Can someone explain why these things could be happening in an FM transmission?

[radiodeviation7]

Hello Everyone

I have noticed that some FM radio stations, particularly low-budget and poorly maintained radio stations always have at least one of the following problems:

1. A mains hum in the audio
2. A high-pitched ringing noise in the audio
3. No bass (not a processing issue)
4. Overly distorted and painful to listen to audio (when looked at in an audio editor, it is full of square waves)

Are these things a common occurrence with low quality equipment or are there specific technical issues causing this? I look forward to your answers!

 

[AlligatorClipz]

Good quality audio takes attention to detail.
Overworked, underpaid, and undertrained people do not have the time, motivation, or knowledge to rectify issues like this.

When listers are used to shitty mp3's out of cell phone speakers, does it really matter?

Apathy in the broadcast industry.
It's like the fall of Rome...

 

[Bongwater]

It comes down to the broadcaster. Metaphorically speaking, serious broadcasters pay attention to their air quality; A sparkling, but not dog confusing high-end. A nice, punchy bass. No humming outside of a Lumineers tune. And watch yer levels.

Others, you have to step over the skeletons of the last EPA inspectors to get to anybody who knows anything in those 100-100,000 watt smog factories these days. (And they KNOW who they are because even they're not listening, it's that bad.....)

 

[Mikey Radio]

There could be many causes for the things you're hearing, and there are lots of places in the audio chain where hum or noise and distortion can occur, or audio quality can be degraded. I think the key is in what was mentioned in the OP: "particularly low-budget and poorly maintained radio stations". When you're dealing with contract engineers who are covering many stations, they may not have time or the ability (or budget / permission from the owner) to do much more than come in and slap band-aids on reported problems and move along. Other times you get well-meaning owners who think they know a little about electronics who buy and use consumer-grade stuff in the signal chain which can sometimes be OK, but in many cases no.

Facilities that have been around for years and are running on a shoestring budget or worse, may have a whole host of older duct tape solutions in place to quickly fix or patch past problems, and have everything from shoddy electrical and wiring to stacks of equipment not mounted in racks to consumer-grade stuff using "wall wart" power supplies and converters to make it work, to transmission facilities that have been infiltrated by birds, rats and a host of other stuff. A few years back there was a discussion thread where people on this site described some of the worst studio and transmission facilities they'd visited, or in some cases had been called to work on. Some were shocking (no pun intended in some cases) to say the least.

 

[PTBoardOp93]

There are *so many* things that can cause these issues. Basically any device in the air chain could cause distortion or hum, for example.

The reason you observe these at small town stations is largely because small stations generally use contract engineers. That contractor probably lives outside the listening area of the station. The station owner or GM (often the same person) has to notice the problem, request the engineer investigate it, and budget the funds to fix it.

Just getting the contractor to come and investigate can be pricey. Most of the contractors I worked with charged something -- perhaps not full price -- while in their vehicle coming to our station and going home. Some had a minimum number of hours per visit, making each visit cost several hundred dollars. So there's a pretty strong incentive not to call the engineer for something the GM thinks is minor.

That also leads to unqualified people doing things they shouldn't do. I learned a lot of things not to do in a radio facility by being 18 years old and not knowing any better!

 

[secondchoice]

Another issue is the "modern" engineer has to deal with "rf and power" issues which can be caused by a digitial issue and purely digital proplems. It took me an hour to figure out a failing hardrive once. I thought it was a bad Microsoft update. I will say PCs have gotten more dependable the last 20 years and cheaper so you have almost every part on hand. Then there are the pure software errors. The pc automation systems can have software issues that can really take time to figure out. I would hate to try to do a lot of work without a laptop with an internet connection just to get copies of manuals and check for software updates.

You can buy quality equipment or two or three sets of junk. If you have a good contract engineer on a retainer, before you buy anything ask him what he thinks you should buy. Pay him to properly install it.

There are too many things that can go wrong to make a blanket statement why stations "sound crappy". However I can almost guarantee: limited or no service by a competent person will eventually make any station sound bad.

 

[pclover]

As people mention many things.

Tho, I figure the hum is often caused by a ground loop somewhere in the chain. It's a common problem with AV equipment as well and often people suggest a hacky and dangerous solution of removing the earth or ground connection. It may solve the problem but now the case is not grounded and somebody could be in for a nasty surprise should a failure result.

If you can't solve the ground loop problem you have to isolate it with a isolation transformer. I'm not how often these are actually used in equipment racks since they cost money.

 

[radiodeviation7]

Thank you everyone for your replies!

The distorted and unbearable audio can also be caused by bad capacitors in the exciter. A radio station here had a similar problem but it was fixed quickly. I'm still wondering what could be causing that no bass issue that I have encountered on some stations? I'm guessing a problem with the pre-emphasis circuitry in the transmitter?

 

[secondchoice]

If you can't solve the ground loop problem you have to isolate it with a isolation transformer. I'm not how often these are actually used in equipment racks since they cost money.

600 ohm isolation transformers are cheap. I've seen them on eBay for less than $3 each when you figure in shipping. Of course you have to buy several at a time to get that price. Is the problem constant? If so then make sure all AC outlets with equipment plugged in has a ground. I would check to make sure all plugins are properly polarized. If that doesn't fix the hum you could start at the first equipment with annolog output terminals. Turn it off and listen for the hum. Work your way thru the audio chain. Also check for anything with the old RCA cables these can get flaky easily, and if there is a patch panel check for loose connections.
If all else fails break out the ocilliscope.

Ground hum is inexcusable on any radio station.

 

[Kmagrill]

Lack of Bass could be several things, but it's not likely a pre-emphasis issue unless to pre-emphasis networks are accidently placed in series with each other. Such things can happen when the audio processing has a pre-emphasis in it and the transmitter also has pre-emphasis enabled. Such situations are pretty rare these days since most audio processing is an all in one box with left and right or AES input and composite output. Exceptions are when software audio processors are used to feed left and right inputs of a stereo generator that has pre-emphasis switched on. In that case, the software must be used without pre-emphasis.

A more likely cause for lack of bass is old equipment with deteriorating electrolytic capacitors. Most equipment needs to be re-capped once in a while in order to preserve its bass specifications. This can be anywhere from five to fifty years. A good rule of thumb is once every 15- 20 years. Dried out capacitors in the power supplies of older equipment can also be a source of hum. However, ground loops are a more likely cause of hum in most broadcast plants. This is especially common where balanced and unbalanced audio are mixed within a plant.

A note on isolation transformers: Not all are created equal. It depends on what the transformer was created to do. For example, a transformer created for matching to a telephone line will exhibit serious low frequency roll off when loaded by 600 Ohms. It may also have high frequency roll off at 600 Ohms, but be more forgiving if the load is higher. Be sure that the transformer selected is rated for 20Hz to 20kHz (or at least 50Hz to 15kHz) when loaded into 600 Ohms. The actual audio performance of the transformer will vary with the load, so be aware. Most likely, transformers of this spec will be more expensive, but they are worth it in the long run.

 

[secondchoice]

I saw a neat little box for around $30 that would do a stereo pair on Amazon or eBay last year.
I am surprised more low budget operations aren't using something like "Stereo Tool". PC's are cheap. You don't need a real radio technician until the transmitter. Chances are there is a local PC repair guy who won't build in windshield time if needed for service calls.