Yes, the FM broadcasting industry uses pre emphasis and de-emphasis techniques to improve their signal-to-noise ratios. It's been correctly pointed out that audio frequencies below the breakpoint are transmitted flat, and audio frequencies above the breakpoint are transmitted pre emphasized. (There have been other such "curves" used to tailor response, such as the RIAA curve in phonograph records, and the NAB curve in tape recording.)
But that isn't the original reason pre emphasis and de-emphasis were used in narrowband radio. The early transmitters were PM (phase modulated), not FM, so they naturally had a 6 dB/octave pre emphasis. PM became the standard modulation method. When FM transmitters came along, their audio had to be intentionally pre emphasized to maintain compatibility with the PM transmitters already in service. In very early narrowband literature, you won't even find the terms "pre emphasis" and "de-emphasis". Engineers simply "rolled off" the audio in the receiver with a single pole filter because they had to defeat the PM transmitter's characteristic "roll-up". The pre-emph and de-emph terms came from the broadcast people. (I wish the narrowband radio industry had better terms for these characteristics. Unlike the broadcasters with their middle-of- the-band breakpoint, in narrowband radio the breakpoints are outside the voice bandwidth.) So, de-emphasis has little to do with signal-to-noise radio and everything to do with making the response correct. If FM had always been used, there never would have been pre-emph or de-emph in narrowband radio.
We must recognize that early narrowband radio was intended for one transmitter, one receiver applications. This business of linking repeaters came much later. We pre-emph the audio to the FM transmitter to simulate PM, but must maintain a narrow bandwidth to be a good neighbor. So, we roll off the audio at, let's say, -3 dB at 3 KHz. If we hop through another similar system, the resulting audio is then down another 3 dB at 3 KHz, or a total of -6 dB at 3 KHz. This narrowing of the audio bandwidth is what everybody complains about in linked systems.
So, the popular answer is to eliminate de-emph and pre-emph in the repeater. Just feed the user's pre-emph'd audio from the repeater receiver's discriminator to the repeater transmitter after the pre-emp stage, thus bypassing the repeater's de-emp and pre-emp circuits, resulting in a "flat" repeater, right?. (Of course, you still have all those controller mods to make.) Everyone then assumes de-emph and pre-emph are evil!!! They must be, since the audio sounds better without them!
But from an engineering perspective, there is nothing inherently evil in pre-emph or de-emph. The transmitter still rolls off at 3 Khz. By feeding pre-emph'd audio to the transmitter, you are artificially increasing the amount of high-freq audio fed to it. You are "peaking" the transmitter so that it rolls up. You are effectively widening the bandwidth.
My response? First, let's at least admit that we are attempting to make narrowband systems wide band. No bones about it. You want a nice, high-fidelity linked system? Go after the transmitters. Replace their audio filters with high order, brick wall audio filters that allow wider bandwidth signals (at the expense of smaller guard bands between channels). Or use wide band links on higher bands. But if you want to continue defeating de-emph and pre-emph, at least admit that it's similar to putting an audio equalizer in line.
One last thing - - FM'ing crystals is really hard (they're nonlinear). FM'ing in a synthesized transmitter is easy. So, if someone tells you he FM'ed his crystal controlled repeater transmitter with a few wiring changes and a capacitor, make him prove it. What audio signal generator did he use to sweep the transmitter? What receiver was used to produce the measured audio? Remember, the proof is in the pudding (lab grade test equipment). If it's really an FM transmitter, the received audio will be at a constant amplitude regardless of frequency. Anything else is modified PM.
73,
Bob Schmid, WA9FBO
S-COM Industries