NEW HD-AM Broadcast Scheme?

[JohnnyElectron]

There are reports about a new way of transmitting HD-AM - AND - keep a 10.2KHz analog AM bandwidth! If true, then I could envision a plan where you could transmit the C-Quam analog AM stereo in the 10KHz window AND keep the HD info in the 'neighbor's yard'. You could have your cake and eat it too if you could transmit both stereo systems at the same time, allowing for extreme distant AM stereo on the analog portion and local on the HD? Question: is it backwards compatible with existing poor bandwidth HD-AM radios ?
Next question: when will they put a switch on the HD tuners for analog only, HD only, auto switch, and an indicator for the 25Hz analog AM stereo pilot and HD stereo AM? Bandwidth wide/narrow switch for AM? Clarion did that in the 80's, and the wide-bandwidth AM (stereo) Delcos of the late 80's/early 90's had an AMST switch they narrowed the down the wide stereo bandwidth.
IF this works, what a great reason to play stereo music on AM clear channel frequencies again, like WSM 650, 1560 NYC, and loads of oldies stations?

Now they need to figure out a way to add AM to the BestBuy portable HD radio!

 

[Tom Wells]

A higher pitched hiss, but perhaps enough it will be reduced some on the host analog decode.
It wil depend on the radio, as it is now. It can only mean more noise upon the adjacents,
and it still can't remove the restriction of maximum -94% modulation. The carrier cannot be allowed to wink out,
or HD will drop. To take advantage of this, a station will not only need to drop the brickwall, but
re-tune their audio to put the sibilant sounds back without significantly changing their signature sound.
If the analog audio is well balanced, there will be no need to side tune to achieve full fidelity.
This would be (ahem) less loudness and spectral packing in the midrange as is currently practiced.
With proper 'sharp skirt" IF response design, actual 10 khz audio could be enjoyed.


WBBM AM 780 Chicago has been hissless for a day or two now, and as a flagship CBS station, I wonder if they are
being refitted for this?


In any case of bw reference in AM, it is essential to be sure whether total RF bandwidth is being discussed, or potential maximum
decoded audio bandwidth, as mistakes in design have occured over this very issue.

It takes twice the bandwidth in RF AM to produce any given audio bandwidth. 5 khz audio requires 10khz "on the dial"
A careful read of old dusty FCC laws and allocation "protections" to AM signals provided over 20khz protection to each signal,
permitting full 10-13 khz "semi-chatterless" audio bandwidth to most stations.

It was nice while it lasted. I recomember when.....

 

[Savage]

I think it's safe to say CBS Radio is tinkering with AM-HD. On several recent occasions WBZ has either sounded like it's transmitting with greatly reduced carrier power - perhaps from their standby nondirectional site, which reportedly has no IBOC - or they're being received with typical strength but no HD saddlebags.

Another mystery: on some occasions WYSL has been almost back to the pre-IBOC clarity, hiss-free...yet KDKA (lower WBZ sideband) is being typically obliterated. Perhaps CBS is trying the single-sideband flavor of HD to confirm Crawford's recent experiments from Catalina?

Then again, on one occasion a few nights ago, the IBOC skywave from WBZ was unbelievably horrible. It was killing WYSL within our COL of Avon, NY, rendering almost 70 mV/m unlistenable. I had never heard the HD noise that bad...the worst ever experienced. I assume this might be an example of the wonderful new modulation scheme....?

 

[RadeoEngineer]

It's nothing more than a new shade of lipstick on the pig.

 

[Savage]

Or a new shade of lipstick on Charles Manson.

 

[ve3jf]

What this change will do is dump the stereo capability of HD-AM, leaving just 20 kb/s mono. So you give the analog an upgrade in bandwidth, and downgrade the digital... the digital remains a simulcast of the analog, and the analog is now arguably superior in quality. Meanwhile, you have done absolutely nothing to reduce adjacent channel interference to other stations, or to improve the digital coverage. Why on earth would you leave the digital on at all, when it provides no new service and it harms other stations? There oughta be a law against this sort of nonsense.

New shade of lipstick indeed.

 

[Tom Wells]

I betcha it still sounds bad. Hasn't happened here yet. WBBM and WSCR hiss/sidetune ratio is unchanged.
I had thought maybe WBBM was gonna get the new rev this past week when they took HD down.

 

[Savage]

Let me get this straight: so HD-AM "Version 7.4" includes...10.2 khz bandpass analog and 4.5 kHz bandpass digital, with everything higher synthesized - and all the digital crap still resides in adjacent channels?

As the saying goes: "Wish I had some of what THEY'RE smoking."

So: NO improvement over analog performance (AM still interrupted by thunderstorms and impulse noise, no increase in coverage, day-night power discrepancies and skywave interference remain unaddressed.) Yet now every AM station transmitting in HD occupies a minimum of THREE channels where it previously resided only on its assigned channel, thus disastrously ratcheting up interference.

I take it back. I DON'T want what they're smoking.

 

[satech]

I don't see the point of this either. Given a strong enough signal to decode the IBOC, the 10 kHz analog is going to sound better than the 20 kbps mono digital, hands down -- even moreso if you add C-Quam stereo to the analog.

 

[ve3jf]

Let me get this straight: so HD-AM "Version 7.4" includes...10.2 khz bandpass analog and 4.5 kHz bandpass digital, with everything higher synthesized - and all the digital crap still resides in adjacent channels?

As the saying goes: "Wish I had some of what THEY'RE smoking."

So: NO improvement over analog performance (AM still interrupted by thunderstorms and impulse noise, no increase in coverage, day-night power discrepancies and skywave interference remain unaddressed.) Yet now every AM station transmitting in HD occupies a minimum of THREE channels where it previously resided only on its assigned channel, thus disastrously ratcheting up interference.

I take it back. I DON'T want what they're smoking.

Well, to be fair, there's no reason why the audio on the digital side would be bandlimited to 4.5 kHz, though it's not entirely clear what the bandwidth would be with the new scheme. The current system is designed to fall back to mono from stereo when reception conditions degrade, and I doubt if that would entail a major drop in bandwidth, as that could be rather jarring. The new scheme just makes the fallback permanent. I think it's safe to say that the bandwidth would remain considerably more than 4.5 kHz. The artifacts associated with a 20 kb/s encoding scheme would also remain, of course.

That said, the whole thing remains a pointless exercise, rearranging the deck chairs on a sinking ship.

 

[ve3jf]

A small correction: according to an article in the March 24 edition of Radio World, the new scheme will still offer the possibility of doing stereo. The audio bit rate remains 20 kb/s, but apparently they will introduce a "parametric stereo" option that is supposed to be backwards compatible with existing receivers. It's hard to see how they could do that without a firmware upgrade for the receivers - my guess is that existing receivers would deliver mono, while new receivers could do the stereo decode.

My other comments still stand - it remains a pointless exercise.

 

[Tom Wells]

WSCR 670 is now hissless and a careful-listen sweep-tune listen of 780 WBBM's napalm sidehiss suggests that the new scheme is in place.
The hiss is the same for the host signal, on a wideband radio...or at least no audio advantage has been taken yet that is audible ( on WBBM). Side tuning to sharpen AF curve for "less frequency distortion" ( let's argue about this), still creates the same amout of
hiss that was introduced as before.

But the adjacents....it's all packed in a little tighter, somehow. The whoosh is NOT as wide, it is less pink and has more spectral density.
The first adjacent sounds as bad as ever, but now with more density, lower audio products, and a tighter bandwidth.
The second adjacent "may" be considerably freed of onerous interference.
If this is so, then WSM should be relieved partially of the scourge of WSCR in the Chicago region.
If this is so, I can tip my hat to ibiquity for a slightly less poisonous version of their product.
Oh, if only that they'd somehow gotten the AM on a synchronous +5 mhz signal and done a self time synching diversity mode.
Or something less kludgey.
Time to boot the military off a lot of useful wavelengths! ( Heavy Chortle and Knee-Slap )

 

[Carmine5]

So while this redesign will improve analog listening (you know, the signal that actually makes money for the station) the one problem it still doesn't address is nighttime skywave interference, a problem that is inherent in HD-AM. But it seems that this is an issue iBiquity and its investors are unconcerned about.

Notice these reply Comments of iBiquity investor Greater Media to the FCC back in '04 regarding the allowance of nighttime HD-AM and skywave reception:

"The fact is that wide-area nighttime skywave service, while a fascinating propagation phenomena and very much a part of radio's historic past, is largely irrelevant in today's world. The number of persons deriving radio service via skywave propagation is tiny. Most persons under the age of 40 do not even know that this propagation mode exists, let alone listen to it. The need for a medium wave wide area skywave service in the U.S. has largely passed....Skywave signals will suffer somewhat greater interference then currently is the case and will have their availability decreased as noted in the iBiquity submissions. This is, by far, the most significant compromise attendant to 24-hour AM digital operation. However, the upside of high fidelity -- robust digital service in the stations' groundwave service area -- is worth the attendant deterioration of the skywave service component inasmuch as the vast majority of these stations' listeners and advertisers are within the groundwave service areas. For AM to have a future, it must transition to digital. To transition fully to digital will require this necessary compromise."

http://fjallfoss.fcc.gov/ecfs/comment/view?id=5511438468

So iBiquity and the HD boosters seem intent on making AM a local only or groundwave service as they sacrifice long range listening in favor of going digital.

c5

 

[audioguy]

The new scheme, if that is what is now running on WSCR, is no better in my opinion on 2nd adjacent channel interference than the old one. What an abomination! IBOC is a plague on the AM band! When will the industry ever get a clue? This is worse by far than static. The funny thing is that IBOC doesn't solve static problems because (just like HDTV), the signal drops out during thunderstorms.

 

[satech]

As far as I see it, the only thing the new AM IBOC scheme does is drastically reduce a station's self-interference to their own signal. Especially with a directional array, you can never completely null out the secondary and tertiary digital sidebands underlying the analog audio, causing a nasty constant hiss on receivers out in the field. The new scheme gets rid of this hiss for the station's own analog listeners -- especially those with wideband receivers -- but does little or nothing to reduce the abhorrent interference to the first- and second-adjacent channels. That is simply inherent to the concept of trying to cram digital sidebands "under the mask," 10 to 15 kHz away from the station's carrier.

 

[Play Freebird]

As far as I see it, the only thing the new AM IBOC scheme does is drastically reduce a station's self-interference to their own signal. Especially with a directional array, you can never completely null out the secondary and tertiary digital sidebands underlying the analog audio, causing a nasty constant hiss on receivers out in the field. The new scheme gets rid of this hiss for the station's own analog listeners -- especially those with wideband receivers -- but does little or nothing to reduce the abhorrent interference to the first- and second-adjacent channels. That is simply inherent to the concept of trying to cram digital sidebands "under the mask," 10 to 15 kHz away from the station's carrier.

That's my take on it as well. If a receiver is tuned off the center of the channel or has asymmetrical response across the IF passband (in other words, the filters are imperfect) the secondary and tertiary digital sidebands (which are sent in conjugate pairs) won't fully cancel even if transmitted through a broadband antenna. Apparently, this is a bigger problem than iBiquity originally assumed. Welcome to the real world.

 

[Tom Wells]

Doesn't seem like WSCR has the new mode yet.
It is not yet like WBBM's sidebands.
Last night on the way to work I tried to listen to WSM and found the same amount of intrusion 2 channels over.

If WBBM has the new version, it makes no difference at all in the actual amount of hiss in the host signal
as received in a wideband, analog, continuously tuned radio. In a digitally tuned radio, with "perfect" center tuning,
it's a toss-up, dependent on the manufactured design audio upper end response.


I beleive WSCR had some issues with bandwidth on their antenna. Maybe it will take a little more work.

 

[Tom Wells]

Looks like I spoke too soon.
Listened to WSM 650 on the way to work tonight and WSCR's iboc sidebands ARE pulled in.
I no longer have to hear intrusive hiss within the acceptable-tune passband of WSM.
(This on a really wideband Motorola from 1972.)
I still have to side tune down to miss the hiss, but not to where the WSM carrier is decreased and the sibilants "chuff".
There is still some vestige of the noise but no more than any other incidental noise, nor is it intrusive.

This is no help for Bob Savage with an offender 10 khz away, and I have no idea how the HD decode is with the new
spectrum density, but it does remove some of the offense to second adajcents, apparently.

 

[ve3jf]

Looks like I spoke too soon.
Listened to WSM 650 on the way to work tonight and WSCR's iboc sidebands ARE pulled in.
I no longer have to hear intrusive hiss within the acceptable-tune passband of WSM.
(This on a really wideband Motorola from 1972.)
I still have to side tune down to miss the hiss, but not to where the WSM carrier is decreased and the sibilants "chuff".
There is still some vestige of the noise but no more than any other incidental noise, nor is it intrusive.

This is no help for Bob Savage with an offender 10 khz away, and I have no idea how the HD decode is with the new
spectrum density, but it does remove some of the offense to second adajcents, apparently.

I really don't see how the digital sidebands could be "pulled in". It's pretty clear that the new scheme does not change the primary sidebands at all - that means that they still extend to nearly +/-15 kHz from the carrier frequency, and they still have the same power level (though the power levels of the upper and lower primary sidebands can now be changed independently, as was done recently by KBRT in California to reduce interference to second-adjacent KFMB). I don't think the new scheme, at least as it's been explained so far, would have any impact at all on adjacent channel interference, but it would reduce self-interference.

 

[Tom Wells]

A few days of listening makes me pretty confident that WSCR is using the new scheme.
WSM continues to be "listenable", if I tune down a bit. I could not escape the hash before on this same radio.
The upper sideband of 650 sounds as bad as before, so it hasn't changed things much for someone with a wideband radio.
It's just a >tiny< bit narrower on the noise, but enough to make difference that I can listen to 650 almost normally.
I hear NO benefit to the audio on the host signal, no matter how I tune, or how carefully I try to center tune.
The 94% max negative mod peaks combined with the (unchanged) hiss still makes them sound flat and dull.
The change to phase modulated sidebands in summer of 2007 was a very audible improvement to the hiss level on the host.