Audio Bandwidth of an AM Broadcast Station

AM Radio discussion. Directional arrays are FUN!
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R. Fry
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Audio Bandwidth of an AM Broadcast Station

Post by R. Fry » Sun Feb 11, 2018 12:28 pm

Basic information for any somewhat non-technical readers of this board ...
_______________

Below is a clip showing the received r-f spectrum of an AM broadcast station, at one instant of time. The carrier plus its upper and lower sidebands are clearly visible.

The two sidebands are mirror images of each other, because they are produced by a process where the audio program modulation creates sum and difference r-f spectra, referenced to the carrier frequency.

The sidebands extend to about 9.5 kHz above and below the carrier frequency. This means that an AM receiver with reasonably flat r-f/i-f bandwidth response and able to pass that complete spectrum from 920-940 kHz could have an audio output bandwidth of about 9.5 kHz. Many listeners to audio of that bandwidth would find it fairly indistinguishable from the audio bandwidth received from FM broadcast stations.

(Granted, the reception of AM broadcast stations is more susceptible to atmospheric and local r-f noise, and co-/adjacent-channel interference.)

Most consumer-level AM receivers are not designed to receive/reproduce program audio having a ~9.5 kHz bandwidth. But that limitation is not a function of the capabilities of amplitude modulation itself, or the useful r-f channel bandwidth permitted by the 10 kHz carrier spacing of AM broadcast stations.

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kkiddkkidd
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Re: Audio Bandwidth of an AM Broadcast Station

Post by kkiddkkidd » Sun Feb 11, 2018 3:10 pm

Richard,

As I have found over the years, a lot of AM stations suffer from either horribly equalized telco program lines and/or horribly mis-tuned antenna tuning & phasing equipment. A lot of the "Hispanic Tee" networks are tuned as a narrow filter with the correct impedance conversion...

Regards,
--
Kevin C. Kidd CSRE/AMD
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AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
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R. Fry
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Re: Audio Bandwidth of an AM Broadcast Station

Post by R. Fry » Sun Feb 11, 2018 4:30 pm

No doubt, Kevin.

The r-f spectrum shown in my post was produced by an SDR directly from the daytime radiated fields of that AM station, received about three miles away from their tower site.

Their performance clearly may not be representative of the performance of other AM stations.

kcbooboo
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Re: Audio Bandwidth of an AM Broadcast Station

Post by kcbooboo » Mon Feb 12, 2018 5:30 am

It's interesting that there's so little energy in the 0-5 kHz range yet so much more in the 5-10 kHz range. Also, the modulation level seems to be quite low. I wonder what sort of program material was being tranmitted at the time you caught that trace.

ClearChannel / iHeartRadio stations seem to have been directed to go to 5 kHz bandwidth many years ago, and my annual Equipment Performance Measurement spectrums show that quite nicely.

Bob M.

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RFWarrior
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Re: Audio Bandwidth of an AM Broadcast Station

Post by RFWarrior » Mon Feb 12, 2018 6:31 am

kcbooboo wrote:
Mon Feb 12, 2018 5:30 am
ClearChannel / iHeartRadio stations seem to have been directed to go to 5 kHz bandwidth many years ago, and my annual Equipment Performance Measurement spectrums show that quite nicely.
Pretty much any station running AM IBOC will be limiting analog audio to 5kHz - after several complaints of self-interference in the early days, it was recommended (mandated?) for that situation. The funny (sad) thing is that it all comes back to Richard's original post - if good engineering practices on AM antenna array design/maintenance had been followed all along, it probably wouldn't have been an issue to begin with. With a lot of the shoehorns out there now, getting 1.2:1 or better out to 15kHz in the array is flat out impossible (or at least, not financially viable, which amounts to the same thing).

It amazes me how many people get that a home stereo, no matter how expensive, isn't going to sound like anything but crap running into a pair of $10.00 Walmart speakers... but insist on doing that exact same thing with their radio station. Your antenna is your speaker - the broader and flatter it is, the better the station will sound (vastly oversimplified, I know, but it's the analogy I use when trying to explain to OMs why the antenna upgrades their consultant is recommending are NOT a waste of dollars).

I have a copy of a paper written in the early to mid 19xx era around somewhere (can't remember which Carl wrote it - Jones or Smith), that explained bandwidth of a directional AM array in wonderful detail... it proved how a properly set up array would maximize coverage area and audio quality. Those same principles were revived during the AM Stereo days and, coincidentally, apply equally well to AM IBOC. Funny how physics doesn't change, isn't it?

Heh, started with a 2 sentence, "yeah, HD Radio operation requires it" response and ended up going on a tear... not enough coffee yet, I guess!
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bmcglynn
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Re: Audio Bandwidth of an AM Broadcast Station

Post by bmcglynn » Mon Feb 12, 2018 7:00 am

kcbooboo wrote:
Mon Feb 12, 2018 5:30 am
It's interesting that there's so little energy in the 0-5 kHz range yet so much more in the 5-10 kHz range. Also, the modulation level seems to be quite low. I wonder what sort of program material was being tranmitted at the time you caught that trace.
That could be related to the Pre-emphasis on the signal that tries to equalize typical AM receivers. Some of the presets on Optimod equipment have sharp equalization centered around 5 kHz to sound brighter.

One of the things that has plagued AM radio has been the lack of receiver performance standards. I remember back in the early 1990's when AMAX was around to address that. The FCC is now looking into AM Revitalization (which, to date, has mostly been about offering FM Translators to AM stations). Tom King of Kintronics has been quite active in this area.

Check out some of the relevant material on their site:
http://www.kintronic.com/resources/technical-papers/
  • Broadcasters Clinic 2015 Presentation - Addresses Bandwidth and other issues
  • NAB-AM-Synchronization-Paper-TK-SFS - Shows the benefits of the industry locking carrier frequency to a GPS reference

Here is an interesting article about audio quality issues from complex lowpass filters:

http://www.radioworld.com/headlines/004 ... d39/313449


See study "NRSC-R101" for a comparison of receiver bandwidth in the market today.

http://www.nrscstandards.org/reports.asp

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Deep Thought
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Re: Audio Bandwidth of an AM Broadcast Station

Post by Deep Thought » Mon Feb 12, 2018 9:04 am

bmcglynn wrote:
Mon Feb 12, 2018 7:00 am
I remember back in the early 1990's when AMAX was around to address that.
That was one component of the program, the others being the NRSC-2 occupied bandwidth mask and the change from 0.5:0.5 to 0.5:0.25/0.25:0.5 mV/m first-adjacent protection, and moving some of the most interference-causing stations to the expanded band.

As usually happens, the FCC lost interest halfway and while the transmission end was implemented the receiver end was not.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Re: Audio Bandwidth of an AM Broadcast Station

Post by radio_guru » Wed Feb 14, 2018 2:17 pm

Never mind that with a FM translator, station owners have no motivating reason to care about their AM audio quality. It can sound like it's coming on a string and tin cans. Doesn't matter a hill of beans as long as the thing is on the air at licensed power and pattern.

I will say I have started using the SW LP filter settings and chopping at 4.5Khz for talk stations. Combined with getting rid of the NRSC 2 curve (always been optional) and all HF above that frequency along with optimized antenna tuning and cusp rotation, the really important audio can be loud, proud, and clean of the IM swirl commonly heard in poorly tuned systems.

One of my stations is 1.1:1 flat out to 15Khz and 1.15:1 to 25Khz. The curve on a Smith Chart is but a very short line +/-5 Khz.

RG

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Dale H. Cook
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Re: Audio Bandwidth of an AM Broadcast Station

Post by Dale H. Cook » Thu Feb 15, 2018 7:28 am

radio_guru wrote:
Wed Feb 14, 2018 2:17 pm
Never mind that with a FM translator, station owners have no motivating reason to care about their AM audio quality.
I have one client who has applied for a translator to complement their flea-power night signal. It will cover relatively little of their 5 kW day signal so they are very keen on keeping that running well as it serves many listeners that the translator won't reach in their terrain-challenged rural county.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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Re: Audio Bandwidth of an AM Broadcast Station

Post by RodeoJack » Thu Feb 15, 2018 10:22 am

Yah... I'd have to argue that one, too.

15 of the stations in my circuit are AMs. Most of them have attached FMs, or have recently received CPs for them under Revitalization. While diligence toward maintenance and compliance varies somewhat between the owners, the worst of them is only marginally deaf toward the quality of his AM signal. In his case, the issue revolves around cascading low-quality MP3 streams on the way to his transmitter, where his demand for aggressive processing makes the audio much worse.

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