Talk:FM Broadcast Radio Frequency Deviation

loudness 1

First, how FM works! FM is Frequency Modulation, the frequency transmitted varies around the centre carrier frequency (e.g. 98 MHz) that you set on your radio — it would stay at exactly that frequency if the station transmitted silence — and it deviates proportional to the instantaneous value of the audio signal transmitted.

The stations want to appear louder , so the audio is processed, by compressing it's dynamic range (the range from quiet to full blast) which makes the quiet parts louder *without* increasing the level of the loud bits beyond the limit that would cause over-deviation and earn a fine.

home fm transmitter is quieter

In practice, commercial stations want their broadcast to sound as loud as possible for listeners, so they process the sound to maximize perceived loudness for a given deviation, using dynamic compression.

Dynamic compression attenuates the loudest parts of the sound and boosts the weaker parts, you can then be constantly at the edge of saturation. The more compression you use, the less natural the sound will feel, so audiophiles usually hate it, however broadcasters have found that most users favored the loudest stations.

broadcast test generators from Rohde&Schwarz - audio gain and deviation can be set separately on some of them

real time dynamic compressor - does compression on the fly for live broadcasts

The professional grade equipment that real radio stations use has very sophisticated circuitry to precisely control the deviation levels without listeners hearing much in the way of artifacts or distortion. If the station wants competitive loudness, their equipment can legally deliver it (that is, there is no risk of interference).

The FM transmitters made for playing an analog or Bluetooth source through a radio are consumer grade devices that have to be simple to be cost-competitive. They have to incorporate the treble boost so that the audio does not sound dull on the radio. However, the only way that they can effectively prevent over-deviation of the carrier is by keeping the volume low. If your transmitter were to over-deviate, the first thing that you might notice is that some treble sounds (like the letter “s,” cymbals, etc.) sound unpleasantly distorted. The manufacturer simply makes the assumption that you will turn up the volume on your radio if needed.