Ok here's a more technical approach on radio mixes...warning you may have to read this few times to get it.....i grabbed it from GEAR SLUTS 
Bob Katz has a perfect article on radio ready. The end all be all I call it.
In fact, I'm gonna read to you verbatim out of my edition of Bob's book. At the end, I'll suggest what you can do as a *mix engineer* to prep a song for radio.
here are the goods.
selected quotes:
"The Truth about Radio Ready"
Quote:
Every radio uses a transmission processor in front of it's transmission signal. It's most important function is to control the peak modulation of the transmitter to the legal requirements of the regulatory bod in each stations nation.Very few stations use a simple limiter, but rather a complex chain to produce a seemingly louder signal
In other words, it'd be good to find out what the specs are within your country. (In the U.S. regulated by the FCC I would imagine)
Quote:
A typical processing chain in a radio signal is as follows:
Phase rotator, AGC, Stereo Enhancement, Equalization, Multiband Compression/Limiting, Pre-Emphasis, HF Limiting and Clipping.Phase Rotator: A chain of Allpa** filters (typically four poles, all at 200hz). It's purpose is to make wave forms more symmetrical and consistent for radio listening.
Simply put, from my understanding, it's designed to seek out the center channel stuff (like voices) and put them right there in your face for radio play. No distortion and it's kind of an added bonus. The trade off is that you loose some transparency in the mix. But it was accepted one.
Quote:
The AGC, short for Automatic Gain Control, is an averaging system within the chain. By recording studio standards, this AGC is required to operate over a wide dynamic range. (About 25 db).
The AGC compensates in level errors the live sound engineer might and will make during any specific broadcast. This is what keeps radio stations loud over a length of time. AGC operate slowly to prevent things like ba** pumping. AGCs are always gated in component processors.
So in essence, the AGC is kind of like a slow referee.
You tell him you wanna play something too soft, he says "nope, I'm bringing it up". You wanna play something too loud, "he says, nope...gotta sound like everyone else. I'm bringing you to their level".He works on averages. He doesn't discriminate and he works for the set standards, not for any one person.
Quote:
Stereo Enhancement is used by some, but not all radio stations. This happens typically after the AGC. Overdone, it can remix the song.
When mixing for radio, stick to the dry side of things. because of the nature of stereo enhancement can bring this out excessively.
The only universal standard is the
strict need for mono compatibility, obviously for broadcast done on mono stations. FM is frequently receieved in mono.
A note. It's my understanding that the "stereofication" process used by AM radio stations involves M/S Stereo processing. Basically this means anything thats not present in your center channel is summed up to the center and shoved to the back. So in other words, if you mix a rock song like anyone normally would, you'd expect your guitars to get lowered dramatically while the vocals, kick and snare (anything panned centered) to be most dominant in that mix.
In FM, there alot of situations where people have to still listen in mono. Any department store or major building that carries those recessed ceiling speakers are more than likely giving you a mono signal. Super markets, gyms, legal buildings, JC Penny, doesn't matter, it's in mono. So that's kind of a big deal.
**EQ and Multiband Compression are pretty self explanatory. This is dependent on the radio stations tastes at this point. For a basic run down on compression or EQ, you can refer to any article on here, as it's discussed extensively.**
Quote:
Premphasis and HF Limiting. FM Radio is preemphasised at 50 milliseconds or 75 microseconds. Preemphasis is high frequency boost thats 3db up 2.1khz or 3.2khz.
Easy to understand.
Quote:
Clipping. In most processors, the clipping stage is the primary means of peak limiting. It's to broadcast processor performance. Because of pre-emphasis, simple clipping dosn't work well at all. Instead, you have "Distortion cancled clipping".
Distortion canceled clipping is a very effective means of peak limiting because if affects only the peaks that actually exceed the clipping threshold and not surrounding material. Also, clipping does not cause pumping.
Some chains might comprise of multiple clipping stages.
So now the question, how do you prep a mix for the radio? Taking all this into account, a quick checklist:
Quote:
1) Avoid fast digital clipping and limiting.
2)Leave short term envelopes unsquashed
3) Let the broadcast processor do its work.
4) The result will be a just as loud, hypercompressed song, but with more punch, clarity and life.
So in other words, when mixing for this kind of stuff, you really have to go minimal. A radio mix is a completely different beast than an record mix.This obviously means they are not mastered in the same way either. There is a heavy difference. How that's done, i can't tell you because I'm not a mastering engineer. But I would imagine it involves some type of "radio preview system" to get a feel of what you might get on the radio. Could be a boom box, could be that you have a friend that works as an engineer at a radio station.
It's for this reason engineers are asked by labels to generate radio mixes for singles. Experienced ones know what that means and how to do it.
Chris-Lord Alge is one of the beast premiere "mixing for radio" engineers out there. So research him and his work. Good luck.
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