5 kHz Audio Bandwidth on AM - Radio World

5 kHz Audio Bandwidth on AM

Clear Channel Reduces AM Bandwidth; Calls on Industry to Do the Same
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Clear Channel Reduces AM Bandwidth; Calls on Industry to Do the Same

The following was e-mailed to Clear Channel Radio engineers by Jeff Littlejohn, senior vice president of engineering, on Sept. 29. It is reproduced with permission.

To: Radio Engineers; Regional Engineering List

Subject: Five Kilohertz Audio Bandwidth on AM

Good afternoon everyone,

Over the past couple of years, I've spent some time doing IBOC testing to determine its impact on the AM band. One such test was conducted in Cincinnati at WLW and in New York City at WOR.

During those tests, we limited both stations to 5 kHz audio bandwidth and then turned on and off the IBOC carriers at 1-minute intervals. I happened to be flying home the night of the tests, so I was not able to go to the transmitter site, but I did tune in on the drive home from the airport. The result was pretty interesting!

When I tuned to 710 AM, I heard WOR(AM)'s skywave crystal clear! Never in the dozen years of living in Cincinnati had I heard WOR's skywave signal. It had always been obliterated by the sidebands of WLW.

Next, I tuned to WLW 700. I couldn't perceive any audible degradation by limiting the audio to 5 kHz.

Suddenly it struck me that radio had lost the battle that was fought in 1987 through 1991. That was the time period when we argued with the CEA, NAB and NRSC about a couple of new AM audio standards, NRSC-1, NRSC-2 and NRSC-3.

I remember complaining about how much limiting our audio sidebands to 10 kHz was going to degrade the performance of AM. NAB wanted to retain 15 kHz audio, CEA members wanted us to reduce our audio bandwidth to 5 kHz. In the end, we settled on 10 kHz audio and later the FCC adopted the standard as the new bandwidth requirement.

Why?

The goal of NRSC was laudable and was intended to result in wider bandwidth receivers (remember the AMAX standard?) but the fact is that nobody ever made these wider bandwidth receivers. Instead, in response to customer complaints about AM interference, the receiver manufacturers continued to reduce the audio bandwidth of AM receivers to eliminate the "chatter" caused by the sidebands of adjacent channels.

The result is that an "above average" receiver today has audio response that is less than 4.5 kHz. In fact most have audio response that is down 10-12 dB at 5 kHz - and the rolloff can start at around 2 kHz.

As far as I am aware, there is only one commonly manufactured radio that has more than 4.5 kHz audio bandwidth, the GE SuperRadio (in Wide Band mode); that one is good to about 6 kHz before it's significantly rolled off. These results were confirmed by tests conducted under a study by ATTC.

So ask yourself: "Why do we broadcast 10 kHz audio on the AM band if nobody can pick it up?"

The only reasons I can find to maintain 10 kHz audio bandwidth is that "It's the way we've done it for the last 12 years."

There are a couple good reasons to reduce our AM bandwidth to match the bandwidth of the available receivers:

1. Increased modulation efficiency. By eliminating the broadcast of the high-frequency energy, we can increase the amount of energy that is in the 20 Hz to 5 kHz region. Let's not forget that due to pre-emphasis, higher frequencies are boosted and will have a more profound effect on total modulation than lower frequencies will.

2. Reduced interference to first-adjacent frequencies (WOAI-1200 in San Antonio interferes with KFXR-1190 in Dallas; WHP-580 in Harrisburg interferes with WTNT 570 in Washington, DC).

3. AM modulation that falls outside of a receiver's usable bandwidth ends up increasing the noise floor within the audible bandwidth.

Given that I can find no good reason to maintain 10 kHz audio bandwidth and that there are substantive benefits to our reduction of audio bandwidth to match the pass band of AM receivers, I want to institute a Standard Operating Practice for all Clear Channel AM stations.

New SOP

Beginning as soon as the next time you go to the AM transmitter site, the following SOP should be put into place.

- For all AM stations operating with modern audio processors - Orban 9200, Omnia 4.5, Omnia 3, Omnia 3 CC - we will reduce our audio bandwidth: to 5.0 kHz audio bandwidth for all AM stations except music-intensive AM stations, and 6.0 kHz audio bandwidth for AM stations with a music-intensive format.

- For stations that do not currently have a modern audio processor that is capable of this reduction in audio bandwidth, we will give favorable review to any request for a replacement audio processor. This doesn't mean you'll get it, but it's likely.

It is true that AM HD Radio is going to require that we reduce our audio bandwidth to 5 kHz in order to properly implement that technology; and we're making serious efforts to implement AM HD Radio at our stations over the next couple of years.

However, this change has nothing to do with HD Radio. It's the right thing to do for AM analog radio service. HD Radio just happened to point out that the benefit existed.

The result of our change will be more competitive AM analog modulation (without the necessity of increased audio processing) and reduced interference to our neighboring stations.

As the co-chairman of the AM Subcommittee of the NRSC, I am proposing this standard practice among all AM licensees. But for today, Clear Channel will be the leading the charge in this effort to clean up the AM analog service.

Thanks for all your help in this effort. Please feel free to ask questions if you have them.

Jeff Littlejohn

Littlejohn also e-mailed a copy of this internal memo to engineering executives of other leading radio groups and asked for their support on this matter at the pending NRSC meeting.

"While HD Radio will make a big impact in improving the profitability of the AM band, it will take a long time to implement and even more time to change out the 800 million receivers that exist today," he wrote to his colleagues. "This suggested change will not improve the audio quality of AM analog, but it will drastically decrease the interference that each of us generates toward the other and may help our listeners 'bridge the gap' to HD Radio.

"I realize that this idea may be too radical for some. I realize that others will simply disagree with the idea. If that is the case, I certainly respect your opinion. However, I believe in this strongly enough to implement it today and I hope you will join me."

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