Dilemma for rimshot stations

[fybush]

But again Bob, what is the real purpose of a 21st century radio station? Its only real purpose is to serve a local audience. Thanks to the laws of physics reflection occurs at medium wave frequencies. There's no real means of effectively preventing that. KDKA, a 50 KW radio station should have no problem covering the Pittsburgh metro area. They at least cover the areas they are able to sell with enough RF to prevent serious interference to their signal. I come to this conclusion because unlike Citadel, there has been no effort to turn off WBZ's HD sidebands by KDKA. Whether that effort was public or private is of no importance. If KDKA suffered serious interference from WBZ something would have been done and it has not up to this time.

I'm not convinced that this conclusion can be drawn as definitively as you'd draw it. The disparity in market size between Boston and Pittsburgh has grown large enough that it's also quite possible that CBS is both aware of serious interference to KDKA (which does exist, well within the metro) and willing to tolerate that interference in order to keep running HD at the much more important WBZ signal.

If that's the case (and I'm hearing, anecdotally, that it is), it's nothing new. WBZ and KDKA have been spaced too tightly for better than seventy years now, back to the days when KD was on 980 and BZ was on 990. They probably could have been better rearranged in the NARBA shuffle - WHO could have gone to 1030, with WBZ on 1040, for instance - but that ship sailed long ago, too. In the fifties, when the clear channel stations made one last pitch for superpower, Westinghouse acknowledged that it was probably impossible to hike both WBZ and KDKA above 50 kW, and told the FCC that it was willing to keep KDKA at 50 kW in order to allow WBZ to go to 500 or 750 kW. That was half a century ago, and Pittsburgh was a much more important market then.

If it were KDKA interfering with WBZ within the Boston metro, I suspect the situation would be far different.

 

[Tom Wells]

WBZ is widely listened to in western MA, and it probably gets ugly about 50 miles out from Boston from KDKA splash.
I'd love to hear stories from someone who drives into Boston from "halfway out" before sunrise.

The data sidebands are intended to have an "average balance" that is not quite balanced.
If they were perfectly balanced, a center tuned detector would hear NO hiss.
ibiquity did upgrade in recognition of this problem, even if we have no proof. My ears, radios and experience have never lied to me yet.
I believe this improved balance was the change for less hiss I documented this past August in the Chicago market.

IF "cheapie" or any radios with narrow IF response ( not just dull audio ) were built to side tune with about 4 khz, a la powerside,
it would certainly be possible to avoid the HD noise if it were possible to SSB the iboc.
How to decide upper or lower in each case would be obvious given some situations, but others still a problem.
And it would complicate tuning for average listeners to figure out whose analog went "up" vs "down".
There would be NO way for step-tuned modern varactor-diode 10 khz "digital" center-tuning radios to avoid the noise which would result.

More stations would be able to use such a system, as overall system bandwidth would drop by 15 khz, making
more existing antennas, networks and DAs compatible.


I am repelled by the idea of "what is a market" for FM being forced upon the AM.
The market is whoever can and chooses to listen.
In FM, the wavelength, power, terrain, elevation, capture ratio, and allocations have made this a pretty tidy, controllable system
where markets are neatly defined. Until skip or other "freak" conditions happen.

In AM, the workable situation was figured out some 70 years ago, accepting and utilizing mediumwave's characteristics, not fighting against them.
Mediumwave behaves in a certain way, and no amount of wishing will change the physics of it all.

No amount of training will ever turn a racehorse into a pack animal , or vice versa.

No one races pack animals, or uses arabian thouroughbreds for pack animals.
A truck is not a car.

An econobox is not a touring car.
If you settle in Buffalo, Cleaveland or NW Indiana, you KNOW you're going to have lake effect snow.
This deliberate willful misunderstanding of the nature of MW is what irks me.

MW is not VHF, and many things which were once prescribed and used as medicines are now known to be toxic.
Hey let's put some merthiolate (tincture of mecury) on that cut, that'll help it heal.

In MW, the very concept of AM modulated data is antithetical to "radio", given 10 khz spacing.
With 50 khz spaced channels, the present method could work, if the stations' BW would permit sidebands between 15-25 khz.
And then it really could be FM quality, not the chunky sparkly effect HD AM currently delivers.
EVERTHING in the 15-20khz IF passband is intended to be AUDIO information detecting to 7.5-10khz audio.
That's MW radio.
Those who don't wish to work with mother nature are certain to have numerous disappointments.

"Communications" can suffice with limited bandwidth, and even benefit from limited bandwidth.
I never did like listening to broadcast radio on narrow bandwidth communication receivers.

 

[fybush]

In AM, the workable situation was figured out some 70 years ago, accepting and utilizing mediumwave's characteristics, not fighting against them.
Mediumwave behaves in a certain way, and no amount of wishing will change the physics of it all.

Thank you, Tom.

I continue to be amazed at the number of otherwise exceedingly intelligent and experienced broadcasters who think they can somehow change those laws of physics simply by wishing it so...and who don't understand why it's useful, from time to time, to keep an eye on the DX lists to see what the propagation gods are up to.

 

[ve3jf]

While the concept of turning off a HD sideband might not be workable, it's a start.

Isn't the problem with turning off one HD sideband self-interference?

It's my understanding that the two HD sidebands are out-of-phase, so that when both sidebands are received on an analog receiver they'll cancel each other out. Turning off one of the sidebands would allow the remaining sideband to be demodulated (as noise) on an analog set. Or am I off-base on that?

In any case, it's not hard to find cases where there are potential interference problems on *both* sides. Indeed, WBZ would seem to be such a case -- dropping the upper HD sideband would certainly fix the problem with regard to WYSL but would do nothing for KDKA. And vice-versa.

You're not quite off-base, but you could get picked off if you're not careful. ;D

The secondary (5-10 kHz from the carrier) and tertiary (0-5 kHz from the carrier) digital sidebands are complex conjugates of each other. The sideband pairs thus form a constant-envelope signal that, in theory at least, won't create any noise in an AM detector. However, this is not true of the primary (10-15 kHz from the carrier) sidebands, and they're the ones that contain most of the power, and cause most of the adjacent channel interference. The primary sidebands carry the core 20 kb/s mono audio bit stream, and they are coded so that this stream can be decoded if only one of the sidebands is received. Moreover, there is a time diversity delay of 4.5 secs between the streams on the upper and lower sidebands. This time offset allows the decoding to ride through momentary interruptions, such as noise pulses or fades in mobile reception when driving under power lines, etc.

So, in principle, the AM IBOC system could still function if only one primary sideband were transmitted, but it would be less robust: no time diversity to help with transient problems, and no frequency diversity to help with interference problems (or problems with pattern bandwidth of directional arrays). Digital coverage at night is already lousy, and dropping a sideband would make it significantly worse. As you point out, if WBZ dropped their upper primary sideband to protect WYSL, they would still be mangling KDKA with their lower sideband. And, it works both ways. Right now, it is likely the upper sideband that is carrying the load in establishing digital night coverage, since the Boston area would have stronger skywave on 1020 than on 1040. So, if that sideband went away, coverage would be considerably worse than it is now, and I'm sure it's nothing to write home about already. Considering the damage it's doing to KDKA, hardly worth it, methinks.

 

[Len14043]

While the concept of turning off a HD sideband might not be workable, it's a start.

Isn't the problem with turning off one HD sideband self-interference?

It's my understanding that the two HD sidebands are out-of-phase, so that when both sidebands are received on an analog receiver they'll cancel each other out. Turning off one of the sidebands would allow the remaining sideband to be demodulated (as noise) on an analog set. Or am I off-base on that?

In any case, it's not hard to find cases where there are potential interference problems on *both* sides. Indeed, WBZ would seem to be such a case -- dropping the upper HD sideband would certainly fix the problem with regard to WYSL but would do nothing for KDKA. And vice-versa.

You're not quite off-base, but you could get picked off if you're not careful. ;D

The secondary (5-10 kHz from the carrier) and tertiary (0-5 kHz from the carrier) digital sidebands are complex conjugates of each other. The sideband pairs thus form a constant-envelope signal that, in theory at least, won't create any noise in an AM detector. However, this is not true of the primary (10-15 kHz from the carrier) sidebands, and they're the ones that contain most of the power, and cause most of the adjacent channel interference. The primary sidebands carry the core 20 kb/s mono audio bit stream, and they are coded so that this stream can be decoded if only one of the sidebands is received. Moreover, there is a time diversity delay of 4.5 secs between the streams on the upper and lower sidebands. This time offset allows the decoding to ride through momentary interruptions, such as noise pulses or fades in mobile reception when driving under power lines, etc.

So, in principle, the AM IBOC system could still function if only one primary sideband were transmitted, but it would be less robust: no time diversity to help with transient problems, and no frequency diversity to help with interference problems (or problems with pattern bandwidth of directional arrays). Digital coverage at night is already lousy, and dropping a sideband would make it significantly worse. As you point out, if WBZ dropped their upper primary sideband to protect WYSL, they would still be mangling KDKA with their lower sideband. And, it works both ways. Right now, it is likely the upper sideband that is carrying the load in establishing digital night coverage, since the Boston area would have stronger skywave on 1020 than on 1040. So, if that sideband went away, coverage would be considerably worse than it is now, and I'm sure it's nothing to write home about already. Considering the damage it's doing to KDKA, hardly worth it, methinks.

Listening to WLW HD about 10 miles from the tower during a thunderstorm, is is apparent that the HD signal in somewhat immune from sporatic lightning. However, during a severe storm with frequent lightning, the digital signal is lost and the analog signal is nearly unlistenable. If the sidebands must be out-of-phase, the vageries of skywave reception must cause the analog signal to be snuffed out when the sidebands arrive in-phase at the receiver. It was strange the other night when WHAM's analog signal was buried under a Cuban station, but the digital signal locked for about a minute.

 

[w9wi]

So, in principle, the AM IBOC system could still function if only one primary sideband were transmitted, but it would be less robust:

Oh, I fully believe the AM IBOC system would function with only one sideband. (though as you say, with considerably less reliability) I've heard brief bits of audio from distant HD AM stations where one side was clear of interference while on the other, analog stations were clearly listenable through the HD station's digital sideband.

My point was that with the other digital sideband no longer present, self-interference might make the HD station's analog signal unlistenable.

 

[ve3jf]

So, in principle, the AM IBOC system could still function if only one primary sideband were transmitted, but it would be less robust:

Oh, I fully believe the AM IBOC system would function with only one sideband. (though as you say, with considerably less reliability) I've heard brief bits of audio from distant HD AM stations where one side was clear of interference while on the other, analog stations were clearly listenable through the HD station's digital sideband.

My point was that with the other digital sideband no longer present, self-interference might make the HD station's analog signal unlistenable.

Okay, so I addressed your point only peripherally. There is no "cancellation" effect between the primary digital sidebands as far as a receiver tuned to the IBOC station is concerned. So, if the receiver is wide enough to pick up audible noise from those sidebands, removing one of them won't make matters worse - in fact, the noise pick-up should actually be reduced. However, the idea of running with only one primary sideband is pretty much a non-starter, for the interference and performance reasons already cited.