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FM squelch tail removal

Discussion in 'Microphones, Speakers & Audio Processing' started by K7JBL, Jul 10, 2019.

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  1. G0GSR

    G0GSR Ham Member QRZ Page

    Yes, there is always a price to pay.
    But in this digital world that we are forced to live in, nothing happens in real time apart from getting older :)
    Even the radio time signal pips have stopped due to delays in satellite distribution and DAB coding/decoding.

    Frank
     
  2. G3YRO

    G3YRO Ham Member QRZ Page

    No they haven't ! They still have them every hour on BBC Radio 4.

    But ever since the lines to the FM Transmitters were no longer Analgue, it's impossible to monitor yourself while broadcasting off-air (which we ALWAYS used to do).

    Will there ever be a time when D/A Converters introduce a negligable time delay?

    Roger G3YRO
     
  3. K7JEM

    K7JEM Ham Member QRZ Page

    The G.711 encoding and decoding used in typical T1 and E1 systems is very fast. We use it on some of our linked amateur systems out in my area, and you really can't detect a time delay, it is probably actually a few milliseconds, but impossible to hear, at least over one or two link hops. A millisecond of time equates to sound traveling about a foot through the air, so if you introduce 4 ms of delay, it is like listening to sound coming out of a speaker 4 feet away from you. IOW, not much delay.
     
  4. G0GSR

    G0GSR Ham Member QRZ Page

    You are so right Roger. I thought they had been removed :(
    But I just checked and DAB is a whole 2 seconds slow!

    Delays are very short over telecoms networks, its the satellite distribution and lossy audio CODECs that add to the delay.
    Now back to wonderful FM...

    Frank
     
  5. K7JBL

    K7JBL XML Subscriber QRZ Page

    Here's the result while listening to a couple of guys on 2 meter FM. I was streaming 146.520 to my YouTube channel. The top waveform (from my FT-991) clearly shows the squelch tail. The bottom trace is the output of the noise gate and you can see the tail is gone. I was playing around with different threshold and delay setting so the audio might drop out once or twice.
    73
    Phil
    K7JBL

     
  6. AI3V

    AI3V Ham Member QRZ Page

    In the commercial world "squelch tail elimination" is done by taking the sub audible tone, and at the instant the microphone unkeys, reverse the phase of the sub audible tone.

    The transmitter stays on for a short period of time

    The receiver senses the phase reversal and mutes the audio before the transmitter stops.


    http://www.repeater-builder.com/micor/reverseburst.html

    Rege
     
  7. ND5Y

    ND5Y Ham Member QRZ Page

    Why does YaeComWood refuse to include reverse burst in their amateur products?
     
  8. W7UUU

    W7UUU QRZ Lifetime Member #133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    Very clever!! I've used Reaper for many years for my home studios - best DAW out there IMO.

    Really cool adaptation for radio purposes.

    Dave
    W7UUU
     
    K7JBL likes this.
  9. K7JEM

    K7JEM Ham Member QRZ Page

    It would be easy to do, but there might be some patents or something that they have to mind.

    Baofeng does an interesting thing for squelch tail elimination. Instead of doing a "reverse burst", they send a few hundred milliseconds of "PL" that is below the standard frequencies, something like 55Hz or so. When the RX station sees this, it ceases to recognize the tone as valid, and immediately mutes the RX. It has good compatibility with most brands of CTCSS, we have some Kenwood repeaters on our system, as well as a Tait repeater, and it squelches both of those very fast.

    But these methods don't work on carrier squelch. The best squelch I have seen for FM noise squelch is the Micor dual level squelch system (or equivalent), which has a "normal" squelch burst for signals less than 25dB quieting (or so), and a very short burst for better quieted signals. The squelch burst on a well quieted transmission is probably 5ms or so, the longer squelch burst is around 120ms. The 5ms sounds just like "click".
     
  10. ND5Y

    ND5Y Ham Member QRZ Page

    On Baofeng and Wouxun the 55 Hz STE tone works in CSQ (CTCSS/DCS encode and decode both set to OFF) and you can't disable the receiver from detecting the tone and muting.
    I don't know how the receivers in other CCR brands handle it.

    AnyTone programming software allows you to set the squelch tail elimination tone to 55.2 or 259.2 Hz. Baofeng only has ON/OFF and and doesn't say what the tone frequency is but it appears to be 55 Hz. Wouxun has no setting, it send the STE tone all the time. Maybe 55.2 is a Chinese standard.

    On both Baofeng and Wouxun the STE tone turns into the standard 134.4 Hz DCS turnoff code if the radio is set to encode DCS.

    A cool trick you could play on Baofeng Beeotches and Wouxun Whackers would be to set your repeater to transmit a custom 55.2 Hz CTCSS tone all the time. That would keep Baofeng and Wouxun receivers muted the whole time the repeater is transmitting. The CCR users would not be able to hear anything unless they keep the monitor button pressed all the time. People with real radios would not have a problem.
     
    Last edited: Jul 14, 2019

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