Actually, the electronic CTCSS decoders react about the same as the old reeds. The physics of the matter causes the filters that can discern for instance - 100 Hz from 97 Hz or 103 Hz to be very narrow, and they ring - even when the driving tone is removed. By reversing the tone phase for a short period of time, the energy in the filter is driven to zero very quickly, and if the tone is removed from the decoder input at the right time, the tone decoder closes very quickly, and you get very short squelch bursts at the end of a transmission.
There is no 'reverse burst decoder' per se in a tone decoder - it is just driven with the out of phase energy long enough to cause it to close very quickly. All tone decoders react to the reverse burst, not just one that is specially configured to react to a reverse burst. I don't know of any special circuitry in a tone decoder that makes it more susceptible to a reverse burst than a normal tone decoder. 73 - Jim W5ZIT -----Original Message----- From: [EMAIL PROTECTED] To: Repeater-Builder@yahoogroups.com Sent: Fri, 19 Jan 2007 2:27 PM Subject: [Repeater-Builder] Re: Reverse Burst Comments (Com Spec RB-1) --- In Repeater-Builder@yahoogroups.com, "skipp025" <[EMAIL PROTECTED]> wrote: > > Re: Reverse Burst Comments (Com Spec RB-1) > > If you add a circuit like the Com Spec RB-1 board to the typical > repeater system using a ts-32/ts-54 board... the tx ctcss is not > disabled or removed before the RB-1 delayed ptt line drops. > > So you have the phase inverted ctcss present for at most up to > 200 ms typical before the tx drop. If you don't remove the ctcss > source the inverted ctcss remains up until the tx off/drop... > > Any of you actually running the RB-1 board with a true reverse > burst type ctcss decoder (built into your radio)? Is a true > reverse burst decoder in your commercial radio completely "fooled" > by the phase inverted ctcss before carrier drop function. > > Or do you actually still hear some minor difference from the > rb-1 type operation vs an original true Motorhead (Motorola) > encoder - decoder operation? > > Thinking out-loud about having to possibly mute the ctcss at some > time after invert and before the delayed ptt drop as a requirement > to get the full/true reverse burst quiet squelch close. > > Any of you been down that road already? > > skipp > Skipp, I find your comments interesting in that the purpose that Motorola had in using "reverse burst" of the PL tone was to quickly damp the mechanical reed in the PL decoder to close the squelch and eliminate the user from hearing the noise burst. But of course you knew that. However, in later model radios there is no mechanical vibrating reed to abruptly dampen and stop the vibrating from being detected. So where is the need for a inverted burst if there are no receivers using mechanical reeds as PL tone decoders? Incidently Motorola did not use an inverted reverse burst of 180 degrees. Their designs used 270 degrees since the PL reed then stopped vibrating faster and the amplitude of the burst was also increased to hasten the reed to stop. Don't modern day receivers use electronic circuitry to detect PL tones, and aren't the detectors not using a ringing decoder? If so isn't the purpose of having a "reverse burst" unnecessary? I can remember many years ago that some hams used a circuit which they refered to as "polish PL" which turned off the PL tone before the xmtr dropped and had no "reverse burst". It seems like I'm hearing more of the same. Where am I going wrong here? Allan Crites ________________________________________________________________________ Check Out the new free AIM(R) Mail -- 2 GB of storage and industry-leading spam and email virus protection.
