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Question about AM reception (sound quality)

snowdog88

snowdog88

We all know that the closer you are to an AM transmitter, the stronger and clearer the station will be on the receiving radio.

I'm under the impression that the stronger the receiving AM broadcast is, the better the high end frequencies in the audio will be, coming close to 10 kHz. The weaker the broadcast, the worse that the frequency response will be, being capped at around 7-8 kHz, as anything above that can't overcome the noise floor. Can anyone verify if this is true, or does it all depend on the quality of the radio's tuner?
 
No. The field strength has no bearing on the bandwidth. The fractional bandwidth (10 kHz) is very small compared to the carrier (at least 530 kHz), so the physics don't make sense for different portions of the same AM signal to propagate differently.

It is possible certain tuners may detect a weaker signal and narrow the bandwidth, but it is not a fundamental property of mediumwave broadcasting.
 
The biggest issue with AM bandwidth (beyond the 10 kHz hard stop) is that many AM antennas have a different j (reactance) on either side of the carrier. On very tightly tuned antennas, that can create issues. The reasons for this can be either wanting easy tuning of the tower or issues like short towers or directional systems.
 
I thought there's a spectral mask on AM, where the frequencies from 5 to 10 kHz on either side of the carrier are reduced, compared to the signal +/- 5 kHz. The reduction of frequency response is noticeable when driving under an overpass, and the fidelity of the signal is reduced as the signal strength goes down.
 
KeithE4 said:
I thought there's a spectral mask on AM, where the frequencies from 5 to 10 kHz on either side of the carrier are reduced, compared to the signal +/- 5 kHz. The reduction of frequency response is noticeable when driving under an overpass, and the fidelity of the signal is reduced as the signal strength goes down.
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As a person who got into radio as a DXer, I can say that distance does not reduce fidelity. From Cleveland, OH, I got 250 watt KIKI in Honolulu with just as good audio as a local.

The only "restriction" on audio over about 5 kHz comes from the design of radios in the last three or four decades. Bob Orban headed an NAB commission in the early 2000's regarding AM stereo and showed that almost every radio made fell off severely at some point as low as 4800 kHz and no higher than 6 kHz.

For that reason, stations contemplating AM stereo at that time realized that reducing the "over 5k" audio would have a minimal effect on analog listeners and, those few that implemented iBiquity AM stereo put in effect limits on audio over 5 kHz.
 
OK. To further explain, I have several tapes that I recorded from 2000 up to 2007 of an AM station with a music format back in the day. Some of the earliest ones were recorded on a Sony CFD-D73, and the rest were recorded on a Sony CFD-8. The ones recorded on the CFD-D73 have better dynamics and frequency response, and the radio was located at a relative's house who was within the local contour of the broadcast. The rest were mostly recorded on the CFD-8, located just outside of the local contour, where the frequency response on those recordings seem a bit lower compared to the CFD-D73. However, there were times when I did bring the CFD-8 over to that same relative's house and made some recordings of that same station, and even those sounded a bit dull, but typical of an average AM broadcast. This was kind of what made me bring up this topic.

I think a lot had to do with the tapes (mostly name brand type I's), as they were previously used in the past, and were likely in a degraded state by the time those recordings were made. The CFD-8 was also showing its age as well, so I'm sure that also played a part. Some of the later recordings had sections where one channel would fade out for several or more seconds, which I corrected or used the other stereo track of the recording (which served as redundancy for the mono broadcast) when I digitized them. And yes, when I digitized these tapes, I made sure the azimuth of the head was adjusted accordingly for each tape for the brightest sound. One was way off, which I was able to fully correct after turning the adjustment screw 3-4 times. The CFD-D73 could have had better circuity in either the tuner or the tape player, as all my recordings made with that model really brought out the quality of the broadcast and sounded bright. Or it could have likely been the station itself naturally degrading over time as well.
 
davideduardo said:
For that reason, stations contemplating AM stereo at that time realized that reducing the "over 5k" audio would have a minimal effect on analog listeners and, those few that implemented iBiquity AM stereo put in effect limits on audio over 5 kHz.
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A family member has a Ford Escape with HD Radio. AM broadcasts are capped at 5 kHz in order to filter out the side-bands, regardless if the station is broadcasting a HD carrier or not.
 
snowdog88 said:
A family member has a Ford Escape with HD Radio. AM broadcasts are capped at 5 kHz in order to filter out the side-bands, regardless if the station is broadcasting a HD carrier or not.
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Most AM tuners have had bandwidth below 5 kHz for decades. A 2006 study from the National Radio Systems Committee found that the average car in-dash receiver had an AM bandwidth of just 3 kHz.

 
In 2019, I drove a (then) new Cherokee which had a infotainment system in it. The AM section wasn't too bad. Pretty impressed with the audio quality and reception. I drove under high voltage lines and the thing choked the audio to a rumble until I passed. It seemed to have a way to filter noise and one of those was reducing audio bandwidth depending on the severity of interference. The AGC was aggressive, but it was set up so well.

I had a Sanyo M W250 with dual cassette and multi-system AM Stereo. There were several AM station stations in my area and I recorded them. The audio in the recording was cut off below 10 kHz as you'd expect, but the reciever had the ability to pass higher frequencies. The M W250 was made during a time when some AMs were still passing audio at 15 kHz.
 
One way radio makers do a cheap "noise reduction" on AM is to pull the audio bandwidth way down when there's static (like lightning) or a low signal.
Just muddies it. You're luck to get 7 KHz audio on most current radios.
In the US transmitted audio bandwidth is individual to the station. Some 5 KHz, some 7 KHz, some 9.5 KHz.
FM they blend the stereo to mono to hide multipath.
 
Obviously I failed to reply here. There was a time (and I have heard older radios that did this) where AM frequency response was much better than it is now. The downside to that better response came in places where two AM stations on adjacent frequencies could be heard clearly (such as Galt, California, with Stockton's 1230 kHz station and Sacramento's 1240 kHz station vying for your attention.) Essentially, what happened in these locations (as well as during evening hours in many more locations because of skywave) was that there was a noticeable high pitched squeal (I believe it's called a hetrodyne but I'm sure I'll be corrected) when attempting to listen to any of the two adjacent stations affected by this problem. I'm guessing here but I believe that narrowing the frequency response helped to ease the hetrodyne issues but the downside of that was to make AM sound quality on most receivers very poor compared to FM.
 
secondchoice said:
Recently an AM station went "all digital." Did they start their data at 5 khz or did they use the entire 10khz channel?
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AM channels in North America are actually 20 kHz wide -- 10 kHz on either side of the carrier, because AM is double sideband. Yes, this causes overlap with adjacent channels, but stations are supposed to be spaced far enough apart and/or have sufficient directional pattern protection for this to not be a problem, at least during the daytime.

Digital-only AM IBOC (HD Radio) stations contain the digital signal within ±5 kHz of the carrier, so that (theoretically) there will be no adjacent-channel interference between digital-only signals on neighboring channels.

Oh, and buried in the technical notes of that early 2000s test by iBiquity "proving that AM receivers don't have any audio response above 5 kHz" was the fact that the transmitter used to conduct the test had its audio bandwidth restricted to 5 kHz! The NRSC later revisited the topic with tests conducted at various bandwidths, and recommended not reducing the transmitted audio bandwidth to any less than 7 kHz, or else it would impair audio quality even on narrowband receivers.

And wideband AM audio can certainly make it through DX reception, especially on a receiver with a synchronous detector. 900 CHML in Hamilton, Ontario (now dark) was transmitting with an audio bandwidth of at least 12 kHz, nearly FM-quality:

 
ted chittenden said:
Essentially, what happened in these locations (as well as during evening hours in many more locations because of skywave) was that there was a noticeable high pitched squeal (I believe it's called a hetrodyne but I'm sure I'll be corrected) when attempting to listen to any of the two adjacent stations affected by this problem. I'm guessing here but I believe that narrowing the frequency response helped to ease the hetrodyne issues but the downside of that was to make AM sound quality on most receivers very poor compared to FM.
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Yes, that's present on many of my recordings as well, acting like a pilot tone. I used a notch filter to remove that squeal (at 10 kHz) in my digital transfers, and I think some wide band receivers were even capable of notching out that sequel as well.

secondchoice said:
So does that means if an non IBOC AM reciever (old) picks up the station all you would hear would be screaming data?
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When there were IBOC carriers on a couple of the AMs in my area, it was hard to tune out the sidebands with my Sony CFD-8, and it required precise tuning to minimize, but not completely remove, the the hissing caused by the carrier on the adjacent frequencies. I imagine that receivers that are capped at 5 kHz completely filter out the IBOC carrier, at the expense of frequency response.
 


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