Why Does My CD Sound Different on the Radio?
By Bernie Nau, Matthew Toledo- 6/12/2008
You've spent a lot of time and/or money tweaking your latest recording. You've played it in your car, on your home stereo, in your computer, and on a boom box. Each time, you adjust the recording until you think it sounds perfect almost everywhere. Then you hand your recording to a radio station. They play it, you hear it, and to your horror, the recording sounds absolutely nothing like it did during all your previous experiments. What happened? You raise your fists into the sky and proclaim, "Why does my CD sound different on the radio?"
The answer is simple: radio stations process your recording. Here are a few things you should know:
1. Phase Rotators: They were first introduced in the 1970's by Circuit Research Laboratories for use by AM radio stations. They are intended to make vocal waveforms more symmetrical, thus reducing clipping and distortion in human speech. (Orban, 1992) In a nutshell, it was designed to make it easier to hear someone talk. Unfortunately for modern recordings, when your CD is mastered, if the peaks are clipped or heavily limited, the phase rotators are going to introduce distortion.
2. AGC: It stands for Automated Gain Control and is sometimes called "normalization." It is a form of audio compression. (Automatic Gain Control, 2008) It is intended to raise the level of CD's that are soft. Inversely, if your CD is really loud, AGC will make your CD softer! It will change your mix, causing bass and midrange pumping, as well as spectral gain inter-modulation. "Virtually all radio station program directors want their stations to stay loud always, eliminating the risk that someone tuning the radio to their station will either miss the station completely or will think that it's weak and can't be received satisfactorily. Radio people often call this effect 'dropping off the dial.' (Foti & Orban, 2001)
3. Stereo Enhancement: meant to make music sound bigger and wider. If you used stereo reverb in your mix, the stereo enhancement will make that reverb louder on the left and right edges of the sound stage. It will also reduce the reverb in the center, where the vocal is. Basically, it will change your mix-reverb-blend. (Tech Tips, 2002)
4. Pre-Emphasis: pre-emphasis raises the level in the frequency range between 2000 and 3000 Hz as much as 6dB and at 15,000 Hz, it is raised 15 dB or more. Radio stations use pre-emphasis as a form of noise reduction. It increases the signal-to-noise ratio of the radio station's broadcast. As you may or may not know, "signal-to-noise ratio compares the level of a desired signal, such as music, to the level of background noise. The higher the ratio, the less obtrusive the background noise is." (Signal-to-noise ratio, 2008) Unfortunately, pre-emphasis will change your spectral balance, making your recording sound much brighter over the air than your CD.
Because of these and other radio processes, major labels often create a separate master for radio play.
If you want your CD to sound great on the radio, ask your mastering engineer to create a separate master for radio. Or, make your master a little softer. If your mastering engineer understands the parameters involved in radio processing, it is possible to find a balance. i.e., loud but not loud enough to trigger the unwanted results.
References:
Automatic gain control. (2008, May 29). In Wikipedia, The Free Encyclopedia. Retrieved 18:01, June 12, 2008, from http://en.wikipedia.org/w/index.php?title=Automatic_gain_control&oldid=215785570
Foti, F., & Orban, R. (2001, June). What Happens to My Recording When It's Played on the Radio? In Master Digital Retrieved June 12, 2008, from http://masterdigital.com/24bit/radioprocess.htm.
Orban, R. (1992). A Short History of Transmission Audio Processing in the United States. In BEXT. Retrieved June 12, 2008, from http://www.bext.com/histproc.htm.
Signal-to-noise ratio. (2008, June 10). In Wikipedia, The Free Encyclopedia. Retrieved 18:17, June 12, 2008, from http://en.wikipedia.org/w/index.php?title=Signal-to-noise_ratio&oldid=218499350
Tech Tips: Stereo Enhancement: how does it work, and is more better? . (2002, February 23). In Sweetwater.com. Retrieved June 12, 2008, from http://www.sweetwater.com/expert-center/techtips/d--02/23/2000.
