Audio Bandwidth of an AM Broadcast Station

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 ~0 to ~9.5 kHz. Many listeners to audio of that bandwidth would find it fairly indistinguishable, audibly, 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.

Interesting stuff there, Richard, even to an electronics moron such as myself. In fact, just yesterday I found out how to lower the thudding bass on the car radio I've owned for four years.

All I can add are two 'ditto's of sorts.
It was maybe three decades ago -- I only read about the study once -- when someone conducted a test with an old, tube AM radio tuned to then-classical WQXR from NYC. The experimenter arrived at the same conclusion: AM radio can sound just about as good as FM radio. 'Average' listeners wouldn't know, hear, or care about any small sonic differences.

Ditto 2 : Noise.
I have what are perhaps the three finest AM DX radios in our town of 6000 people. There are two longwires here and two air-core loops. The radios may as well be three Seeburg jukeboxes without records in them. In the daytime, the wife and I can hear maybe six stations with any clarity. The noise is horrific.

Thanx for the post!

I preface this by saying I have no idea where or even if this fits in. I'm not a technically inclined guy....

I'm in the midst of a month at our usual getaway spot west of Pensacola directly on the gulf. About 55 miles to my east is WFTW, Fort Walton Beach, FL, which produces a good signal via saltwater path at 2.5kw ND day power. The audio that comes out of my GE Superadio-II from this station is phenomenal! As close to FM quality as I've ever heard on AM. Anywhere. If I had to describe it in a single word, that word would be "brilliant". It clearly stands out above everything else on the AM dial. And in fact, it strikes me as something of a waste of audio bandwidth because the format is all talk (mostly canned).

Any idea what might be going on?

You need to hear a station like that on a McKay-Dymek tuner (flat almost all the way to 10K, with a switchable 10 KHz Notch ("Whistle") Filter. Or, as a second, try a Denon TU-680NAB.
I rebuilt a McKay-Dymek years ago, and hooked it up to the matching directional antenna, and was amazed at the sound. I was listening to a sports-talk show, and I could hear which chair had a squeaky wheel on it. Music stations, few as there were locally, were unbelievable.

I believe that some of the newer standards (NRSC, and the ones for IBOC) limit the high-frequency response on analog AM, though.

A radio providing full bandwidth will sound great on strong signals, and also on weaker ones with low noise levels and no adjacent channel interference. However a radio with limited bandwidth will sound much better when there is noise or adjacent channel splatter, particularly from IBOC hash. The only way to have the best of both is selectable bandwidth, either manually switchable, or automatically adjusted according to signal level and noise. Most manufacturers don't want to spend any more than the minimum on the AM section, and making the bandwidth narrow with higher audio frequencies rolled off is the cheapest way to do it.

@cyberdad: Probably a combination of things. WFTW's transmitter is practically in the Gulf of Mexico (technically Santa Rosa Sound), probably giving you a very good signal strength for the distance. The antenna plant is probably properly tuned (easier to do when non-directional, which WFTW is). And secondly the engineer of WFTW probably has juiced up the mid-range to make the audio sound "Bright". That's an effect which I find galling on a talk format, but quite like on music-on-AM formats.

PTBoardOp94 said:

@cyberdad: Probably a combination of things. WFTW's transmitter is practically in the Gulf of Mexico (technically Santa Rosa Sound), probably giving you a very good signal strength for the distance. The antenna plant is probably properly tuned (easier to do when non-directional, which WFTW is). And secondly the engineer of WFTW probably has juiced up the mid-range to make the audio sound "Bright". That's an effect which I find galling on a talk format, but quite like on music-on-AM formats.

Click to expand...

Thanks for the explanation. The word "bright" also came to mind when I experienced the audio. And one of my second thoughts was that it did sound sort of weird on a talk show.

The advantages of digital over FM over AM are not the potential frequency responses, as the reductions of noise, distortion, and interference.
AM modulation monitors and crystal radios within the 100mv/m contours of single-tower stations sound great.
Signal to noise ratios have improved with each change though, from noisy, to fluttering, to perfect.