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AM Stereo Sound

Leonard Kahn had an article in the 1960 NAB Engineering Handbook about Full Carrier Compatible Single Side Band. It was used to lessen adjacent channel interference between WINS, KDKA, and WBZ. Nothing new under the sun I guess. In that article, he discussed how the single sideband phase modulated the full carrier. It was in a way that could be represented as a spinning vector from the carrier vector going CW or CCW depending on whether it was USB or LSB that was used. Hence if there was both a USB equal to the LSB there was no phase modulation. If USB and LSB were not the same, they HAD to phase modulate the carrier in some way. I think that this was the "secret" that Leonard Kahn used to talk about that allowed other AM stereo detectors to detect the Kahn system. All that was needed was the pilot added, which accidentally happened from difference frequencies between pure audio sine components and intermodulation of audio frequencies. All that had to happen was it to persist long enough to activate the detector. Meanwhile, Leonard Kahn maintained that it was not based on phase modulation, that the sidebands were completely independent. But R-L had to phase modulate the carrier. Not sure about the linearity, but I suspect it was quite linear also.

Anyone have another theory or explanation?
 
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SSCFC (SSB w/ full carrier) is easy to do...but really does not help any....with most AM radios using an envelope detector, it sees both sidebands....reducing one sideband will not improve reception on the other unless the rcvr is made to do so..(BTW SSBFC is easy by running the audio and carrier into an unbalanced modulator...which passes the carrier through...then filter out the unwanted sideband)...can be done using phased techniques too...
Kahn's ISB system had issues with antenna arrays AND digital radios...if the array was not flat over the entire channel, the freq response of one channel would be different than the other...NOT good for stereo...also Kahn never licensed rcvrs for his system....Sony came out with a 2 chip reverse engineered method to get around that...Digital radios killed the "tune low for left channel and high for right channel" two radio idea..so with few radios able to decode Kahn, he was fighting a losing battle....
Belar's AM/FM system was the easiest to implement but they had problems in distortion in the rcvrs...the AM was normal L+R mono...the FM was 400uS preemphasized fed with the L-R at low level deviation...the use of FM though also made the transmitter not stay within the 20Hz frequency tolerance for AMs...thus would create some wild whistling hetrodynes with cochannel issues....
The AM/PM systems like Harris and Motorola didnt have the stability issue but suffered platform motion bad (where the skywave phase shift would cause the center to shift to left then right and back to center...later versions in the rcvr overcame that for the most part)...
As to audio fidelity, yes I know the NRSC is what caused the AM station to limit audio to 10kHz...one AM I worked at in the 70s was capable of passing up to 30kHz audio...but we filtered out above 15 with a six band EQ ahead of the transmitter (No we werent running stereo)...
AM stereo was a good idea...but done wrong..NOW with DSP and Noise Blankers, it could be done right...but the manufacturers dont want to spend the extra $$ to do it right..with the image of AM now, noone would buy it anyway except those of us who know the potential!

(I have several Sony AM stereo radios..used to listen to WLS at night back in the 80s...and they still run CQUAM on 890...also KKBQ AM in Houston on 790 was Kahn then CQUAM in the 80s as well...) and no there was no front/back separation / quad..only left and right channels...
 
I would've loved to have heard AM stereo, but none of the stations in my area were experimenting with it. It seems to have been a great opportunity lost. Now the only thing on AM is talk radio, and there's no point doing that in stereo...the sound would only come out of the right speaker! :)
 
I don't know about the Kahn system, but the only AM Stereo broadcast I heard was WSM during a trip to the SE US in the late 1990's. Even on my SRF-42, it sounded good. Like a mellower sounding version of FM stereo. There was a little bit of platform motion, but not all that much.
 
I believe WSM used the Kahn system, at least at first.

My observation was that any station with asymmetrical side bands sounded bad, not specifically Kahn. If you didn't use very resonant 90 degree ground plane towers, it had better frequency response. Nondirectional sounded better than directional, since a phasing network is also inherently asymmetrical with freqeuency.
 
WSM was using C-QUAM by the late 80's. I had a radio that only detected C-QUAM and it always lit the light when I could hear it.
I never heard any platform motion on an SRF-42, no matter how weak the signal.
 
Part of the platform motion had to do with fading, and fading would be much less on 650 than on stations at the high end of the band. Not being sharply resonant and lopping off sidebands without compensation, being nondirectional and not having extensive distribution and phasing networks, being a Clear Channel (Webster's Sense 1) station, and being on a low frequency had much more to do with the lack of platform motion and sound quality on WSM than the AM Stereo system that it used. In fact, Leonard Kahn hand picked the stations that would sound best. He really didn't want those 5 kW, 3 90 degree tower in line DA, stations above 1200 kHz. That said, Leonard Kahn did invent AM Stereo in many people's opinion. He just didn't market it well. It was like a marketing scheme developed by Sheldon Cooper, no offense intended, he just needed non geniuses to market his product like Penny.
 
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Schroedingers Cat said:
I believe WSM used the Kahn system, at least at first.

My observation was that any station with asymmetrical side bands sounded bad, not specifically Kahn. If you didn't use very resonant 90 degree ground plane towers, it had better frequency response. Nondirectional sounded better than directional, since a phasing network is also inherently asymmetrical with freqeuency.
Click to expand...

A lot of AM Stereo stations started as Kahn because it seemed a great way to get listeners to hear it with two analog tuned AM radios...The sideband issue affected Kahn's audio response more than anyone else since the lower sideband was L and upper sideband was R....if one reactance curve was off, the freq response was different than the other channel...Also Kahn had NO stereo separation above 7-8kHz...it started to show less sep around 5-6kHz.....whereas the AM/FM and AM/PM systems showed full separation to max audio. Any of the other systems besides Kahn could be adjusted to overcome any phase issues in the directional pattern....Biggest issue was the out of channel emissions....Kahn argued in a lawsuit that Motorola's CQUAM went well beyond the FCC mask limits...and HE WAS right....The Harris system met the requirements...and the Belar was close....However, Kahn refused to license receiver designs for his system....so with Motorola making car radios already and being able to easy add the AM Stereo decoder chip in it, it was easy to see who was going to be the winner in that fight...790 in Houston during the Q Zoo days was a Kahn system then a CQUAM...I heard both and the Motorola definitely sounded better...and it even sounded fuller than the FM did!! It has 8 towers and two patterns...the switch to Motorola definitely improved the separation and the overall frequency response.

BTW WLS still broadcast in CQUAM.....(they started with Kahn but switched after a few years)...even when LS was running IBOC daytime, they would run CQUAM at night...during the Big 89 Rewinds, they were in full CQUAM and sounded fantastic..I have the audio off the air monitor in the studio...complete in CQUAM with reverb and compression....sounds as good as a FM or CD to my ears!
 
My AM station ran C-QUAM. I kept the sidebands legal by reducing the phase-modulated L-R level. It reduced the maximum stereo separation slightly but the RF signal was clean.
One problem with the Kahn system was that adjacent channel 'splatter' could affect each channel differently.
 
JeffM said:
I would've loved to have heard AM stereo, but none of the stations in my area were experimenting with it. It seems to have been a great opportunity lost. Now the only thing on AM is talk radio, and there's no point doing that in stereo...the sound would only come out of the right speaker! :)
Click to expand...

Ba-dum-PRASH! (rimshot)
 
WJIB AM 740 Cambridge MA broadcast on AM stereo in the past, maybe still does. "The memory station"

WBZ 1030 Boston is on an HD 3 of WBZ-FM 98.5...was listening a few yrs ago to one of the their talk shows
(Steve Leveille who has since retired) and got to hear his intro music in stereo for a change...
 
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