AM Stereo

[ContinuousWave]

And Greg Buchwald of the Motorola Cquam team said this on FB in the AM stereo group:
"That is so incorrect. First, the quadrature carrier is suppressed. If we stopped there, we would have linear QUAM much as Harris produced. That will result in proper carrier level but the envelope term is√ (I^2 + Q^2). Therefore a single tone that contains L-R will have second harmonic distortion. Further, multiple tones (or music /voice) that are stereo contain IM distortion. CQUAM modified the signal by multiplying the resultant QUAM signal by the cosine of the instantaneous phase term. That restores the signal envelope to L+R and no envelope distortion. In reality, L+R is simply applied, in time equalized fashion (and that is the real magic in the exciter) to generate the proper L+R term. The receiver, therefore must divide the incoming signal prior to L-R by cosine phi. This is usually done by feedback utilizing the E and I detectors to extract the cos phi term from the phase modulated signal (not from E as Kahn had sued patent infringement over and lost...another story as I presented that proof to the judge). The IF signal is divided by cos phi and the Q detector now demodulates a clean L-R signal. Misunderstanding of this concept has historically been fairly common. One of the smartest broadcast engineers, Al Resnik, once made the identical statement at an NRSC meeting. I really liked Al but he was an early Kahn, then Harris fan before switching to cquam. I went up to the board, preapologized for what I was about to do, and then showed his error. We spent the evening laughing about it over a few drinks! Al was chief at WLS before taking over for ABC in NY."

 

[ContinuousWave]

John Pavlica, Jr added this comment:
"Only the original Harris system was designed for synchronous detection, and it was Kahn and his 90 degree warped audio that had the diode distortion issue. AFAIK, the Motorola CQUAM, Magnavox and Belar systems didn't have any 'penalty' for diode detectors on old mono radios and the signal covered just as far as plain mono analog. This is misinformation based on my old booklets and the FCC test results which graded each system for frequency response (Harris & Motorola winners; Kahn loser) envelope/diode detection distortion (Kahn loses again). It is true that CQUAM uses the I and Q phase detection, but so does iBoc for that matter. "

 

[ContinuousWave]

You can post any replies to the AM Stereo CQUAM group on FB if you wish to join.
There are currently 844 members and healthy discussion daily about new stations going Cquam and receivers, etc. along with sound technical discussions

 

[frankberry]

Agree, but listeners perception of a loss of coverage is what matters.

The issue with a perceived increase in noise is most likely due to a slightly different noise profile on each received channel. That could make the noise more noticeable than on a mono receiver.
As for coverage on a mono receiver, in my tests, there was absolutely no difference when I switched between the Delta AM stereo exciter and the transmitter's internal crystal oscillator. No difference whatsoever. Of course, as I explained in an earlier post, I had my systems adjusted so that the L-R audio did not cause excessive sidebands and then increased the L+R amplitude modulation so I could hit 125% positive peaks. I lost a little bit of stereo separation but gained a couple dB of loudness.

 

[frankberry]

You can post any replies to the AM Stereo CQUAM group on FB if you wish to join.
There are currently 844 members and healthy discussion daily about new stations going Cquam and receivers, etc. along with sound technical discussions

Now 845 members.

 

[kenglish]

So, did no one ever feel the need to find a quantifiable reason why AM Stereo sucked? I thought that was what defined engineers and scientists.

 

[davideduardo]

So, did no one ever feel the need to find a quantifiable reason why AM Stereo sucked? I thought that was what defined engineers and scientists.

It was not so much that it sucked but that it was too late to matter.

After all the legal hassles that postponed AM stereo from possibly debuting in late 1978 or early 1979, a confusing FCC non-decision allowed what became a slow roll-out nearly five years later.

By that time, nearly all music format listeners in the sales demos had migrated to FM and the remaining music AMs either appealed to older listeners or were trying to make declining formats work some how.

As a sidebar, it should be noted that by the end of the FM growth decade of the 70's, most stations had optimized their facilities to the best power and height available. In the process, and in most markets, that meant that the "average" FM covered much more than nearly all of the AM stations. For example, in Cleveland, Ohio, only one AM (50 kw 1100) covered the market as well as nearly all of the full Class B FM stations. The rest of the market's AMs were either limited at night (850 and 1220) or just plain limited (1260, 1300, 1420, 1490, 1540). So the average FM was vastly more "listenable" than nearly all the AMs.

Receiver manufacturers did not sense a demand for AM stereo, so there were not many radios out there that could detect it.

By the end of the 80's, while quite a few AMs had the system installed, there was no perceived benefit from it.. As the equipment aged, it was disconnected. And some directional stations noted unpleasant artifacts in the null areas and others objected to the platform motion that was noxious to some listeners in mobile environments... sort of like airsickness.

So it was not so much that it sounded bad; in fact, it did not. It just was too little and way too late.

 

[kenglish]

Receiver manufacturers did not sense a demand for AM stereo, so there were not many radios out there that could detect it.

I've always felt that the receiver manufactures (and retailers) are the biggest killers of broadcast-related technologies. That, and the fact that the broadcasters themselves rarely promote their technology advancements.
If the public doesn't know about it, they don't buy it. If they aren't buying it, no one sells it. If no one is trying to sell it, no one (other than geeks in their garages) will make it.
Manufacturers won't waste their efforts on technologies that broadcasters themselves won't promote.

 

[Radig]

I've always felt that the receiver manufactures (and retailers) are the biggest killers of broadcast-related technologies.

That seems to be the situation with DRM (Digial Radio Mondiale).

 

[frankberry]

AM stereo could sound very good. Back in the day, I drove a new Cadillac which had an AM stereo radio. The radio had an extended high frequency response. The FCC's NRSC 10kHz mandate was the first nail in the AM stereo coffin. It took the "sparkle" out of the audio.

 

[Theater of My Mind]

What took the "sparkle" out of the audio was the glut of terrible sounding radios with narrow frequency response that rolls off by 3,000 Hz.

In my opinion the FCC should have mandated much higher equipment standards and the NAB should have done a better job of lobbying for it. Same with AM Stereo and then with HD Radio. I know there were, and still are, free-market politics involved but when you let the market decide, the real deciders are the manufacturers who won't spend one cent more than they have to, and then you end up with a market full of crap.

 

[davideduardo]

That seems to be the situation with DRM (Digial Radio Mondiale).

Manufacturers produce product that they think will have demand. AM radio in the US can't create demand.

On the other hand, in India there are factories being built to produce DRM radios as that nation has been installing a network of DRM stereo AM stations to cover the whole country in the majority of the significant languages.

There, rather than having to install thousand of FMs and repeaters, they are going with powers up to a million watts to cover vast areas, along with transmitters in the higher population cities to get good coverage of the whole nation.

The manufacturers there are expanding manufacturing capacity to keep up with demand.

 

[Kelly A]

I actually got to witness some SW DRM tests. I thought it was pretty impressive, and easier to integrate than the other systems.
But, as David says, if the listening public has abandoned the medium for greener pastures, there won't be anyone to hear it, nor will manufacturers be willing to supply products to hear it. Manufacturing and marketing products to an audience which doesn't exist, is bad for business.

 

[Theater of My Mind]

Manufacturers produce product that they think will have demand. AM radio in the US can't create demand.

There was still healthy demand for am radio in the 70s into the 80s when there was an opportunity to create those standards though, that's the point.

On the other hand, in India there are factories being built to produce DRM radios as that nation has been installing a network of DRM stereo AM stations to cover the whole country in the majority of the significant languages.

There, rather than having to install thousand of FMs and repeaters, they are going with powers up to a million watts to cover vast areas, along with transmitters in the higher population cities to get good coverage of the whole nation.

The manufacturers there are expanding manufacturing capacity to keep up with demand.

It looks like India's national broadcaster in taking the lead on that so while I don't know the politics over there, it seems the government is involved in creating and promoting the standard, not unlike what we saw in the US with the transition to digital TV.

 

[Kelly A]

There was still healthy demand for am radio in the 70s into the 80s when there was an opportunity to create those standards though, that's the point.

But not if the majority of music listening had already fled in droves to FM.

It looks like India's national broadcaster in taking the lead on that so while I don't know the politics over there, it seems the government is involved in creating and promoting the standard, not unlike what we saw in the US with the transition to digital TV.

India's broadcast stations are indeed government controlled. If their government/station owners decide to switch their station's modulation type, they just do it.

 

[semoochie]

AM stereo could sound very good. Back in the day, I drove a new Cadillac which had an AM stereo radio. The radio had an extended high frequency response. The FCC's NRSC 10kHz mandate was the first nail in the AM stereo coffin. It took the "sparkle" out of the audio.

Part of the problem was requiring a pre-emphasis circuit for existing radios that was intended to be de-emphasized in new, wideband radios but they were never built!

 

[bossbill]

I'm with Kelly on this subject. But I'm new at this radio stuff... not quite 60 years in the biz.

 

[davideduardo]

But not if the majority of music listening had already fled in droves to FM.

India's broadcast stations are indeed government controlled. If their government/station owners decide to switch their station's modulation type, they just do it.

There are private stations there, but they are individual FMs in the larger cities . The new national service that has been rolling out for several years is AM, which is an efficient band for large area coverage in a country that does not have as strongly "independent" sub-divisions as the US does with its states.

India's issue is its multitude of languages, so having national services in each of the principle tongues is the efficient way to go.

Even so, India is only about a third the physical size of the US, but has four times the population. So the average density is 12 times more congested. That means that even the "rural coverage" of a big high power digital AM is going to be important.

As you say, in the US by the time AM stereo was authorized (sort of) music listening was mostly on FM. In India, the limited FM authorizations are very recent... last 20 to 25 years... and not numerous such as in Europe or the Americas.

And broadband Internet is not financially possible for a huge percentage of their population. Over 1.2 billion people there make less than U$S 500 a month and half of those make under U$S 250 a month. So free, easy to use AM radio is very efficient.

 

[davideduardo]

There was still healthy demand for am radio in the 70s into the 80s when there was an opportunity to create those standards though, that's the point.

The interest in AM radio by younger persons died in the mid 70's; by 1977 half of all listening was to FM and more than 75% of music listening was on the "new" band.

As I mentioned before, once the FMs maximized their facilities in the early 70's (for the most part) nearly every market had far more full coverage FMs than AMs, so there were more choices and better quality.

It looks like India's national broadcaster in taking the lead on that so while I don't know the politics over there, it seems the government is involved in creating and promoting the standard, not unlike what we saw in the US with the transition to digital TV.

India's government severely controls broadcasting, with very limited private radio. By "control" I mean they own the stations, and are much more than regulators. Because of all the different languages, the government takes on the job of making sure every group is informed.

 

[landtuna]

There, rather than having to install thousand of FMs and repeaters, they are going with powers up to a million watts to cover vast areas, along with transmitters in the higher population cities to get good coverage of the whole nation.

Then we are to assume we can expect a certain population living near the transmitting antennas to glow in the dark?