A monitor’s audio output will sound excessively bright or harsh if a de-emphasis audio circuit is not included. Monitors traditionally do not have this, but often a simple capacitor and resistor modification will do the trick. The idea is to undo the high-frequency boost that is a part of the audio processing, per the National Radio Systems Committee (NRSC) standard. As you probably know, the transmitted audio has increased high-frequency response to overcome high-frequency rolloff in most receivers. The goal is to restore flat frequency response to the listener. Some audio processor manufacturers are using non-standard pre-emphasis curves to suit their taste. That complicates getting a realistic feel for frequency response. At least they are trying to make the best of receiver frequency response roll-off.

ON A SCOPE
An article I wrote regarding the operation of oscilloscopes, “Your Scope Is a Tool for all Seasons,” appeared in the Jan. 13, 2013, edition of Radio World.  To refresh your memory, a scope has a display where a dot that travels from left to right is deflected up and down with voltage. In this case, we will look at a transmitter’s RF output.

Fig. 6: Traditional analog audio processing used diodes to clip the negative side of audio before it went to the transmitter so it would not attempt to overmodulate the negative modulation while allowing positive modulation to go to 125%. The downside is that it added as much as 6.5% harmonic distortion in the process. Add the receiver’s problems to the mix and you have a whopping 10.2% distortion. Ouch! You’d never allow that on FM.

Newer digital processors reduce but may not eliminate the problem. Yes, the station can be a bit (about 0.9 dB) louder on the dial, but it is irritating to many listeners. They don’t know how to describe it, but oops, there goes another tune-out! Again, some people hear it and some don’t. Best not to penalize the station with high modulation.

Fig. 7 shows the transmitter being modulated at over 100% negative modulation. I’ve moved the scope’s trace up a bit so you can see detail. Negative peaks go flat to the center, which is no carrier at that instant. Modulation like this will not pass the required NRSC occupied bandwidth nor will it pass my ear test for listenability. It is tiring to hear.

Fig. 8 is where you want to be. No more than 95% negative modulation, the sweet spot between loudness and listenability.

It is a shame to lose listeners for that last 5% (about 0.5 dB) of modulation. Few if any will hear the loudness difference. Likely most will hear grit in the audio of transmitters modulated to the max. You can make up much of the modulation percentage difference with careful adjustments of the audio processing, before it goes to the transmitter. Software-defined receivers eventually will solve much of this problem, but we need to deal with today’s radios.

When I was installing AM stereo years ago, negative modulation was usually set at 95% and positive modulation at 95% for stations to sound clean. It was positive +125% if the client preferred it. That extra positive modulation comes as “forced asymmetry” where the negative audio peaks are soft clipped so the positive peaks can go higher. Ouch!

Surprisingly, bad-sounding audio with less than 100% modulation will usually fit into the NRSC occupied bandwidth mask, in the FCC required annual measurement. That is because of the required 9.5 kHz low-pass filter in audio processing.

AM stations competed in loudness wars to beat the other guy years ago. Now it is time to give listeners a pleasant experience with natural-sounding audio. Don’t drive them away.

I grew up in a broadcasting family that owned two AM stations and no FM. Success was dependent on keeping listeners. Loudness was not the answer.