AM IBOC impact on host analog signal.

[Len14043]

A few months ago RBruceCarter claimed that AM stations running IBOC suffer from analog signal loss. I was skeptical because I didn't notice any loss of range on AM stations running IBOC while traveling extensively throughout the country. Still skeptical, I took several measurements using my CC Shortwave radio that is equipped with a signal strength meter. Specifically, measurements were taken comparing IBOC and non-IBOC stations close in frequency. The RF gain control was used to get a good baseline during this non-scientific test. The signals of WLW (IBOC) and WJR (non-IBOC) were measured at a point almost precisely between their towers, and WJR was slightly stronger than WLW. In another test about 250 miles away from WSCR (IBOC) and WGN (non-IBOC), WGN was slightly stronger. I see the WSCR/WGN test as more creditable because both towers were about the same distance and direction from the receiver. The WLW/WJR test could be suspect because WJR appears to be in an area where the ground conductivity may be better. My point is this: either the meter in my radio is more sensitive at higher frequencies, or perhaps there may be some validity to RBruce's claims.

 

[audioguy]

Subjectively, HD AM stations do sound weaker, but I don't know that there's a difference in the actual received signal strength. Car radios don't have "S" meters. The point is, how the station sounds DOES matter. The thin, pinched audio makes it hard to listen to. When I was working in radio the PD would have had a cow if his station sounded that way-- it would never have survived for one single day.

 

[Tom Wells]

They sound weaker because of the hiss that would normally only be heard in fringe areas, now heard everywhere, and because the
modulation can now no longer be allowed to ever have any -100% peaks. If -100% mod occurs, EVER, there is no longer any carrier for the sidebands to reference, and HD lock drops out. So they are required to keep the audio density lower.
The S-meter may or may not show a decrease, but they're no longer using the whole AM modulation capability of the carrier.

If my part 15 AM sounded as bad as iboc AMs, I'd shut it down and would not be back on the air until it was corrected,
and I'm not even trying to make money on my signal. To pollute a commercial signal in this way AT THE SOURCE is unforgivable.

 

[Savage]

Back in September 2007, the weekend before IBOC was authorized to roll out at night, WBZ's Mark Manuelian briefly cooperated with WYSL in performing local field tests of the Boston station's signal as their HD exciter was cycled on and off. Simultaneously I recorded the audio on a digital Marantz recorder connected to the audio port on WYSL's Potomac FIM-41, monitoring WBZ at a WYSL directional monitoring point by the side of the road.

You don't find many receivers better than an FIM-41. The effect of turning on the IBOC was dramatic, which I personally monitored on expensive WESC e-8.4 headphones. Apparent loudness dropped by a factor I would estimate at more than 6 dB, a mushy unpleasant form of distortion suddenly appeared and a constant multifrequency whine was superimposed on the analog signal. I was on a cellphone with Manuelian when the HD came on and I let out an exclamation I don't think Manuelian appreciated. He demanded to know what I was hearing. I said, it's your signal and your station, but I sure wouldn't want mine to sound that way.

Human hearing is complex, and the psychoacoustic impact of various forms of audio can vary widely, hence the decades-long debate over "apparent loudness" of TV commercials vis-a-vis surrounding programming. Suffice it to say, the interaction of the COFDM carriers with the PWM/PDM analog modulation schemes in modern AM transmitters is generally not a good thing. "The proof of the pudding" is an analog AM pudding which sounds lousy. And that's comparing the IBOC-plagued signal with a narrowband choked-off non-IBOC signal only 5 or 6 kHz wide. If you open your transmitter up to the full 10 kHz it was designed for - actually, WYSL's Nautel AMPFET-25 factory proof shows it can transmit flat out to 14 kHz - it's no contest. In any case, it's not hard to fathom how IBOC junk can cut down on analog coverage, especially outside the city-grade contour. IBOC helps the analog signal submerge in general QRM/QRN instead of punching through, like a quality, well-modulated and controlled signal will, at least as detected on the usual POS consumer radio.

IBOC-AM: a bad idea which never should have been authorized. BTW, another long-respected chief engineer of a well-known group joined the StopIBOC Alliance yesterday.

Some day I'll have to post the WBZ airchecks with the IBOC on-off for you guys to hear. Have an air sickness bag handy when listening.

 

[rbrucecarter5]

A few months ago RBruceCarter claimed that AM stations running IBOC suffer from analog signal loss. I was skeptical because I didn't notice any loss of range on AM stations running IBOC while traveling extensively throughout the country.

The only way to properly do this test is to listen to the same station - or monitor with a field strength meter - with IBOC on and off. Fortunately I had a station that did it - not for me to do a DX test, but they installed IBOC, then had technical problems, then put it back on, then finally canned it completely. The station is WBAP. I had a nice, repeatable, quiet listening location over 200 miles away and could compare before and after. Because there was an intermediate gap, I also had two different times of the year, and I was able to test at different times of the day. The bottom line is that WBAP's signal over Houston degrades badly when they turn on IBOC. I wasn't lucky enough to have a field strength meter with me, but the difference is dramatic. Another poster on here jumps all over me about out of market listening be irrelevant, so I always have to add a disclaimer about $5 radios IN the market being able to receive the station. The coverage loss with IBOC is real, dramatic, and large. Since the overwhelming majority of listeners are using analog radios, and will be for the foreseeable future, if I owned a station, I'd shut down IBOC and concentrate on a great analog signal chain with clean audio and high frequency response - until or unless the IBOC system catches on with the public (add that to your list of things that will never happen).

 

[DaveWilliams]

When WPEN started running IBOC on their daytime site, I was listening to them on a GE Superadio III. In the wideband mode, the audio was flat and muffled and the IBOC hiss was audible on the main carrier. BUT when they were running their night site with NO IBOC, it sounded fanastic! I also tried the Radio Shack HD Radio, on AM, ONE station running HD was receivable and that was only by attaching a 100 foot longwire antenna to the set and then the HD was switching between the analog and HD signal! The station was 5000 watts on 580 and less than EIGHT miles away! Hope the people that bought this system are satisfied with the results and revenue its generating for them!

 

[BRNout]

When WPEN started running IBOC on their daytime site, I was listening to them on a GE Superadio III. In the wideband mode, the audio was flat and muffled and the IBOC hiss was audible on the main carrier. BUT when they were running their night site with NO IBOC, it sounded fanastic! I also tried the Radio Shack HD Radio, on AM, ONE station running HD was receivable and that was only by attaching a 100 foot longwire antenna to the set and then the HD was switching between the analog and HD signal! The station was 5000 watts on 580 and less than EIGHT miles away! Hope the people that bought this system are satisfied with the results and revenue its generating for them!

I have had the exact same experience with WLS. When they have their IBOC buzzmaster on during the day, their audio is flat and muffled and I hear the hiss in the background of the audio. However, when the jammer is deactivated, the audio becomes clean and crisp. An amazing difference that is constantly downplayed by HD radio proponents. Look guys, we're not crazy.....

 

[rbrucecarter5]

I have had the exact same experience with WLS. When they have their IBOC buzzmaster on during the day, their audio is flat and muffled and I hear the hiss in the background of the audio. However, when the jammer is deactivated, the audio becomes clean and crisp. An amazing difference that is constantly downplayed by HD radio proponents. Look guys, we're not crazy.....

They attempt to eliminate hiss by phase modulating the IBOC signal from 5 kHz to 10 kHz, then revert to amplitude modulation from 10 to 15 kHz. There are some problems with that approach:

-- if the phase response of the antenna system is not flat, phase modulation will appear as amplitude modulation. And getting an AM array, especially a directional one perfectly flat in phase is next to impossible.

-- if the radio is not perfectly tuned, the phase modulation will appear as amplitude modulation. Given the little tiny tuning dials and complete lack of anti-lash mechanics on cheap radios, it is virtually impossible to tune a radio precisely on frequency.

-- cheap radios with sloppy wide IF designs will pick up the amplitude modulated 10 - 15 kHz IBOC signal. Given that these cheapies also have tiny speakers which are really effective as tweeters, you will hear that hiss as well.

-- I have long suspected that the presence of the sidebands is fooling receiver AGC into limiting gain and therefore the station appears to have much less coverage.

 

[RememberWHEN]

As mentioned by Tom Wells, rbrucecarter and others, IBOC contamination of the host analog is several poisons all mixed together. Modulation density must be reduced as Tom mentioned. So the IBOC station is no longer as 'loud' as other signals in the market with the same field intensity. I wouldn't think the S meter would show any difference. The meter is indicating a 'relative' strength referenced to the AGC in the receiver.
With regards to sideband 'pairs' and rbruce...IIRC...the sideband pairs are out of phase with each other and should cancel. 'Course...that isn't happening in the real world. This is probably what BRNout is hearing. He apparently is in city grade coverage and probably has a moderately wideband receiver. So there's an increase in the 'background' noise...like the 'self noise' of a superhet receiver one would hear 80 miles out from WLS during the day. This gives the illusion that you are listening to a 'distant station' when in reality you're not.
And then there is the required restricted bandwidth (< 5kHz) of the analog modulation to prevent interference with the digital sideband pairs. Now there's a good idea...NOT. Two tin cans and some string.

 

[rbrucecarter5]

the sideband pairs are out of phase with each other and should cancel. 'Course...that isn't happening in the real world.

I was lucky enough to stumble onto a IBOC AM station not running any analog - and I tuned up and down the channel to hear what the hiss would be like. I didn't have to go but a few Hz off center channel to start hearing the sidebands, and they were there with a vengeance 100 Hz off channel. The wierdest thing happened right on channel - a faint warbling sound, like something on the two sidebands was different. Wierd stuff, and it only showed up when I was almost exactly tuned. I've never heard anything like this on an analog signal that runs dead air - a fairly common occurrence on brokered stations.

I have all this stuff on tape, too.

 

[BRNout]

the sideband pairs are out of phase with each other and should cancel. 'Course...that isn't happening in the real world.

I was lucky enough to stumble onto a IBOC AM station not running any analog - and I tuned up and down the channel to hear what the hiss would be like. I didn't have to go but a few Hz off center channel to start hearing the sidebands, and they were there with a vengeance 100 Hz off channel. The wierdest thing happened right on channel - a faint warbling sound, like something on the two sidebands was different. Wierd stuff, and it only showed up when I was almost exactly tuned. I've never heard anything like this on an analog signal that runs dead air - a fairly common occurrence on brokered stations.

I have all this stuff on tape, too.

This is fascinating stuff. Rbruce, what station was running IBOC without analog?

 

[radioskeptic]

In Reply #3, Savage wrote:

You don’t find many receivers better than an FIM-41. The effect of turning on the IBOC was dramatic, which I personally monitored on expensive WESC e-8.4 headphones. Apparent loudness dropped by a factor I would estimate at more than 6 dB, a mushy unpleasant form of distortion suddenly appeared [emphasis added] and a constant multi-frequency whine was superimposed on the analog signal.

And in Reply #7, rbrucecarter5 wrote:

They attempt to eliminate hiss by phase modulating the IBOC signal from 5 kHz to 10 kHz, then revert to amplitude modulation from 10 to 15 kHz. There are some problems with that approach:

-- if the phase response of the antenna system is not flat, phase modulation will appear as amplitude modulation. And getting an AM array, especially a directional one perfectly flat in phase is next to impossible.

-- if the radio is not perfectly tuned, the phase modulation will appear as amplitude modulation. Given the little tiny tuning dials and complete lack of anti-lash mechanics on cheap radios, it is virtually impossible to tune a radio precisely on frequency.

And finally, in Reply #9, rbruce wrote:

I was lucky enough to stumble onto a IBOC AM station not running any analog - and I tuned up and down the channel to hear what the hiss would be like. I didn't have to go but a few Hz off center channel to start hearing the sidebands, and they were there with a vengeance 100 Hz off channel. The weirdest thing happened right on channel - a faint warbling sound, like something on the two sidebands was different. Weird stuff, and it only showed up when I was almost exactly tuned. [Emphasis added.] I’ve never heard anything like this on an analog signal that runs dead air - a fairly common occurrence on brokered stations.

I think there’s a connection here.

That “faint warbling sound” Bruce described is the accidental AM resulting from poor phase response in the station’s antenna system, slight mistuning of the receiver, or both. The sound you hear is actually the intermodulation product of several very low frequencies – bass and/or infrasonic. And it cross-modulates the audio, when there is audio – just as, when there’s ripple in the B+ of an old tube radio with a failing electrolytic across its rectifier, that ripple cross-modulates the audio. But effect of the “warbling” sound, which is constantly shifting, is less easy to identify, if no less grating on the ear, than the effect of a constant 60-cycle (half-wave) or 120-cycle (full-wave) B+ ripple.

Does that hypothesis sound reasonable?

 

[radioskeptic]

In Reply #13, RememberWHEN said:

And then there is the required restricted bandwidth (< 5kHz) of the analog modulation to prevent interference with the digital sideband pairs. Now there's a good idea...NOT. Two tin cans and some string.

You’re being too kind, RememberWHEN. Last month someone (it may have been you—I don’t remember) had made that comparison, and I cited it in a post on the Philadelphia board about Radio Disney. I think you’ll enjoy it, so I’m quoting it here:

But fifth-rate (and worse) frequency assignments and power levels in most markets aren’t RD’s only problem. They’re making things much worse for themselves by further compromising their already inadequate signals with the Hiss-O-Matic system, better known as “HD” AM. ..

Of course, hiss isn’t the worst thing about the system. The dull, muffled analog audio is. It’s been described as sounding like a pair of tin cans connected by a string, but tin cans actually offer slightly better articulation. To me, it sounds more like what you’d hear if someone was talking through the empty cardboard tube from a roll of toilet paper, with a handkerchief over the end of the tube to further muffle whatever consonant sounds survived the journey! It’s no wonder that kind of sound is driving whatever audience RD has to other platforms, especially the internet.

(In case you want to look it up, it's from Reply #13 on the thread “Radio Disney good scores high with Moms and Kids, but WWJZ??” Yes, that thread title is absolutely sic, with two question marks.)

 

[rbrucecarter5]

This is fascinating stuff. Rbruce, what station was running IBOC without analog?

It was KMKI Dallas. They used to run a lot of brokered stuff, sometimes they got careless and ran dead air, usually Saturday mornings.