for this application). Basically, the transmitting station must transmit a PL / CTCSS tone and the receiving station must use the same Rx PL / CTCSS tone. If the PL / CTCSS tones do not match, the receiver remains muted and the operator does not hear the transmitting station. Some multiple repeater systems use PL / CTCSS to select which repeaters are used. If the receiver uses carrier squelch (i.e. no PL / CTCSS) then all stations on the frequency are heard regardless of any transmitted PL / CTCSS. The "Private Line" part of the PL name is not to be taken literally because there is really nothing private about it when it comes to monitoring.

The designed feature of not hearing stations that do not transmit the matching PL / CTCSS tone can cause a problem. When someone does not hear any activity on a frequency they assume it is not in use and they can transmit. With receive PL / CTCSS the frequency can actually be in use, but the receiver does not hear it. This creates the potential problem of two or more stations unknowingly transmitting simultaneously and interfering with each other. The solution is the accessory called the hang-up box or clip. When the microphone is removed from the hang-up box or clip, the PL / CTCSS receiver squelch is disabled allowing the operator to hear everyone on the frequency. It's a natural, pick up the microphone and find out if it's really clear to transmit. Put the microphone back and the unwanted signals without matching PL / CTCSS disappear.

PL / CTCSS reverse burst:
Reverse burst only applies to radios using coded receiver squelch. Reverse burst actually changes the way the transmitter operates for the benefit of the receiving station. Radios using carrier squelch do not need or benefit from reverse burst.

To understand reverse burst, first you have to understand how PL or CTCSS coded receiver squelch operates. The normal setup for receiver encoded squelch is STD/STD which mutes the receiver audio and runs the squelch setting wide open (maximum noise would be heard on a frequency without any activity, except the audio is muted). This helps ensure maximum receiver sensitivity. When the radio receives a signal with a matching PL / CTCSS signal it will unmute the receiver audio so you can hear the signal. Since there is now a received signal present, the noise previously mentioned is absent. However, when the received signal disappears at the end of the transmission, there is a brief period of time before the receiver can detect the loss of PL / CTCSS and mute the audio again. This brief period of time causes a short burst of noise in the receiving radio speaker commonly called a squelch tail.

Reverse burst causes the transmitting station to go through a special process every time the transmission ends. When the transmitting station releases the PTT the transmitter will still transmit for about 180 milliseconds. During this 180 ms time, a transmitted PL signal phase will be delayed by 120 degrees (other radio manufactures use an incompatible 180 degree phase shift during a 150 ms Tx time). This phase shift gives the receiver time to decode and recognize the loss of a valid PL or CTCSS and mute the receiver audio before the transmitted signal disappears, which is while the receiver is still quiet. This eliminates the squelch tail noise burst in the receiver speaker.

The reason for using a phase shift for the PL reverse burst probably came about because in the old days mechanical resonate reeds (called Vibrasponders) were used to detect PL tones. The sudden PL signal phase shift of the resonate frequency PL signal would more quickly reduce the vibrations of the reed, rather than letting the vibrations wind down on their own as in if the resonate frequency PL signal was simply changed to a non-resonate frequency or removed. Without the reverse burst phase shift, the old Vibrasponder PL squelch would probably not close very quickly.

The different PL reverse burst phase shift used by Motorola as compared to other manufactures is why Motorola PL reverse burst usually does not work with other brands and why their CTCSS reverse burst usually does not work with Motorola radios. Keep in mind that PL and CTCSS do work together, the reverse burst is the only thing that can be incompatible.

The effect of loosing the squelch tail is very pleasant to the ears of the human receiver.

On radios equipped with Securenet encryption, the PL/CTCSS tone and its reverse burst is automatically disabled when transmitting encrypted audio.

Standard PL / CTCSS tones:

Standard PL / CTCSS Tones (42 total)

Tone Frequency - Motorola Code
67.0 - XZ

97.4 - ZB

141.3 - 4A

206.5 - 8Z

69.3 - WZ

100.0 - 1Z

146.2 - 4B

210.7 - M2

71.9 - XA

103.5 - 1A

151.4 - 5Z

218.1 - M3

74.4 - WA

107.2 - 1B

156.7 - 5A

225.7 - M4

77.0 - XB

110.9 - 2Z

162.2 - 5B

229.1 - 9Z

79.7 - WB

114.8 - 2A

167.9 - 6Z

233.6 - M5

82.5 - YZ

118.8 - 2B

173.8 - 6A

241.8 - M6

85.4 - YA

123.0 - 3Z

179.9 - 6B

250.3 - M7

88.5 - YB

127.3 - 3A

186.2 - 7Z

254.1 - 0Z

91.5 - ZZ

131.8 - 3B

192.8 - 7A


94.8 - ZA

136.5 - 4Z

203.5 - M1



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