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  1. #1

    I have questions regarding FM broadcast audio chain Plz help

    Hello all.

    I have a LPFM broadcast station here in the USA.

    The music we have consists of MP3s, cassette tape, CDs and Vinyl records.

    First and foremost the major problem with this station is the audio levels either being too loud or too soft.

    This is more noticeable with the MP3 collection which is played using a computer and various automation software we tried. We have so far tried Zara Radio, RadioDJ and SAM broadcaster.

    From the computer's sound card the audio is sent to a (Radio Shack) Realistic 32-1101A Stereo mixer. Now from there, many pieces of audio processing equipment has been tried. The goal is to stop the MP3s from over driving the transmitter with peaks and under modulating the transmitter with valleys.

    We purchased a Behringer Autocom Pro-XL MDX-1600 processor in hopes of stopping the MP3s from over driving the transmitter.

    At times, the music sounds like the BASS is pumping or has the effect of as if the cone of a woofer speaker is damaged and has excess vibration/distortion. This is more common in Hip-Hop music.

    We tried adding a 5 band EQ into the Autocom's side chain trying to regulate the lows from pumping and add more mid range to bring the voice up out of the noise. Trouble is, the more we raised 1,000 Hz above zero, the more distortion we added to the singer's voice. Any words that had the SH sound to them sounded distorted while those parts of a word were said.

    The Autocom has a DE-ESSER. However, when ever it is activated, the SH words sound 100 times worse!

    Now the output of the Autocom goes directly to our FM transmitter.

    We recently purchased a Technical/Pro EQ7153 Duel 21 Band Equalizer and we tried adding it to the Autocom's side chain in place of the Realistic 5 band one. We ended up with more problems with one song playing okay while the next drove our transmitter into over modulation and distortion conditions to the point that the FM stereo pilot lights on our FM radio receivers were blinking on and off.

    The major problem with our station is that we are a non-profit self supporting one and can not afford a technician to advise us on how to properly set this system up.

    The controls on the autocom are confusing and getting any help from the behringer form board has proven to be a waste of time, no one has been of any reasonable help explaining what the 21 controls on the autocom pro-xl does and which ones should be active and which ones shouldn't be.

    We tried and tried again to make adjustments to that autocom and nothing seems to be effective against this over modulation and under modulation issue we are plagued with.

    I am here hoping someone can offer us some sound advice on how to get this issue under control. I have to ask, is it better to have an EQ in the autocom's side chain or should the EQ be tied in at the autocom's input or output instead?

    We have another transmitter on order which is supposed to have a built in automatic modulation level control at its input stage, the current FM transmitter we are using does not have a built in automatic modulation level control. I am hoping the new transmitter helps us gain more control.

    Can anyone who is reading this offer some inexpensive solutions or advice on how we can get this set up to sound good?

    Also the Realistic 32-1101a mixer does not have any audio level adjusting controls on the output jacks. Because of this, we can not use any audio levels that make the VU meters move very much past a slight fluctuation of their resting points. If we introduce enough audio from the computers sound card to make the 32-1101a's two meters reach the red 0% line the result is the autocom is being over driven to complete over saturation levels.

    I am curious if adding two potentiometers to the mixers outputs would help eliminate this and what type of potentiometer we should use. A Linear or Taper type and what Ohms works best for line type audio circuits?
    Sadly we can not locate a schematic diagram for this Realistic Mixer. This link is the only one we could locate for this mixer. Mixers&Reuse=N

    Any help you can provide us with would be much appreciated.


  2. #2
    Senior Member SC358's Avatar
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    May 2005
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    Hi Bruce,

    Actually, I believe the answer is simple but you're not going to like it (maybe)...

    Since the problems are more noticeable with MP3, I will address that.

    First and foremost, MP3 is not broadcast quality. I will not take the effort to explain to you what MP3 is all about, I'm sure you already know.

    Cut your loses and get rid of the MP3 collections because they are not made equally to begin with. Although it is a standard format, there is no standard for listening since the algorithms are
    preselected and automatic. So while you try to effectively fix one bad area, it does not work in another.

    Second and second most, your company who does not and cannot afford an engineer should look to invest in another business...
    I'm sorry, that's more of a personal opinion rather than tech advise, but you can't have a business and not have the right people on board - it only spells failure to me.
    You're company has invested too much as it is because of this issue.

    Best of luck.

    Relationships are based on compromises - behavior accepted is behavior repeated.

  3. #3
    What SC358 said, pretty much. The big boys have very sophisticated equipment that lets them run really close to the limit without going over, and they can heavily process audio without completely wrecking it. And they don't use MP3s as source material.

    But let's try to improve things. Doesn't your FM transmitter have an audio level control? You do need to knock down the output of your processor, but it's hard to say how to do that without more information about the transmitter. Getting a transmitter with a built-in limiter will improve things. Anybody who is on the air needs a limiting system that simply will not permit the transmitter to be overdriven under any conditions.

    When it sounds like the bass is pumping, that's because it is. Cuts that have extreme bass will cause the limiter to temporarily turn down the gain on every beat, and everything else will get turned down along with it. If every drumbeat punches a hole in the vocals - well, you can tune across the FM band and hear lots of stations doing that.

    Most stations nowadays don't take music directly off records, tapes or digital files. They copy these sources onto their hard drives, and perform whatever processing they need to as they do it. That way, when it comes time to put them on the air, they're all pre-adjusted for the right volume etc.

    So what transmitter are you using? What transmitter are you switching to?

  4. #4

  5. #5
    Hello, sorry for the delay, last time I looked at this topic, there was no responses.

    Since my last post we replaced the radio shack mixer with the Behringer VMX 1000.

    We used XLR balanced connectors on the output and we replaced the MDX1600 with the ULTRA-DYNE PRO DSP9024.

    Ultra-High Performance Digital Stereo Mainframe Powered by Two 24-Bit High-Speed Signal Processors.

    We are currently learning how to set this thing up properly.

    Now, we choose to place the EQ7153 Dual 21 band EQ in between the mixer and the DSP9024 to have more control over the audio spectrum.

    The original transmitter was a Ramsey FM100 (Not to be confused with the currently available FM100B). The new transmitter is the CZE-T501 which has adjustable audio input and auto modulation. Although the auto modulation feature does not seem to respond fast enough, so the audio exhibits a noticeable annoying fluctuation. It does have 75us Pre-emphasis and is well filtered and is adjusted to provide us with a part 15 compliance of 245 microvolts of the allowable 250 microvolts/meter at 3 meters of the FCC part 15.239 Rules. Just wanted to clear that legal matter up.

    We have audio editing software and have tried using MP3 gain to level everything to 89.0.

    I tend to realize that the sound card of the computer may not be the best piece of hardware, I believe it's an Aureal 8820 Audio card.

    If we converted the audio files to a different format, what would be the end result in converting those MP3s to another format? I tend to believe, if you start with a compressed format, there's really no way to recover the originality of the music.

    I am not expecting good responses, regarding our upgrade choices, but this is a learning curve for us.



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