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NRSC 1, 2, 3 AM broadcast standards

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NRSC 1, 2, 3 AM broadcast standards

In the 1980's licensed AM broadcasters became concerned with the dwindling listener numbers with the overall acceptance of FM and its signal quality, low noise and superior receiver design.

The National Radio Standards Committe was formed for the purpose of recommending solutions to qualitative signal challenges of radio. AM radio was its first arena for research, analysis and standards to be implemented to improve signals.

NRSC-1, published in July 1998, embraced the age old challenge of high frequency audio quality in AM broadcasting. A modified 75 u/sec preemphasis/deemphasis curve was recommended, much like the long used system used in FM exciter and receiver designs. In addition, the standard recommended limiting audio bandwidth to 10 Khz (-30db @ 10.5 Khz., -40db @ 11.0 Khz., -50db @ 15. Khz.). This "audio masking" scheme reduced percentage of modulation at 15 Khz. down to as little as .32 percent. This produced substantially less 2nd adjacent channel sideband interference in wideband receivers. Other advantages included better signal to noise in the "listenable" pass band of most average AM receivers and a finite audio standard for AM receiver manufacturers. NRSC-1 was a voluntary standard.

NRSC-2, published in June 1998, further described the reduction of AM broadcast spectrum bandwidth as a product of limiting audio frequency bandwidth. This original voluntary standard is referred to today as the AM "audio mask".

NRSC-3, introduced in June 1990, established audio bandwidth and distortion standards for AM receivers.

All of these standards have now been accepted by the FCC as mandatory for licensed AM broadcast stations.

You will find copies of the published studies on the NRSC standards site:

http://www.nrscstandards.org/Standards/nrsc-1.pdf

http://www.nrscstandards.org/Standards/nrsc-2.pdf

http://www.nrscstandards.org/Standards/nrsc-3.pdf

So, you might be asking, what the dickens does this have to with Part 15 broadcasting? Well, if you don't want to interfere with licensed radio stations and want to be accepted as just another station on the dial, you might consider complying with the standard. Does your station use preemphasis, audio masking and reduced bandwidth? And can you transmit your signal without "splattering" outside the 10Khz bandpass? All that audio above 10Khz is probably being spalttered all over the spectrum due to the narrow bandwidth and high reactance of Part 15 antenna systems.

I will point out Part 15 stations are NOT required to comply with any of the NRSC standards. However, if a local broadcast engineer captures your signal on his air monitor and spectrum analyzer, he might be motivated to make a phone call to the FCC. I have read where several Part 15 AM broadcasters have "solved" this problem with equalization and keeping the modulation percentage to a bare minimum. All of which makes your signal muffled sounding and not very listenable in comparison to the big boys.

My solution has been the Innovonics 222 AM audio processor which takes care of everything but the bandpass of the antenna and it's associated tuning network. This processor produces nearly perfect modified 75 u/sec preemphasis, limits audio bandpass at 10 Khz and keeps the average percentage of modulation at an acceptable level.
This piece equipment of does more to extend the range of my station legally than any other improvement I have attempted.

I welcome your feedback. What is your experience?

NRSC, etc

KK7CW and all,

Thanks for the fine writeup concerning this. I agree that limiting AM audio to 10 kHz. is a good idea. Another reason for this is many AM radios won't produce audio above this frequency and the transmitter power is just wasted at high audio frequencies.

A properly tuned high Q antenna actually reduces splatter, but your approach is much better...control the audio.

In addition to high modulating frequencies, the surest way to cause splatter on AM is to overmodulate the transmitter. This causes clipping of the audio peaks on the envelope producing strong harmonics which are multiples of the audio frequency and they appear added to and subtracted from the carrier frequency. For example, a 1 kHz. sine wave audio tone at >0 to 100 % modulation produces a 2 kHz. bandwidth. Over 100 % modulation which clips the top of the envelope or the nadir of the envelope will produce strong sidebands out to about the 7th audio harmonic which gives a bandwidth of 14 kHz. Audio programming up to 5 kHz. with clipping will yield a bandwith of 70 kHz. instead of the usual 10 kHz. So, in addition to limiting the audio bandwidth, the modulation level has to be set to avoid clipping.

I believe you have a good approach with audio processing. I don't have an audio processor so I use an equalizer program set to cutoff at 10 kHz. for my AM work. Such an equalizer program could also be set to give preemphasis but I have not done so. For those who want to try this approach, the corner frequency corresponding to 75 uS. is 2120 Hz. where the boost is 3 dB with a positive slope of 6 dB/octave above this.**EDIT. This is based on a 1 zero response. When I reviewed the NRSC-1 document I noticed that their response curve for AM has a slope of about 3dB/octave.***

I use a SSTRAN AMT-3000 which has built in modulation limiting but it still has to be adjusted properly. I use an oscilloscope for this. If one does not have a scope, listen to the audio on a receiver and make sure it is not distorted. Increase the modulation until distortion is heard and then reduce it until the signal is clean. This is not perfect but it is close enough for part15 work.

Neil

More NRSC...

The question that could be asked here, If narrowing the pass band of the antenna system would have reduced interference and made AM signals sound better in the receiver, why didn't the NRSC come up with the same recommendation?

Here is the answer. When an AM signal is overmodulated past the negative peak or past the ability of the amplifier or power supply to produce a clean positive peak, the distortion to the radio sine wave produces harmonic, spurious signals that can be detected by a receiver. You will notice I did not say the receiver will always detect the signal. The bandwidth and associated receiver circuits would have to be able to pass the signal onto the listener. Just because you can't hear distortion "off air" doesn't mean harmonic, spurious radiation doesn't exist on the signal. The only fool proof determination of over modulation product is with a spectrum analyzer which is beyond most Part 15 stations budgets. Even receivers listening at harmonic frequencies of the fundmental may not be able to determine the source of the signal when combined in narrow band I.F. sections of most receivers.

Part 15 antennas have very high capacitive reactance causing the bandpass of the antennas to be very narrow. However, concluding that none of the energy produced by over-modulation, high frequency audio or mistuned antenna systems will not pass through a narrow bandpass is not correct. When broadcasting complex assymetrical audio through an AM transmitter, under some circumstances, introduces spurious products at the fundamental carrier frequency. One of these products could be "carrier shift" that causes distortion at the receiver as well.

So, the reasons behind the recommendations of the NRSC were to cover all types of antenna system designs including narrow band antenna systems. Most directional AM antenna arrays of "non-contemporary" design are the real world example of just such an antenna system. There are hundreds of these extremely narrow bandpass systems across the country.

And, I should mention here, the primary reason to be a broadcaster is to have pairs of ears LISTEN. Excessively narrow band signals create audio products in the listeners receiver that is transmitted to the ears. Listeners do make qualitative decisions as to whether they will listen to your station. If the audio is tubby, harsh, thin, distorted, a whisper or whatever, radio listeners are fickle. Broadcasters have learned over the decades to cater to listener tastes. The NRSC took years of research and made recommendations based on engineering and listener research. As a life long broadcaster, the mind and ears of my one listener is the most important judge of how well I am doing. Maybe its time to ask our listener what works?

Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org

Marsh Johnson, Sr. - North Bend, Oregon, USA

AM unwanted emissions.

Hi Marshall,

In my previous post I mentioned antenna Q in response to a statement that the high Q antenna increases splatter. It does just the opposite since it acts as a narrow band pass filter. You are correct that the splatter will not be eliminated this way but I point out that if antenna Q is a factor, splatter will be reduced rather than increased. Because it is hard to control the Q and tuning of a coil loaded short antenna, this is not the approach to use to reduce splatter which needs to be prevented at the transmitter. A high Q antenna will supress the wanted sidebands and make the audio sound poor. I use a coil loaded resonant antenna and had this problem so I "spoiled" the Q with a shunt resistor.

It is true, as you said, that a receiver may not hear the splatter while tuned to the signal. I maintained a college carrier current AM system and would check for splatter by tuning the receiver off the signal. With experience, I could determine by listening off frequency if the transmitter was overmodulated on the negative peaks.

Spurious signals such as splatter are caused by improper transmitter adjustment (or perhaps poor design) as you discussed, and it is very important not to overmodulate and secondarily to limit the audio bandwidth. The NRSC preemphasis curve boosts the higher audio frequencies which with a deemphasised receiver gives a better signal to noise ratio. This curve also limits the AM audio bandwidth to 10 kHz. thereby limiting the AM transmitted bandwidth to 20 kHz.

A spectrum analyser is a great tool for monitoring the transmissions, but an oscilloscope display of the modulated carrier observed by a "trained" eye is a low cost practical monitoring means which should be affordable by most hobbyists. They are available in the $50 range, though a can of contact cleaner may be needed for the scratchy controls.

I agree that one wants to provide a high quality audio signal that listeners will enjoy and I have nothing to add to that other than it begins with a properly operated transmitter.

Neil

Pure spurious for the curious...

Neil and all,

Thank you for your input. I need clarification of the statement,"it is very important not to overmodulate and secondarily to limit the audio bandwidth." Limiting audio bandwidth is exactly what the audio mask does in the NRSC standards. Could you have meant to hard limit the amplitude of the audio wave?

In my experience, whenever an audio wave is "clipped" or hard-knee limited, a square-ing of the waveform is produced thereby producing harmonics in the audio that will, to a degree, be imposed on the transmitted signal. This can produce sideband distortion that sounds like splatter. It is in fact audio distortion showing up as splatter in the receiver audio. I have experienced this receiver product on several occasions, with a sufficient amount of tail-chasing to follow. Eventually, I found the processing was the culprit. Limiting "audio bandpass" has been used for decades to enhance transmitted signal to noise ratios for AM receivers. Additionally, in the old days when we used tubes and huge high voltage transformers, the components in the amplifier circuits smoothed everything out making for smooth warm sounding audio. Today, with digital and solid state circuits being more true to "garbage in, garbage out", audio processing byproducts are more easily transmitted over the air.

Limiting audio bandwidth is used in several radio services around the world to reduce interference and to better untilize scarce radio spectrum.

By narrow-banding the antenna circuit to use it as a final bandpass filter, consider this. By supressing the RF and audio bandwidth with the antenna system you also waste a substantial portion of the sideband waveform as heat. The sideband of the signal carries the intelligence of the signal. So that, even though the transmitter and audio chain are operating perfectly, the antenna becomes the "fly in the ointment". My suggestion to take a look at the NRSC standard is to ultimately put more signal and better audio out of our stations. This not only produces real-world credibility with the FCC, but also with our customers...our listeners.

And finally, AM deemphasis is recommeded for use ONLY in wideband receivers. Most cheap comtemporary AM receivers have a maximum IF bandpass of only 4-6 Khz. The purpose being to limited the amount of atmospheric noise sent to the audio section of the receiver. Wideband receivers have a bandpass in excess of 10 Khz and use modern day digital signal processing (DSP) circuits to get rid of the noise.

Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org

Marsh Johnson, Sr. - North Bend, Oregon, USA

Regarding overmodulation

Hi Marshall,

I believe we are in agreement about this but let me respond to your query:

>I need clarification of the statement,"it is very important not to >overmodulate and secondarily to limit the audio bandwidth."

By "overmodulate" I had in mind clipping on the negative peaks which pinches off the carrier. When this happens very wide band audio harmonics are produced in the sidebands resulting in wide band interference. Sharp limiting on the positive peaks can have the same result.

By "secondarily limit the audio bandwidth" I was ascribing less importance to this than to the carrier pinchoff situation since this will not produce audio harmonics and the RF bandwidth will be less than overmodulation produces. My comments are for part15 and not commercial stations. I do agree and recommend that part15 AM operators should limit their audio bandwidth to 10 kHz., but I don't see a lot of harm if they don't. High audio bandwidth would be a waste of power in the sidebands to no good effect at superaudible frequencies.

Hope this helps.

Also, thanks for your comment about receivers. I have not kept up with that technology so I found it informative.

Neil

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