brian@sdcsvax.UUCP (Brian Kantor) (07/04/85)
In article <86@biomed.UUCP> lagasse@biomed.UUCP (Robert C. Lagasse) writes: >the question: When the PL switch is off, the receiver audio is muted >by the normal squelch circuit which allows you to hear receiver hiss for >about half a second after the repeater shuts down, then all is quiet. When >PL is on, receiver audio is heard, but right after they finish speaking, the >reciever is muted and no squelch tail is heard. Are they or are they NOT >shutting off the transmitted PL tone BEFORE thaey allow the repeater >transmitter to shut down? This is what it sounds like. True sub-audible PL as done by Motorola uses a technique called ``reverse burst'', in which the transmitter emits PL for 120 ms after you release the microphone, but 180 degrees out of phase with that which was sent during the main part of the transmission. Its frequently louder too. The idea is to cancel the vibration in the mechanical decoder reed at the receiving station, so that the tone squelch will shut before the received carrier goes away, thus eliminating the squelch tail. (Note to ham radio newcomers: the ``squelch tail'' is the burst of white noise heard as an FM receiver squelch delays closing for a short period of time. It is NOT (as I have heard ignorant people say on local repeaters) the delayed drop-off time of the repeater. It is perfectly possible (and common) to have a squelch tail when there is no repeater involved at all. More recent Pl systems use a quick burst of digital code sent at the beginning and end of each transmission. This DPL can address lots of different stations both by group and individually, so its more flexible. By combining it with dual-rate squelches (close fast on strong signals, more slowly on weak signals that may be fluttering), even if the trailing code burst is lost, the majority of squelch tails will be eliminated. >Next question: Why does store bought 2m ham gear usually try to sell itself >with a PL tone encoder ONLY? Most rice-rockets can have a decoder added to them, but since the great majority of them have really shitty low-frequency audio response (small speakers, little coupling caps, etc), most repeater operators choose not to repeat PL tones through their repeaters. It is possible to do so, by either of two methods - a tone coupler will repeat PL tones through (under the squelch, so that reverse-burst will continue to work), or by having an encoder on the repeater transmitter. So although its possible to have a decoder, since most people won't have one, and the tones will really distort audio in their receivers, repeaters generally don't emit tone. Tone bursts in the audible range, DPL, touch-tones, etc as selective station calling devices have been used in ham radio off and on for years, and I have not heard of their legality as such being questioned, but they are unpopular for a number of reasons: They are annoying as hell to listen to, and the general air of cliquishness they generate usually pisses off enough people that they record and play back the tones to annoy the members of the clique. (My, aren't repeaters FUN?) Better to move to another band and put up a private system. >Last question: on commercial FM gear, is the PL tone transmitted by a >mobile set filtered out before the signal is retransmitted by the repeater??? See above. >N 1 A L G Bobby L @ MGH PL=Private Line, QC=Quiet Channel, CG=Channel Guard, and they're all trademarks of Motorola, RCA, GE. Touch-tone belongs to some part of what is left of the Bell System. Brian Kantor WB6CYT, repeater baron extraordinaire (ret.) decvax\ brian@ucsd.arpa akgua >--- sdcsvax --- brian ucbvax/ Kantor@Nosc "You know, I really like my RCA walkie - its the only radio I've ever owned that withstands pounding nails with it and keeps on ticking...."
ron@brl-tgr.ARPA (Ron Natalie <ron>) (07/08/85)
> (Note to ham radio newcomers: the ``squelch tail'' is the burst of white > noise heard as an FM receiver squelch delays closing for a short period > of time. It is NOT (as I have heard ignorant people say on local > repeaters) the delayed drop-off time of the repeater. It is perfectly > possible (and common) to have a squelch tail when there is no repeater > involved at all. > On a repeater however, you hear the squelch tail from the repeaters receiver being retransmitted. If you have remote receive sites, the problem is compounded. You can get around this by delaying the repeated audio by enough time that when the COS from the reciever dies, the squelch tail in the delay line is dumped. Of course, there isn't a whole lot you can due about the squelch tail in the user's receiver. -Ron
rpw3@redwood.UUCP (Rob Warnock) (07/12/85)
+--------------- | > (Note to ham radio newcomers: the ``squelch tail'' is the burst of white | > noise heard as an FM receiver squelch delays closing for a short period... | ... Of course, there isn't a | whole lot you can due about the squelch tail in the user's receiver. +--------------- Actually, with some fancy coupling between the squelch and AGC circuits, it should be possible to eliminate most or all of the "squelch tail". The squelch tail occurs because the AGC gain opens up faster than the squelch closes. By using a longer time-constant on AGC decay than on rise, and by tying the time-constant to whether the squelch is open or closed (you want a long decay if squelch is open), the "tail" will be heard as QUIETER than the transmission, not louder. Of course, this means that the squelch cannot be controlled simply by the AGC voltage (as is often the case in simpler designs), or you'll get "motorboating" oscillations in the squelch. A possible algorithm is to use AGC voltage to OPEN the squelch (and switch the AGC time-constant to "slow"), but then use some ratio of actual input R.F. to "estimated" input (i.e. the AGC voltage) to trigger the CLOSING of the squelch. Careful design is needed in mobile rigs (or anywhere fast fading can occur) to make this work, and of course impulse noise has to be treated carefully as well. Note that the "slow" AGC decay (with squelch open) need not be "slow" by human standards, only "slow" compared to the time-constant of the squelch-closing detector. Note that a squelch tail will still exist -- it just won't be so annoying. p.s. Hmmm... maybe the problem is not the AGC, but the squelch open/close detector. Mayebe all you need to do is make the squelch OPEN slow (compared to impulse-noise rejection time) and CLOSE fast (compared to AGC recovery time). That would get rid of the "tail" entirely. Rob Warnock Systems Architecture Consultant UUCP: {ihnp4,ucbvax!dual}!fortune!redwood!rpw3 DDD: (415)572-2607 USPS: 510 Trinidad Lane, Foster City, CA 94404 require
karn@petrus.UUCP (Phil R. Karn) (07/15/85)
> Actually, with some fancy coupling between the squelch and AGC circuits, > it should be possible to eliminate most or all of the "squelch tail". > The squelch tail occurs because the AGC gain opens up faster than the > squelch closes. You're forgetting that FM is a nonlinear modulation method. An ideal discriminator reads signal PHASE and is completely insensitive to RF level. Moreover, there is limiting ahead of the discriminator, so the "gain" in an FM receiver up to this point depends instantaneously on the signal level. When the input signal to an "ideal" FM receiver goes away, the pre-discriminator gain instantaneously goes to infinity. (In a real receiver, it goes just high enough for front-end thermal noise to saturate the limiters). Thus, you have squelch noise that starts immediately upon loss of signal. The AGC is there just as an engineering convenience to keep the front-end intermod products down when receiving a very strong signal. It plays no part in keeping the audio output level constant, as it does in linear modulation reception such as SSB. Phil