Time for new Speed Limit? 150% AM

[speakerman]

AM fidelity is an oxymoron. I gave up once they forced the NRSC 10kc filter. AM Pre-emphasis made up for some of the HF roll-off of the HI-Q IF sections. Fuel to the fire has been added by having to carry the IBOC load also. The only way I can see the AM band can be made "High Fidelity" is to drop the analog and go total digital just as terrestrial TV is getting ready to do shortly. 10kc is not a lot of bandwidth to work with however the new advanced coding schemes could get the job done for good stereo.

It would be a shame to see all of the analog detectors made obsolete overnight and it would never be done as AM does not have the cable company (the majority of TV viewers) to act a a go between. Total digital conversion would immediately eliminate all of the current listeners as a converter box would not work in this situation.

The listening public (especially the younger demographics) have a plethora of multi-channel true high fidelity choices they use daily. I would bet the majority of the upcoming demographics do not even consider AM an option in their media choices. It's like asking a young kid to dial a number for you on a rotary set.

 

[FredRichards]

You can't solve a listener issue with a technology solution when it's a content problem. I would love to see better fidelity on AM radio, and as 40+ veteran of this craft I know it is technically possible.

I don't think going to a higher modulation limit would help. We should have learned a lesson about more is better when we made all Class IV stations 1kw day/night as the means of overcoming on channel noise. Bob nails the point well about processing and transmitting content that can not be recovered on conventional receivers.

Think of radio being like Sears Craftsman tools. We have some of the best tools available to us. But many people in the industry continue to drive three-penny nails into a 2x4 with a wrench because they can. We don't need a bigger wrench. We need to learn that the hammer works the best for driving nails.

 

[Tom Wells]

We need to make fun of even using nails when fine joinery and screws are called for.
Once you develop some really fine, small, high quality nails and develop a method suiting such I may be interested.
I am not inclined to have "nail capentry" forced upon me when I can see it looks awful from 10 feet away.

If AM is not analog anymore then it's the DM band, and I will call it what it is.

You can't solve an engineering problem caused by the FCC's lack of teeth on pt 15 unintentional radiator rules by dumbing down
your signal quality.

The real problem is bad engineering in all the other devices. Why has the FCC no teeth in enforcement of design type acceptance
for traffic lights, etc? They need to be putting the screws to industry other than radio, if they're not in compliance.
We could have the FCC raking in huge fines from the folks causing the interference.
As I was growing up, this what I thought the FCC charter was about, keeping the service viable by maintaining the standards and
compliance.

The mod increase limit would be another band-aid, because 125% sounds pretty darn good on most radios and gives
good punch without driving too many radios into overload. And as (R Fry?) pointed out, once you've gone asymmetric
that in itself IS distortion by definition. An ear-candy tradeoff that hurts as much as it helps if studied analytically.
It gets used because it's louder. If done well, it doesn't sound weird, but it's very subjective case by case, and
this is where processor manufacturers rightfully make their money.

Any louder is just "shouting" when we should be policing the noisemakers instead.

 

[FredRichards]

Agreed, Tom. Excellence should be sought by all means. However, it's not like we don't have the means. The tools are there.

I think Part 15 is only a minute segment of the issue. As I tune the dials I hear noise from devices that don't qualify as radiation devices. My charger for my cellphone, the LCD display on my postage scale. Even my portable minidisc spews garbage by the gallon, all affecting the quality of what the public may want to listen to. The computer on my wife's car normally takes a readable signal to noise once the car is started. Perhaps there should be legislation to tighten noise restrictions to RF devices. Can't you just hear the power companies howl?

Still, I think radio could do better to produce fine carpentry, rather than making fruit baskets and claiming they are choice furniture.

Using my kids as an example, they don't listen to radio. They listen to MP3s from the web. The issue is not noise, or fidelity of sound. Their needs are content. Same with me, if I listen to radio, I listen to an out of town station on the fringe. It's not because of their fidelity, but because locally there is no content of my choosing.

I'm all for doing more to better sound quality. But it's moot without having a product that the public wants, and needs.

 

[OKCRadioGuy]

IF enough HD radios ever get put out into the market, at some point some of the AMs can just choose to switch over to 100 percent digital. I hate to say it, but if some AMs would go ahead and go dark the band would be better off. This is a case of where less would be more. If enough go away adjusting out the band allocations for the remaining would be a good idea. Nothing is going to be a magic bullet other than utilzing virgin spectrum to put pure IBUZ or DRM on the air. Personally I'd rather see DRM as the licensing for radios isn't a cost problem. Too bad someone didn't have the forethought to reserve the expanded AM band for digital instead of doing what they did with it. It would make a nice local service band at 1k of digital IMHO. Of course channel 5 and 6 TV band would be the BEST place at this point, but politics will prevent anything sensable from happneing with that idea. Another thought occured to me on where to park digital for AM. Will any TV stations be using 2-4 after conversion? It's not ideal, but certainly a lot better than the current AM band in most cases. Nice pure DRM on that band should work pretty good. Occationally skip might be an issue, but then again, isn't a little problem from time to time better than the crap AM staitons currently put up with?

I would like to see continued amplitude modulated service on the band with at least the 50kw stations. I think that's the most useful thing about the current AM band. WWL proved it's still invaluable during emergencies. If the band was eventually just left to the big blasters and the smaller stations went to a new digital band we'd have the best of all worlds.

 

[robgrayson]

I'm going to step up and demonstrate my ignorance for the world to see....

That there was no limit on postive peak modulation before the 125% limit was imposed is news to someone who came up in the biz instructed that it was illegal to put out over 100% on both + and - peaks.

If you're aiming for the target on my back, please hit near the center to avoid collateral damage.

 

[K4WRF]

I remember the CBS Labs 4400 peak limiter had a selector on the Pos Peak of 100, 120 and 300% (no pos. clipping), the latter stenciled in red. The irony was in that day, few if any stock transmitters were capable of it due to power supply or RF amplifier limitations. Most of the transmitters of that period were rated only at about 30% continious. Regardless of all that, to get that much positive swing would require you to really dig into the clipping diodes to the point the distortion becomes quite noticable, that ole loudness vs. distortion trade-off.

w/

 

[rrsounds]

I would like to see continued amplitude modulated service on the band with at least the 50kw stations. I think that's the most useful thing about the current AM band. WWL proved it's still invaluable during emergencies. If the band was eventually just left to the big blasters and the smaller stations went to a new digital band we'd have the best of all worlds.

OKCRadioGuy,
Your dream of high-powered only stations for analog AM (which I totally agree with) would allow better signal to noise, which AM desperately needs in order to be viable. It would also allow a return to full-service skywave coverage at night, which has been all but impossible for the past 30 years or so. As the number of these analog stations would have to be limited to only a handful nationwide, I would also suggest a limit of one per owner.

While we're dreaming, perhaps it would be time to reallocate MW channels in the Americas, putting the new clears at the low end of the band, where they could really sing. On double-wide 20 (or 18) KHz centers, with full bandwidth audio.
And as you said, put all the new, digital-only DRM AMs in the remaining portion of the band.

Of course there is an alternative positive future scenario for the AM band: power load management.

Kind Regards,
David

 

[amfmsw]

The demise of local radio, Class IV am's, even Class A FM's, would be a disister! I can't believe anyone could suggest such a thing. Will the 50kw in "Big City" air the H.S. Marching Band Car Wash fundraiser "now in progress at Bob's Gulf Station, go help out the kids"? Where I live, not a single one 50kw in that city has a decent signal here, day or night. And it's not a problem of nearby adjacent channels. It's from normal signal decay over the 60 mile trip, plus the contemporary noise floor, plus crap radios with crap antennae. No amount of "filtering" will cover that, only noise blanking, a good rf amp, tuned IF, agc, decent bandwidth, and a real ferrite core. (read GE/RCS SuperRadio). A 5kc audio on a signal that doesn't reach...why not make it 3Kc? It doesn't matter, there's only poor signal.

And, absolutely, content is king. But a format of Puddle Of Mud on AM will never bring young listeners. Their H.S. baseball and football games will, and does. Putting them on the air on weekends, with H.S. News & Sports while playing Puddle Of Mud, will. AND, it's sellable! Local. Only local owners, on local stations, even with small signals, will be able to serve the community. Isn't that what drew us to radio in the first place? The Clears from out of town couldn't care less.

So I guess the solution, or progress, isn't mod levels or HD, it's back to receiver standards. NRSC? Can you help us? Anyone?

 

[w9wi]

I think Part 15 is only a minute segment of the issue. As I tune the dials I hear noise from devices that don't qualify as radiation devices. My charger for my cellphone, the LCD display on my postage scale. Even my portable minidisc spews garbage by the gallon, all affecting the quality of what the public may want to listen to.

Part 15 is (largely) about devices like these that don't intentionally radiate radio signals.

Hams have succeeded in getting the FCC to take electric utilities to task over noisy power lines. It's taken persistence as most utilities seem inclined to completely ignore noise complaints - and have gotten rid of units that used to chase and fix these problems. It probably helps that in the ham case, the *receiving* is done at a specific fixed location. We don't demand they fix the entire city, just our neighborhood.

I wonder if...

If you ran some kind of contest, for your listeners to name the noisiest place in town, got your engineer to verify the source of the noise, then got insistent with the FCC over that specific location, would you get action? (I strongly suspect you would)

Then, promise to make it a regularly-scheduled feature.

 

[rorban]

rob, with all due respect, "Manufacturers who were heavily lobbying for a 5kc filter"... really? To hell with them.

Unless broadcasters want put their money where their mouths are by getting into the consumer receiver business, AM quality is going to be defined by the receivers that existing consumer electronics companies market. The Delco representatives in the NRSC explained that their main concern was warranty claims (which could eat up a lot of the profit margin on a vehicle). Consumers perceived interference as "noise" while not caring much about fidelity. Radios in new cars that were perceived to be unusually subject to noise or static often went back to dealers.

The laws of physics mean that wider band radios exhibited objectionable noise and interference at closer distances from a given TX than did narrowband radios. Consumers did not understand the concept of FCC protected contours; they just wanted to be able to hear their high school football games.

Hence the vicious circle -- the more broadcasters boosted high frequencies to overcome narrowband radios, the narrower the radios got because the boosted high frequencies created interference to first and second adjacent channels. The NRSC standard was an attempt to break the circle.

 

[rorban]

Very intersting that the 6-7 khz compromise resulted from having to accomodate the weakness of "an additional audio processor".
Were there any tests using radios that did not suffer from intermodulation distortion?

Why were the faulty designs produced? Was the clipping really in a processor in the radio or a badly designed AGC circuit?
Poor IM distortion performance is one of the finest weaknesses of semiconductors in RF receiver AVC design.
You just don't have this problem when you apply AVC to control gain in vacuum tubes.
There's a whole lot about this I've forgotten...had something to do with third-order intercept points...
It is possible to do though, I have quite a few transistor radios that have no problem with this, but they're all pre-1985.

Three receivers were used for the subjective tests, representing the 20th, 50th, and 80th percentile bandwidths of the 30 receivers objectively tested. These were also chosen for low distortion and low noise floor so that these were less likely to be confounding factors in the study.

The tests were done with a transmission chain typical of a well-engineered major-market AM station with a top of the line audio processor and very low distortion transmitter. The processor was set to a "middle of the radio" factory preset. The "IM distortion" I was referring to was caused by the modulation control clipper in the audio processor used in the transmission chain -- it had nothing to do with the radio.

The study is here:

http://www.nrscstandards.org/AMB/AMSTG full report.pdf

It should answer the questions you posed and many more.

 

[Tom Wells]

I have read this over before, a quick look at the chart of sample radios is very telling.

Why is the 100th percentile bandwidth radio 10 db down at 9600 khz?


The 50th percentile is barely above plain-old-telphone response.
I won't even bother listening to such radios. And many of the analog ones shown I'm SURE have higher response if slightly side-tuned.
If the responses of this study are to be taken seriously, we'd have to wonder why FM shouldn't follow the same recommendations.
Voice and music on FM must be absolute torture for these people. Do they use ear plugs of muffs to accomplish the
preferred mellowness? How can they stand live speech, for that matter?

All I know is I have always rejected the matress-over-the-speaker effect for most normal AM listening.

OK on the processor not being in the radio, that makes more sense now.
Sure do wish everyone could hear what an unmodified 1962 Chrysler-Mopar radio sounds like on wideband AM.
You really almost can't tell that it's not FM.

 

[OKCRadioGuy]

What I was trying to say is MIGRATE class IV's and even regionals to DRM on either virgin spectrum out of the current AM band or even part of it as someone else suggested. I would think it would take several years to get enough digital radios out there to kill the analog signal. After that let the clears have the analog and sundown the locals to the more useful digital signal. EVERYONE would win by this method. They clears would become more useful and the small local stations could actually serve their communities better because they could be heard though all the man-made noise we seem to be subjected to these days. At some point even the clears might be converted to strictly DRM also, but I'd think that would be a further down the road thing. Many of the big stations continue to make money with talk type formats and are less likely to be riddled with noise in their local service area so there would be less advantage for them to jump into pure digital as fast. For the record I have been involved with a local H.S. football team's broadcast for over 15 years on a 50KW blowtorch here in OKC. We have been very successful over the years serving the community. I can think of several other big stations around the area that also have either a big local team worth carrying or the "game of the week". In fact I would have to say there are more that DO carry H.S. football at least than don't of the biggies around here.

 

[amfmsw]

OKC..I now understand your point better. Thanks for explaining.

rob, the GM excuse is a ruse. If they truly built their radios to those specs for that reason, why were they also offering AMAX C-Quam Stereo wideband radios at the same time? I owned one of each, in an '87 Pontiac (crap) and a '89 Cadillac (awesome). The explanation to NRSC has no merit. Zero. It was, in my opinion, the tail wagging the dog. And we both know the "whistle" on AM would be better removed with a notch filter. It's like "Doctor, I broke my finger" being fixed by putting me in a full body cast!

Your quote: "AM quality is going to be defined by the receivers that existing consumer electronics companies market. The Delco representatives in the NRSC explained that their main concern was warranty claims (which could eat up a lot of the profit margin on a vehicle)." WHY? To many of us on the outside, it is plainly a matter of building a cheap radio, in both price and quality. We were let down.

Are our TV's not made to FCC standard? Are not FM multiplex receivers FCC standard? Why not our AM receivers? Yes, some are better than others, but all meet a minimum standard of some type. Not so AM. This industry, and ALL it's elctronic suppliers stand on the shoulders of AM broadcasters. It's beginning to hurt.

The NRSC forced us into this standard, it's time for relief. I've had this same discussion with other Board members, and have read that report several times. And please fully understand, it's not just about bandwidth. Yes, our listeners want to hear constanants in speach, but they also deserve a circuit with noise blanking. The noise blanking was a plank in the AMAX standard.

You, Frank and others have created wonderful advancements in the transmission of audio on AM, but what's the use or purpose of expense with 3-5Kc radios? IBOC is not the answer. Only requiring manufacturers match our AMAX standards, or better, will suffice. "Or better" because those are already about 20 years old, and Armstrong's Heterodyne is 80 years old. But I guess it's just a case of "it's the way we've always done it". Respectfully, I feel the NRSC is toothless, so FCC mandates to manufacturers are what's needed to enforce NRSC standards.

 

[LA_Guy]

I'm going to step up and demonstrate my ignorance for the world to see....

That there was no limit on postive peak modulation before the 125% limit was imposed is news to someone who came up in the biz instructed that it was illegal to put out over 100% on both + and - peaks.

If you're aiming for the target on my back, please hit near the center to avoid collateral damage.

The FCC rules prior to the 125% rule only specified 100% NEGATIVE modulation 'peaks of frequent recurrance'. They didn't even mention positive peaks.

 

[robgrayson]

a-HA!

 

[rorban]

I have read this over before, a quick look at the chart of sample radios is very telling.

Why is the 100th percentile bandwidth radio 10 db down at 9600 khz?

Top of my head response -- it's probably one of the Wal-Mart cheapies with a minimal amount of IF filtering. If so, it will squeal like the proverbial stuck pig after sundown because of 10 kHz beats.

Making a usable wideband radio used to be expensive because of the need for a stable 10 kHz notch filter and a quick rolloff above 10 kHz. DSP-based radios make this cheaper, but they are still not down to the $20 price point that many customers look for.

 

[rorban]

rob, the GM excuse is a ruse. If they truly built their radios to those specs for that reason, why were they also offering AMAX C-Quam Stereo wideband radios at the same time? I owned one of each, in an '87 Pontiac (crap) and a '89 Cadillac (awesome). The explanation to NRSC has no merit. Zero. It was, in my opinion, the tail wagging the dog. And we both know the "whistle" on AM would be better removed with a notch filter. It's like "Doctor, I broke my finger" being fixed by putting me in a full body cast!

I would have liked to see Dick Kennedy's annoyed reaction if you told him that to his face at an NRSC working group meeting. (Kennedy was a feisty senior Delco radio design engineer who did not suffer blowhards gladly.) In fact, the AMAX specification was based on the now-deleted NRSC-3 standard for receiver performance. This was created well *after* the NRSC-1 and NRSC-2 standards had already been hammered out by the NRSC working group of which I was a part. If you are going to make public allegations about Delco's "real" motivations, the least you could do is to get your basic chronology correct.

Speaking of C-QUAM, Motorola's Greg Buchwald, a C-QUAM mover and shaker, was also a stalwart part of the NRSC working group that created NRSC-1 and NRSC-2. He was the one who first suggested the modified 75 us NRSC pre-emphasis characteristic.

Moreover, I presume you understand that pre-NRSC first and second-adjacent interference is more than a "whistle." For 15 kHz transmitted audio bandwidth, the sidebands of the second adjacent carrier will encroach into one of the the 5 to 10 kHz sideband regions of the desired station (upper or lower sideband, depending whether the 2nd adjacent is above or below the desired transmission) and require a lowpass filter, not a notch filter, to remove. The sidebands of the first adjacent carrier will encroach upon one entire sideband of the desired transmission and the 0 to 5 kHz region of the remaining sideband.

 

[CW]

I have read this over before, a quick look at the chart of sample radios is very telling.

Why is the 100th percentile bandwidth radio 10 db down at 9600 khz?

Top of my head response -- it's probably one of the Wal-Mart cheapies with a minimal amount of IF filtering. If so, it will squeal like the proverbial stuck pig after sundown because of 10 kHz beats.

Making a usable wideband radio used to be expensive because of the need for a stable 10 kHz notch filter and a quick rolloff above 10 kHz. DSP-based radios make this cheaper, but they are still not down to the $20 price point that many customers look for.

IF it is a Walmart cheapie, it:
1) Wont have an external antenna connection...thus rely on the bar antenna inside...making it deaf
2) not much sensitivity to begin with.....lucky to hear the LOCAL stations in daytime
3) will probably not have any 10kHz beats because it cant hear the distant adj channels OR local noise will kill it...

Been there, done the research...

I HAVE a Siny XRA33 FM/AM Stereo....picks up all four formats...and HAS a narrow/wide filter....a lot of times even at NIGHT, I ran it in the wide mode listening to WLS while driving the back roads of SETX...and NEVER heard a 10kHz beat but heard DAMN good Top40 music with JRL, etc back then...only when I started to get near an 880 north of Houston did I have to switch to narrow....and that was 30miles out...5 miles from the 880, WLS was killed by the sideband splatter no matter WHAT filter selection was used...during the day, KKBQ 790 sounded RICHER than their 92.9 FM simulcast.....
(and that was both KAHN at the time)....never heard the 10 kHz beat on them either...even with XROCK 80 booming in