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A Mushy Sounding AM Radio Station Mystery - How Can I Solve It?

E

everydayguy

The mom and pop company I work for is looking into leasing or buying a nearby small market AM. I have some limited engineering experience in FM but have never tackled an AM station.

The owners think there's room for a niche music format on the AM.

When inspecting the facilities, eveything seemed fine in the audio chain. Yet when we listened to it on several different radios, the audio sounded lifeless and dull with the music sounding blurred, unable to hear distinct instruments. Like it was broadcasting through a wet sock my boss said.

The transmitter is a fairly new BE and they are using an older model Orban AM processor. They are using a Marti STL. The TX facilities were fairly new and the radials were only a few years old and intact. It appeared that good engineering practices had been in place, that is, it certainly wasn't a "dog".

I know that many people advise against music on AM, but a couple of nearby AM station are playing a music format and they sound lively and clear -- at least for AM.

So my question is -- where to begin to figure out why the station sounds muddy and dull.

Thanks in advance.
 
Lots of possible problem areas from the TX through to the antenna. Also, if any portion of the plant is Stereo perhaps you have a phase reversal somewhere before the audio is summed.

Listen to the audio where it goes into the TX. That will tell you if the problem is the studio (hopefully) or a problem with the TX/Antenna combination.

Best of luck.
 
If the Marti STL is discrete mono, there is usually pre-emphasis applied to the audio at the STL transmitter, and de-emphasis applied at the STL receiver. Check to make sure the pre-emphasis didn't get disabled at the STL transmitter. Sometimes the pre-emphasis is applied in the STL-protection processing in front of the STL...somebody could have accidentally disabled it there.
 
Start with the program source material.
Then take a CD player (and I don't mean an mp3 player) to the transmitter and feed it directly in to the processor.
 
ORG is right...start at the xmitter and back up thru the chain....you will find the answer shortly and know exactly what the problem is...Tnx...JBI
 
How does it sound on the station's modulation monitor in the studio? Switching from program to air should sound fairly close with the main difference being no high end above 10khz, but there should be lots of clean high end up to 10khz.
 
Some models of the STL-10 Marti's had switches setting pre-emphasis (transmitter) and de-emphasis (receiver). If the settings match--even if there is no pre-emphasis in the system but the path is clean--should work fine. But if there is a mismatch in those settings, for example, 25 usec pre-emphasis at the transmit end and 75 usec de-emphasis at the receiver--it could sound just like you describe.
 
The Optimod may not have the 10kHz NRSC filter installed.
Someone may have installed the 5kHz filter.
Another thought. With four towers, it could easily be a system bandwidth problem.
Put the transmitter on a dummy antenna and listen to it.
 
Thanks to everyone on the board who offered some great suggestions about what to look for at the "mushy-sounding" AM.

Listening to the signal again this morning, music and voice have too much bass, not much mid-range and no higher frequencies. It doesn't "sparkle" like a well engineered AM can despite its limitations. The signal is fine..it's the sound that's terrible. Looks like my work is cut out for me.
 
optimod needs a refurb.another idea,call scms and get a free 30 day demo of the dsp-x mini am processor($1400).One great way of finding out if processing is the issue..Bill Sacks can refurb the orban. a college station here runs a WORN out 8100a,sounds like crap.they have no money,you know the story...
 
Which pre-emphasis module does the 9100 have installed? There were numerous different ones offered, each identified by a different color. I forget which color is which, but you can look it up in the manual (can be downloaded from ftp.orban.com). The two most common were the NRSC curve (rising boost up to +10 dB at 10 kHz) and the next one above that (a quicker rise up to +10 dB at 5 kHz, then staying flat at +10 dB up to 10 kHz) which is essentially NRSC plus an extra mid-range boost.

Also if you're playing music from stereo sources, make sure you don't have phase cancellation when the two channels are combined into mono. Try to balance the left and right channels as exactly as possible through the audio chain until it gets to the point where they are combined.
 
Good advice above. I'd take a set of headphons and a set of clipcords and listen at the console output, Marti input, Marti output, proc amp input, proc amp output, mod monitor output. At one point it will go to junk, the box before that point is suspect. Often, it's as simple as level control )the Martinizer is famous for coming apart if it's overdriven).
Luck.-
 
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