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Swing Kit Vs. NPC (ept3600)

Discussion in 'CB Radio Modifications' started by LeapFrog, Jan 14, 2017.

  1. Mustang 131

    Mustang 131 Well-Known Member

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    Tall man, love reading this shit. I don't have time to transmit much less experiment. So I'll have to take your word.


     
    Tallman likes this.

  2. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    I rarely quote anybody.....

     
  3. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    ...It can explain those uniquely classified injuries caused by falling out of bed...

    :)
     
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  4. Tallman

    Tallman W9WDX Amateur Radio Member, KW4YJ EXTRA class

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    Did you?
     
  5. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    No, but it did cut off Father Fish'es sermon that night...
     
  6. Shockwave

    Shockwave Sr. Member

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    I have to say this is exactly what I've seen on most of the "NPC" CB mods. Although, I've been able to see it on the scope because many rigs keep the carrier held at the higher point after modulation, just long enough to see what it's actually doing once you know what to look for.

    Real NPC as used in commercial AM stations requires fairly aggressive clipping of the negative peak, followed by filtering. It never changes the carrier level to prevent cutoff. It is also true that many AM receivers with simple detectors will start to distort when the signal reaches more than 150% positive peaks. But even 125% positive peaks are a bit louder to the ear than a symmetrical AM signal.

    I would say the bigger concern is correct use of the regular AMC circuit in the radio and resisting the temptation to completely disable it. Back when I had a customer that wouldn't accept anything less than a fully clipped modulation limiter, I'd simply increase the range of the AMC by changing resistors so that it would activate just before the modulator started showing signs of distortion.

    Yeah, it still hit negative peak cutoff but it's a big improvement over the complete mud you get when the modulator itself is flat topping on every audio peak and the words start running together. If you can't get them to reduce the modulation to where it's clean on the scope, you can still reduce distortion and improve the sound quality without shaving enough of them "swing watts" off that they will complain.

    There is a point where you just start over driving everything from the mic pre-amp through the final stage. No more power is created, only increased distortion. You want to prevent that condition from happening. The customer might even notice that the little bit of compression this gives, does increase talk power by allowing any extra gain to bring the average modulation level up before distortion becomes objectionable.
     
    #36 Shockwave, Mar 25, 2019
    Last edited: Mar 25, 2019

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