Mastering a mix for radio play: This post is written to clarify the compression process used by radio stations prior to airplay and discuss some mixing and mastering techniques to combat the unwanted effects broadcast compression.
When a song is broadcast over FM airwaves it is first “run through” or processed with a transmission audio processor in front of its radio transmitter. The processors main function is to control the peaks of the audio prior to transmission to satisfy the legal requirements set forth by the regulatory body. The signal chain can be quite complex – for the purpose of this post I will not get into the technical explanation. The end result of the compression is a constrained peak modulation while significantly decreasing the peak to average ratio of the song. This results in the station sounding louder, or an increased perceived loudness, within the allowable volume limitations. In layman’s terms, it is squashed to the point that the impact and dynamic range is compromised.
Generally speaking, there is not a lot a mixing engineer can do to combat the FM compression. If you get your song into radio rotation, assuming that it is mixed and mastered as intended, there will be a compromise in sonic quality. There is nothing you can do to completely eliminate these effects.
With that said, there is a mixing issue that often becomes a problem with independent releases which can be remedied with simple mixing techniques. The problem I am referring to is called “audio pumping.” This is when the FM compression noticeably pushes the overall volume up and down. For example, a loud cymbal crash or bass hit will generate so much amplification that the radio compression will reduce the overall volume of the song for a second or two and then fade the volume back in as the cymbal crash of bass hit fades away. This is typically is caused by too much unwanted and unnecessary low-end eating up the headroom of the song.
For example, if a cymbal crash is associated with a kick drum which they often are, you may be surprised as to how much low end your overhead mics are capturing from the drum kit. A simple low-pass filter on the overheads and VU meter can demonstrate this effect. One will also notice that the reduced low end on the overhead microphones will actually sound cleaner and “fuller” than without.
This process is not limited to overhead mics. I often hear “audio pumping” on poorly EQ’s vocals, acoustic guitars, and kick drums. If the unwanted low end is not “cleaned” or equalized properly from a song the FM Radio station will expose it.
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