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Explanation of Reverse Burst & "And Squelch"
By Kevin K. Custer W3KKC
Also see the article on the generic use of "And Squelch" in non Motorola applications.
The concept of reverse-burst is difficult
for some to follow.
If you have trouble understanding what is written here, read it again or several times as it may come to you.
Reverse burst is usually generated on commercial 2-way transmitters to eliminate the squelch tail noise burst in systems using PL, or Private Line. "Reverse burst" is a Motorola term, and GE calls the same thing "STE" for Squelch Tail Elimination.
Reverse burst or Squelch Tail Elimination ("STE") is a process that uses a change of the phase of the PL tone encoder for a short period of time after the user unkeys the PTT button. The term "reverse burst" is used to describe the deliberate phase change for a specific abount 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.
Think of the PL decode reed as a precise frequency sensitive transformer - in fact it's physical constructions is very similar to a tuning fork with each side of fork magnetized and surrounded by a coil. Since you have two windings coupled by a tuning fork, you have a precisely (mechanically) tuned transformer. One leg of the fork is excited by the incoming audio, the other leg wiggles back and forth inside a coil, and the voltage generated opens the squelch for as long as the vibrations take place. The actual physical construction uses one leg and one magnet, and the action is the same.
Motorola shifts the tone phase by 120 degrees for about 180 milliseconds, Kenwood and the other manufacturers shifts the phase of the tone by 180 degrees for about 150 milliseconds. The vibrating reed type of decoder responds to both formats with no problems, but the digital signal processors in modern receivers are (or can be) very picky about reverse burst phase shift and duration.
In commercial service where PL is used the receivers squelch is sometimes controlled solely by the PL decoder, and the setting of the squelch control is irrelevant to when the receiver unsquelches. Most amateur equipment does not have the ability to transmit reverse burst, but in most applications it is advantageous to use the "and squelch" feature if using the original Motorola PL decoder board in your MICOR receiver. Why? Because Motorola developed "PL" or private line for commercial users that would operate in an environment where ALL users transmitted "reverse burst."
Consider a user having no reverse burst while transmitting PL into a receiver equipped with a Motorola reed PL decoder, then the user unkeys, the reed will continue to vibrate (like a tuning fork) until it naturally coasts to a stop. Since the squelch circuit audio gate stays open as long as the reed is vibrating this produces an annoying long squelch tail noise burst.
Enabling "AND squelch" allows the receivers regular noise squelch circuit to work as in any other receiver, but in conjunction with the PL logic signal. Thus a user, without reverse burst capability, transmitting PL will be muted as soon as the carrier is dropped, even though the PL reed is still moving and slowly coasting to a stop.
A receiver set-up for "AND squelch" uses the PL AND the squelch control to control the unmuting of the receiver audio. If you are going to use an outboard controller to manage the functions of the repeater, and / or, if you want the squelch control to control squelch sensitivity, you will want to use "AND squelch."
Some manufacturers, for example Scom, interface to the receiver with two different logic signals - a carrier sense line and a seperate PL decode line. This feeding two signals to the controller offers very versatile functionality of the repeater controller - it offers four modes: Carrier-only, Carrier AND the PL decoder, Carrier OR the PL decoder, and PL decode only. The two most useful in the amateur world are carrier only and Carrier AND the PL decoder. There are other controllers on the market that offer similar functionality.
Since, in most cases, an external repeater controller is used to manage the functions of control, the radio is run in carrier mode full time. The PL decode output is simply used to feed a logic signal to the controller and the controller manages the function of controlling / muting the audio. The controller does the "AND Squelch" function, not the radio. The MICOR receiver was never intended to be used on a system using PL and not employing reverse burst, so the stock receiver does not have enough circuitry in it to do this type of "and squelch" by itself, or by using an external controller that supports the "and squelch" function. Many controllers do the "AND squelch" function, if yours does not, "AND squelch" can be accomplished with a simple AND gate (which can be implemented in logic, or in something as simple as a relay). The output of the AND gate will then control the gating/muting of the repeated audio.
In a MICOR receiver being used in a system not employing reverse burst and having an original MICOR PL decode deck, you must cut the connection between the PL deck and the squelch shunt switches or the receiver will produce the squelch tail burst from both the external speaker and from the controllers repeat audio.
Why? As an internal function of the receiver, the PL signal is fed to the shunt switches and overrides the COS signal. The PL signal will force the COS on, thus sending a COS to the controller that is slaved from the PL deck forcing the controller to pass audio until the reed coasts to a stop by itself. Obviously you want a PL signal and a COS signal that operates totally independently from one another when using an external controller, or when adding circuitry to the receiver to allow the use of "and squelch" in non reverse burst operation.
"Conversion" In Mobile Audio & Squelch Boards (TLN4310A & B), "And Squelch" is accomplished by cutting the circuit trace, or jumper wire if present, between IC 202 pin 8, and P 201 pin 3 (pin 3 on the PL interface pins).
In Station configurations (either TLN4725A or TRN6006A or B Audio & Squelch boards), "And Squelch" is accomplished by cutting jumper wire JU-204 between IC 202 pin 8, and P 201 pin 3.
These additional jumpers and components are only necessary when NOT
using a controller, I.E. Factory Control Cards being used:
Insure that CR 955, CR 956 and R 952 are installed and jumpers JU-956, and JU-957 are omitted on the Receiver Interconnect Board.
NOTE: If you are using an outboard controller, the only necessary modification is to cut jumper JU-204 and run the PL logic signal and the COS to your controller, (if it does AND type squelch gating, most newer models do).
"AND Squelch" is necessary when using the MICOR Muteboard because the PL decoder will interpret the COS signal and create a squelch noise burst if user is transmitting PL and does not employ reverse burst.
The "AND Squelch" modification is also necessary when implementing an "AND gate" to create a composite squelch signal that requires both COS and PL to activate but will drop if either goes away. This is a nice feature when you have a controller that does not have a dedicated PL input. Putting the controller in PL then can be accomplished by using a logic output signal from the controller. The controller can be put into carrier access by replacing the PL signal with a constant voltage so the COS signal alone will activate the controller. A relay or an "OR" gate could be used on the PL input side of the "AND" gate to control carrier access.
A note from Mike WA6ILQ:
Many of the early Motorola tube-based radios used relay contacts to select between two windings on a transformer, or between the plate and cathode of a tube, or between the collector and the emitter of a audio buffer transistor, any method producing a 180 degree phase shift of the PL encoder. Later the designers realized that 180 degrees was not the optimum shift - that 120 degrees worked better - and later radios used a 120 degrees phase shift for about 180ms. So some manufacturers - Kenwood, Vertex, Icom, Johnson, Ericsson, etc. used a 180 degree shift, while Motorola used 120 degrees. Late-model radios use digital signal processing chips to encode and decode the CTCSS tones, and (unfortunately) if programmed to look for one format of reverse burst will completely ignore the other. And that is why on some radio systems a radio from one brand both causes and suffers long squelch bursts when communicating with a repeater or a radio from another brand. It's not the fault of the radio, it's the fault of bad programming in the DSP-based PL decoder that should have been designed to recognize either mode transparently to the end user, but doesn't. To rectify this situation the designers of some of the newer Motorola radios - the "Professional" series (HT750 / HT1250 / CDM750 / CDM1250, etc.), for example, offer a "check box" in the programming software labeled "Non-Standard Reverse Burst" that when selected causes the Motorola radio to encode using the other phase shift and to recognize it on decode. This feature is selectable on a per channel basis so that the radio can be squelch-tail-less on both types of systems - as long as the radio is programmed correctly.
A note from Fred Seamans W5VAY:
Kevin: A few comments on your write up on Reverse Burst. In the early 50's Mot. had their large reeds in their equipment and GE used a RC network for CTCSS operation in the Progress Line. Both were all tube designs. Neither had reverse burst at this time. In the late 50's GE switched to reeds made by Bramco Corp to obtain better temperature stability. Then in the early 60's Mot. introduced the Motrac Line and GE introduced the MASTR Progress Line. Both used reeds in the CTCSS systems. Also the Electronic Industries Association developed standards for CTCSS systems. GE's equipment complied with these standards, however Moto did not adopt them. The Mot. normal settings required as much as 1 kHz deviation of the CTCSS tone for proper operation while GE's required +/- 750 Hz deviation. In the mid 60's GE switched from reeds to RC networks and Moto stayed with reeds. Moto even developed a rotational mass reed in an attempt to reduce shock and vibration from opening the squelch circuit. Both mfg's used 180 degree reverse burst in their equipment at that time.
In the late 60's when MICOR and MASTR II were introduced, both had solid state, high Q active filters in their CTCSS systems. This High Q network caused ringing difficulties. Moto adopted a 120 degree phase reversal and GE used 235 degree phase reversal to stop the ringing of the High Q filters.
GE's CTCSS systems were always a Tone "AND" Carrier required to open the squelch.
I cannot speak to the current systems in use today as I retired from any design work in the early 90's.
I hope these comments will fill in some gaps you may have on GE.
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This web page created July 3, 1998 by Kevin Custer W3KKC
Comment added 6-Oct-2003 by Mike WA6ILQ
Fred's comment added 15-Aug-2004 by Mike WA6ILQ
The information presented in this web site and on these web pages is © Copyrighted 1995 - current by Kevin Custer W3KKC and multiple originating authors.