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Very simple Audio compressor for FM transmitter

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Willen, Dec 6, 2013.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The smart (?) designers of the digital telephone (then later video) conferencing product used the Macintosh operating system which caused a nightmare when used with operator consoles using Windows.
     
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Here is a microphone amp with AGC.
    There is a 12db preamp. The output amp has a gain 8,18 or 28.
    The AGC has a gain range of 0 to 20db.
    You can set the speed to fast to slow.

    Fun reading.
     

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  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The Maxim mic preamp with AGC IC will allow you to scream, yell or cheer without distortion.
    But I would not use it with music if its gain is too high and its AGC is actively pumping the background music level up and down all the time.
     
  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I have not used the MAX part. I have used all the others.
    I should determine how to put a meter on it. If I remember 84 posts back this is for a small transmitter. Any limiting is better than none.
    When I set up compressors/limiters for radio/TV I would set the levels so that during the loudest part of music the meter showed 1 to 3db of gain reduction. During live "cheering and screaming" I hoped for less than 10db. The idea is to not work the limiter hard but to have it just on the edge of working. Yes pull back the screaming but not effect the normal audio.

    For HAM radio I would probably hit the compressor hard because we want more RMS power.
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Then why do all the FM radio stations have the gain at maximum so their compressor/limiter causes background music levels to jump up and down with the bass beat, and each bass beat hits hard then quickly fades away?
    Frequently two FM radio stations play the same music at the same time. I can hear small differences but their levels are always the same: too high.

    Cheap radios have poor selectivity so the regulations space radio station frequencies fairly far apart to avoid interference. The poor selectivity in a radio allows it to produce over-modulated FM loud and clear.
    A weak little FM transmitter would not cause interference when it is over-modulated because the capture ratio on an FM radio passes only the strongest signal. Then the weak little transmitter does not need AGC.
     
  7. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Because the boss uses his car radio to compare the loudness of the competition to his station. The noise in a car is so high that the noise level of the radio can not be heard. But a 1db louder signal causes a very loud phone call to the engineer. (ask me how I know)
    In the US the FCC watches for ever modulation. The transmitters I used had hard clippers so they did not over modulate. If there was not a limiter/compressor the hard clipper stepped in and clipped the audio. (distortion) The limiter should turn down the gain so the clipper did not get hit hard.
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I found a good example of a song that uses way too much compression. It is from an English group called Disclosure:
    Following 1 minute and 20 seconds the singer's voice and background music pumps up and down like crazy by the bass beat.

    So maybe it was recorded like that? Or you Tube and radio stations over-compress it?
    The girl in the video cannot choose her boyfriend or her girl friend.
     
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  9. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Sounds bad at 1:20.
    I found several other copies and they all sound that way at 1:20.
    Maybe they want it that way.
     
  10. Willen

    Willen Well-Known Member

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    What could be the reason about exact 1:20? Seems interesting? Do these all generated/converted through same machine/software?
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    They want it to sound that way because most people hear over-compressed music on the radio and think "it is normal to sound like that".
     
  12. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I don't know. I was looking for non u-tube examples. It could be that there is only one recording and it spread across the internet.
    Does anyone have this on CD and we can compare. It is real likely that some 7-year old sent this to u-tube.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The song heard on many FM radio stations sounds exactly like it does on You Tube. Lots of level pumping.
    It seems that the background level reductions are delayed a little from the bass beat or maybe the compressor has a slow release time.

    This other song also has a strong drum/bass beat but it does not sound over-compressed on You Tube nor on FM radio stations:
    www.youtube.com/watch?v=ij_0p_6qTss
     
  14. canadaelk

    canadaelk Active Member

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    You guys veered way of topic! The problems you are talking about is operator incompetence (RTFM), not equipment shortcomings. A properly adjusted (even full audio-band compressor) will do just fine in most situations! E
     
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  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Maybe radio stations do not have a sound man anymore. They simply turn up the volume and let the compressor pump the level down to legal full blast.
     
  16. Willen

    Willen Well-Known Member

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    Hi all,
    Once in the past with the thread, I searched many JFETs, MOSFETs alternatives to make a simple AGC (audio limitter/AGC). Later I got a nice gift (better chips) by canadaelk. Later I got whole professional set of limitter/compressor too. I am VERY VERY much thankful to 'canadaelk'!

    Though, I am still interested to simpler AGC to experiment. Along with the thread we searched/tried to fix a limitter with easy 'available' JFET or MOSFET like 2N7000 but it was incomplete. Today I read the tiny e-book and it's using 2N7000. It looks pretty simple and looks nice!
    What about it for 'general' purpose like to play songs through toy FM Tx or recording voices?
     

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  17. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    From what I built; I like the MOSFET versions better than JFET. Mostly because the MOSFET is linear at a higher signal level.
    My opinion; Any limiting is better than not limiting. (ahead of a transmitter)
    Too much limiting caused a loud distorted noise from Canada. (AudioGuru) LOL
     
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  18. Willen

    Willen Well-Known Member

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    Maybe poor one is better (at least for experiment) than nothing. Here I attached a LTSpice simulation drawing for expert to test the circuit in precise way. Put command and C8 and R9 as your wish, inject input signal and do not forget to change 2N7002 into 2N7000. I didn't get 2N7000 model in LTSpice so just used 2N7002:
     

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    Last edited: Nov 7, 2015
  19. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,

    They also make a little component that is made just for audio purposes. It is a small package with a single small LED and photo resistor in a light tight package.
    When the LED has current flow, it shines on the photo resistor and the photo resistor decreases in resistance. The two leads for the photo resistor come out one side and the LED leads come out the other side of the package. The two resistor leads are used for the resistor to set the gain for the amplifier. This creates either a compressor or an expander depending on how you condition the audio signal to drive the LED. The resistor is of course linear for the most part and works with bipolar signal levels.
     
  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A photo-resistor is too slow to control the gain of a high quality audio circuit such as a compressor. But it is OK for a telephone.
     
  21. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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