(AM) C-QUAM vs. Mono

[kfbkfb]

Various articles and statements about the C-QUAM AM stereo system over the years have said that the range of the signal is lower than mono AM (for the same transmitter power level).

AFAIK, the signal strength (at a given distance) of the L+R portion of the C-QUAM signal is the same as a mono AM signal.

(KCJJ 1630 AM stereo [did the matrix quad broadcasts encoded for Dolby Pro-Logic 2 music decoding in 2011] switched back to mono in 2012, they stated that their signal coverage was less w/AM stereo)

Does C-QUAM reduce the coverage of an AM station?


Kirk Bayne

 

[ContinuousWave]

NO!! It does NOT...it was a false rumor started by Kahn in his fight against CQuam..
My AM has the same range in either mono or CQuam stereo. Anyone who says it does is a liar and full of it

 

[Kelly A]

Does C-QUAM reduce the coverage of an AM station?

Depending on certain factors, the loss of perceived coverage for some CQUAM stations wasn't actual measured coverage, or lack of field strength. There had been stations who received numerous listener complaints because of either how the station sounded to them. Their assumption was the station suffered from less coverage than it had before installing AM stereo. That was the primary reason back in the day, I personally removed several CQUAM systems. After doing so, listeners seemed to notice the change in a positive way.

 

[kevtronics]

KCJJ's claim of increased coverage when they switched to mono was because they were transmitting +/-95% modulation in C-Quam and then upped it to +125%/-100% in mono. Even though C-Quam has no trouble transmitting that modulation level as well.

It was the defunct Magnavox AM Stereo system which couldn't exceed -95% negative modulation because it would cause a popping sound in the receiver. They defended it by saying -- and rightly so -- that the actual difference in loudness is minimal and even with a pure mono signal, exceeding -95% causes a drastic increase in distortion in ordinary envelope-detector AM radios. Nonetheless engineers shied away from the Magnavox system because of that limitation.

 

[ContinuousWave]

Actually Motorola recommends -75% modulation maximum while the positive can go 125 without any problem. The Magnavox system had horrible distortion... Which makes me wonder why the FCC chose it is the standard in the first place

 

[Kelly A]

KCJJ's claim of increased coverage when they switched to mono was because they were transmitting +/-95% modulation in C-Quam and then upped it to +125%/-100% in mono. Even though C-Quam has no trouble transmitting that modulation level as well.

During testing with Bob Carver back in the day, we discovered the problem wasn't that the station couldn't run asymmetrical modulation, it's that the receiver chips had problems with demodulating it without audio performance suffering. Received THD and IM rose significantly, and stereo separation went into the toliet if the station exceeded 100% positive and 98% negative.
That was much of the push back by station management. The mono competition was much louder, plus got that little extra fringe coverage advantage by heavy asymmetrical processing. We tried to explain that stereo doesn't work well in that environment, so the choice was always made between stereo and mono with 125% positive peaks, mono won every time.

It was the defunct Magnavox AM Stereo system which couldn't exceed -95% negative modulation because it would cause a popping sound in the receiver. They defended it by saying -- and rightly so -- that the actual difference in loudness is minimal and even with a pure mono signal, exceeding -95% causes a drastic increase in distortion in ordinary envelope-detector AM radios. Nonetheless engineers shied away from the Magnavox system because of that limitation.

Wives tail. From my experience testing the Magnavox system, we never found any popping over 95% modulation, positive or negative. You're right in a sense that Magnavox wasn't willing to guarantee stereo performance over 95%, especially negative, but it seemed to work fine. Magnavox was originally chosen by the Commission, but Leonard Kahn started crying fowl and suing people. Ultimately Magnavox determined AM stereo wasn't a business they wanted to be in anyway, and backed out of the running.

 

[Greg Strickland]

Was an audio processing change made? In an otherwise good audio system, in a competent market, better audio quality nearly always results in lower loudness. I am thinking installation of stereo may have motivated tweaking the sound. Today, good chance some AM stations sound a bit better when they acquire a FM translator, because the audio chain may be changed, or the FM sound of the format alters their mindset when adjusting the AM.

 

[frankberry]

During testing with Bob Carver back in the day, we discovered the problem wasn't that the station couldn't run asymmetrical modulation, it's that the receiver chips had problems with demodulating it without audio performance suffering. Received THD and IM rose significantly, and stereo separation went into the toliet if the station exceeded 100% positive and 98% negative.
That was much of the push back by station management. The mono competition was much louder, plus got that little extra fringe coverage advantage by heavy asymmetrical processing. We tried to explain that stereo doesn't work well in that environment, so the choice was always made between stereo and mono with 125% positive peaks, mono won every time.

Wives tail. From my experience testing the Magnavox system, we never found any popping over 95% modulation, positive or negative. You're right in a sense that Magnavox wasn't willing to guarantee stereo performance over 95%, especially negative, but it seemed to work fine. Magnavox was originally chosen by the Commission, but Leonard Kahn started crying fowl and suing people. Ultimately Magnavox determined AM stereo wasn't a business they wanted to be in anyway, and backed out of the running.

In my "real world" experience, when the C-QUAM modulation monitor indicated "high angle," that meant that you were nearing the limit of the L-R phase modulator. It would occur at about 85% modulation. Exceeding that level could cause distortion of the L-R audio.
My solution was to increase the L+R amplitude modulation level. It would reduce the stereo separation a bit, but the station would be as loud as anybody else on the dial. The result was about 25dB separation, rather than about 40dB when the L+R was adjusted for maximum stereo separation. 25dB isn't that bad. It's comparable to the separation on vinyl records.

 

[ZantennaG1]

@kfbkfb I would probably be able to run some tests on 610 KCSR's stereo, however what I have found already mirrors what the posts above have said. I don't lose any mileage on their C-QUAM system, but I do get some sort of high-pitched noise if I'm between 0.5 and 1.25 mV/m.

 

[DavidEduardo]

Depending on certain factors, the loss of perceived coverage for some CQUAM stations wasn't actual measured coverage, or lack of field strength. There had been stations who received numerous listener complaints because of either how the station sounded to them. Their assumption was the station suffered from less coverage than it had before installing AM stereo. That was the primary reason back in the day, I personally removed several CQUAM systems. After doing so, listeners seemed to notice the change in a positive way.

I had CQUAM removed from KTNQ in LA around 1995 as the combination of platform motion and problems in our deeper nulls was just not worth it, considering the lack of compatible radios. We noted a marked improvement in the null areas which were critical to covering the LA market.

There was a later solution to the platform motion but it was too late: by the time it arrived none of us would spend on new AM stereo devices since the car makers and radio fabricators had moved on.

 

[Kelly A]

In my "real world" experience, when the C-QUAM modulation monitor indicated "high angle," that meant that you were nearing the limit of the L-R phase modulator. It would occur at about 85% modulation. Exceeding that level could cause distortion of the L-R audio.
My solution was to increase the L+R amplitude modulation level. It would reduce the stereo separation a bit, but the station would be as loud as anybody else on the dial. The result was about 25dB separation, rather than about 40dB when the L+R was adjusted for maximum stereo separation. 25dB isn't that bad. It's comparable to the separation on vinyl records.

Bob and my tests were "real world" from a consumer receiver/demodulation perspective, not a modulation monitor. Ultimately, that's where the rubber meets the road.

 

[Kelly A]

I had CQUAM removed from KTNQ in LA around 1995 as the combination of platform motion and problems in our deeper nulls was just not worth it, considering the lack of compatible radios. We noted a marked improvement in the null areas which were critical to covering the LA market.

Similar to the complaints I heard too. Stations that had relied on listeners in a null seemed to suffer from reception difficulties while in stereo, but not mono. Unlike FM, there are just so many factors with AM stereo that can harm or negatively effect the listening experience. Most stations felt the commitment to stereo wasn't worth the hassle.

There was a later solution to the platform motion but it was too late: by the time it arrived none of us would spend on new AM stereo devices since the car makers and radio fabricators had moved on.

The second generation Motorola units claimed to have solved platform motion, but based on listener complaints in fringe areas or nulls, it was still a factor.

 

[frankberry]

Bob and my tests were "real world" from a consumer receiver/demodulation perspective, not a modulation monitor. Ultimately, that's where the rubber meets the road.

A modulation monitor is the standard. Receivers are not.
Audio performance testing of any broadcast transmission system begins with measurements from a calibrated modulation monitor.
If those tests show that the performance meets an acceptable level but tests from a receiver do not, the problem lies with the receiver.
That is the whole point of my comments.
From what you have written, I'm guessing that the receiver(s) had issues with bandwidth, non-symmetrical upper and lower sideband response. This is not unusual in receivers.