Audio Phase inversions on the FM broadcast chain


Audio phase inversions can cause big problems to your FM broadcast signal!

Audio phase inversions are a relatively common problem in our experience.  In fact only today we just noticed a station near us that has one. We strongly suggest all station managers do some checks to ensure they do not have an Audio phase inversion. They lead to two pretty major impacts on FM.

Impacts of Audio phase inversions

  • Stereo Listeners – not all ears are sensitive to it, but those that are hear a really weird stereo – a bit like ultra-wide stereo, but not in a good way. It can also make the stereo signal quieter.
  • Mono Listeners – the mono elements in your programming (eg vocals on songs, voiceovers on adverts) can get almost or completely cancelled out.  It sounds like you are playing the karaoke version of songs, and commercial music beds with no voiceover message!

To explain these effects You need to go back to wave theory that you did in Physics at secondary school – constructive and destructive interference etc.  The addition of FM broadcast reception with its constant switching and blending between stereo and mono as reception conditions change, can make this a confusing problem to pin-down.    But let’s not go there – just think about making some checks and getting it fixed if you have the problem!

Causes of Audio phase inversions

The most common cause is a mis-wired audio connector, although some equipment has phase invert buttons that can be set incorrectly.  So a unbalanced RCA type connector may have the audio signal and earth wires reversed, or a balanced XLR connector may have its two signal wires swapped.

Fixing Audio phase inversions

The proper way to fix the problem is to inspect every audio connector in the chain to your FM transmitter and fix the offending beast.  If they are all correct, then start checking eack box in the chain.  The lazy way is to take any audio connector in the chain at random and swap the wires over. (not recommended!)

So we suggest you find yourself a FM radio that you can manually switch between stereo and manual and check you do not have this problem – make it your new years resolution!  Alternatively if you do not have access to such a radio, drive to a suitable area on the fringe of your reception area where you can drive between stereo and mono reception locations.


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