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| | |-+  Audio compression/limiter
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Author Topic: Audio compression/limiter  (Read 8818 times)
jerry814
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« on: February 11, 2011, 08:13:42 PM »

Any of our members use an audio compression/limiter for the audio feeds? As Airband is double-sideband AM, received volume is directly related to received signal strength.  Received volume is all over the place. Anyone tried a compressor?  They have their own issues, so I am not going to go there without some advice.

A simple design would be nice, but if it has issues, then might have to look further.  Any assistance woud be appreciated.

Jerry814
« Last Edit: February 12, 2011, 05:36:08 AM by jerry814 » Logged
captkel
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« Reply #1 on: February 12, 2011, 06:45:21 AM »

I use a Behringer compressor model: BMDX2600X-P, and a Alesis Equalizer: model M-EQ-230 on two of my feeds. One of my feed radios has a hum...yes it's the radio. The noise gate on the compressor eliminates this when there is no audio from the radio. Eventually I will purchase a 4 channel compressor, I have 3 feeds at the moment. The one I have been thinking of getting is made by Samson: model SCOMPLUSX-P. It looks like it's a little cheaper since I last looked. shocked
well now...Sam Ash is about 3 miles from my home, may just have to go get it grin grin grin

KEL

I don't see the Alesis Equalizer on there web site anymore however they have others or you can go elsewhere. 

http://www.samash.com/p/Behringer_Composer%20Pro%20XL%20MDX2600%20Compressor%20with%20Expander-GateLimiter_-49983308



http://www.samash.com/p/Samson_Scom%20Plus%20Stereo%20Compressor%20Limiter_-49990240

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jerry814
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« Reply #2 on: February 12, 2011, 07:10:48 AM »

Thanks for the reply.  I will look into this. Jerry814
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jerry814
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« Reply #3 on: February 14, 2011, 05:40:58 PM »

Anyone have experience with a compressor/limiter mnanufactured by RDL?

We are using their products for EMS/Fire Station alerting audio systems.  Don't have any personal experience with these products.. Do they work ok?  I know they are used for FM Public Safety communitions.  How do they work for Airband Double-sindband AM communications?  Any real-world experience would be very welcome!  FM is easy, AM is not!. Thanks,  Jerry814
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captkel
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« Reply #4 on: February 21, 2011, 01:02:37 PM »

I purchased the Samson compressor and it's up and running on my 3 feeds.
I'm still fiddling around with it. Tell me what you think of the audio quality.
Note: I'm pretty far away from the ground stations so signal strength will vary.

KEL grin
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jerry814
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« Reply #5 on: February 25, 2011, 04:47:22 PM »

I have both problems.  You are far away and have low volume at times. So do I.  I live exactly 8 miles from the antenna farm at KCVG.  Aircraft on the ground and the controllers sound good.  If the daily Polar Air Cargo Freighter 747-400 flys right over my house at 0530 est, enroute to Alaska, they just blow the audio out of limits. Thank you double-sideband AM!  So I get audio from aircraft 15 miles south of KCVG, and audio from a PAC 747 1 mile from my antenna.

This really needs a decent compressor, but compressors have their issues (audio pumping we call it).  I have little experience with these things.

Logic says the compressor should have a built-in delay, so it can level the audio before presenting it to the output connection.  And should be DSP.  These are commercial devices, and cost hundreds of dollars.  Anyone recommend an inexpensive processor?

Jerry814
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captkel
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« Reply #6 on: February 25, 2011, 09:47:15 PM »

Thanks for replying: The ones made for commercial FM radio stations sending the digital signal (HD Radio) use exactly what you are talking about...there Look Ahead compressors and they do cost a lot of $. Thousands of $ if I remember correctly. Good luck. The Samson I bought has 4 channels which is good for me; having 3 feeds. The Behringer compressor I have I liked better as far as adjusting options goes. The Samson attack and release times are automatic, I would rather have control. Oh well...still playing with it and the compressor too grin

KEL 
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AB9IL
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« Reply #7 on: May 20, 2011, 11:35:49 AM »

It is unfortunate, but many receivers have poor AGC, which would otherwise even out much of the audio.  Most aircraft tend to run fairly high percentages of modulation, so if the carriers are the same strength, the audio would be reasonably consistent.  For that the RF levels must be high enough to activate the receiver AGC circuit.

Can you work with your feedlines and antennas to increase the raw signal strengths?  That would help, then some compression followed by a limiter would even things out for the listeners:

RECEIVER -->COMPRESSOR-->LIMITER-->SOUNDCARD-->COMPUTER-->INTERNET

Real audio can be pretty bad sometimes.  I do a lot of flying between China and Singapore, and some ATC transmitters have a low modulation percentage, while others are higher.  The Chinese tend to be low tech, and use narrow audio filters and heavy clipping on their transmitters.  So one time you turn your volume up to hear ATC, then get blasted by strong aircraft on the frequency.  Then you get a hand off to another facility where the engineers set the percentage pretty high, but clip the hell out of the audio...

So if you do a bit of processing on the receive side and signals are still not totally consistent, you're not doing bad - radio is tough to tame sometimes.  As long as you don't overdrive your soundcard, things should work out alright.  Good luck!
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Scotty
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« Reply #8 on: July 04, 2011, 03:37:47 PM »

For the Las Vegas feed, I'm using Aphex digital processing going into DBX 160A compressor limiters.  Very happy with the end audio quality.

Scotty
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