AM HD COVERAGE

[ve3jf]

I posted some thoughts on coverage under the "WJR" topic, but since that thread has wandered far afield, I decided to start a new one. Musing further about the AM-HD debacle, it occurs to me that the coverage of an AM-HD station can be divided into three distinct zones. Let's look at the value proposition for your average 50 kW blowtorch:

Zone 1 (the "inner circle"):
In this zone, digital reception is generally available day and night, and analog reception is also good (though somewhat degraded by restricted bandwidth), with the usual exceptions due to localized noise sources, building penetration issues, etc.

Zone 2 (the "middle ring"):
In this zone, digital reception is generally available during the day, but it disappears at night. Analog reception is still good (maybe, but see below).

Zone 3 (the "outer ring"):
This zone extends out to, and somewhat beyond, the nominal NIF contour (well beyond it during the day, of course, and at night it depends on one's tolerance for interference). There is no digital reception possible here. Analog reception (at least in pre-IBOC times) is generally okay, though there are more exceptions due to local noise sources and the like.

I would guess that the area covered by Zone 2 is considerably more than Zone 1. Overall, depending on how the population is distributed, it seems likely that the majority of listeners with HD receivers will lose their digital service and drop back to analog after dark. The area of Zone 3 is larger than the other two zones combined, though population densities are probably much lower here. This is where the big losers are. These unfortunate souls have no possibility of getting digital service even if they're willing to buy new receivers, and their nighttime analog service is badly degraded or destroyed by the hiss from adjacent channel HD stations. In many cases, in fact, this interference will intrude into Zone 2, so users with HD receivers here may see their digital audio replaced with hissy analog after dark.

So, digital is only a simulcast, and is only available where analog reception is already good. Meanwhile, analog reception is degraded over the majority of the nighttime coverage area.

Somehow, this doesn't sound like a sound business proposition to me.

Barry

 

[JohnnyElectron]

I have only reliable HD-AM coverage in Zone 1. My two AM-HD's that are great in the day are still very good at night in HD -provided you 'tweak' the loop antenna. Mobile coverage is 98-99%. In their city of license, they are just fine (except some might claim 'exhaustion' from the digital audio processing).

WJR in Zone 2 (before they shut if off) was 95% on non-thunderstorm days, but only 50/50 at night AM-HD coverage.
All other 50KW Detroit stations are/were 95% analog days; 75% analog nights; but 0-10% HD lock days, and only 0-5% HD lock at night (at best).

Zone 3: Only two blowtorches even give me their PAD info (KMOX and WHAM).

In other words, where I could use the HD-AM digital to help me dig out a signal when the analog signal is low, gets no help from HD, as their coverage is much less than their decent Zone 2 analog coverage.

The only thing that might work is ALL digital on AM, but that destroys its simplicity and ability to cut thru the worst of what nature can give us. I want the FCC to give us an ALL Digital AM hour sometime soon just to see what happens. My test would be various digital signal levels based on their analog levels: TOH: Station is 25% of analog value; Quarter after: 50% of Analog Signal level; Bottom of the hour: Full Bore until :45; then Quarter of to TOH: only 10% of analog power level, but then again, all digital - no hybrid for just one hour. Try it at 10am Sunday morning, then again 2AM Sunday nite/Monday AM.
One hour a piece.
Any takers?

 

[ve3jf]

The only thing that might work is ALL digital on AM, but that destroys its simplicity and ability to cut thru the worst of what nature can give us. I want the FCC to give us an ALL Digital AM hour sometime soon just to see what happens. My test would be various digital signal levels based on their analog levels: TOH: Station is 25% of analog value; Quarter after: 50% of Analog Signal level; Bottom of the hour: Full Bore until :45; then Quarter of to TOH: only 10% of analog power level, but then again, all digital - no hybrid for just one hour. Try it at 10am Sunday morning, then again 2AM Sunday nite/Monday AM.
One hour a piece.
Any takers?

Some all-digital tests could be quite interesting. I'm not a big fan of the iBiquity all-digital design, though. Aside from its proprietary elements, it has a number of technical shortcomings. Among them:

1. It's still not strictly "on-channel", since it occupies nearly 20 kHz of bandwidth. So, it will still cause (and suffer from) nighttime 1st adjacent interference.

2. It has a ridiculously short OFDM guard interval. This means it has no hope of working well over skywave. Even if skywave service is no longer desired, it will still have big problems decoding in the mixed groundwave/skywave reception zones that will still exist for the stations with significant night power.

3. It retains the use of an unmodulated central carrier that contains much of the transmitted power.

A DRM signal, configured for 10 kHz bandwidth, has none of these problems and would be a vastly superior choice if (when) the current hybrid system falls by the wayside.

Barry

 

[PhilJSmith67]

2. It has a ridiculously short OFDM guard interval. This means it has no hope of working well over skywave. Even if skywave service is no longer desired, it will still have big problems decoding in the mixed groundwave/skywave reception zones that will still exist for the stations with significant night power.

Exactly. This explains why I am only in "Zone 1" for ONE of my local blowtorches (890 WLS @ 19 miles), while my others between 35 and 45 miles (WSCR, WGN, WBBM) almost never lock at night, even when the analog portion remains solid and clear. Meanwhile, when the skywave from KMOX or WTAM is strong enough, each remains locked for a minute or so at a time, even when the analog audio isn't totally clean.

Of course, now that Citadel has silenced IBOC from WLS at night, the only HD Radio I ever get after sundown is from WLW, KMOX, WBZ, WLAC, WTAM, WHAM or WHAS. It's usually for only a brief time, too. I have to say, the Route 66 show on KMOX last night really did sound nice, although it would also sound great in wider-band analog AM with C-QUAM stereo.

 

[rbrucecarter5]

All I know is - AM stations that convert to IBOC have LESS analog coverage than they used to. I've observed it over and over again - fire up the IBOC sidebands = lost analog range.

 

[JohnnyElectron]

As rbrucecarter mentioned - loss of analog AM coverage is more pronounced with HD-AM. The poster child again, is WJR. Their analog analog has much less dynamic range, much lower modulation, and of course less frequency response. WJR may be a more dramatic example as they went from a 10KHz analog stereo signal to a degraded 5KHz HD signal. Their HD audio - although stereo, doesn't sound as wide and as full as their analog stereo did.

 

[RememberWHEN]

On the Indiana board there was a question concerning the challenge of listening to WOWO / Ft. Wayne in the Cincinnati area at night since the downgrading of that signal. Responders addressed the pattern change. power reduction, and groundwave/skywave issues all of which are contributers to the challenge.

My point was (also) since WOWO has gone to IBOC the station audio has suffered greatly in terms of both perceived (read that as REAL) fidelity and NO 'PUNCH' (density). The "Historical WOWO" audio signature was very good!!

Does IBOC raise a fuss when you try to analog modulate greater than 100% positive?? Can anyone address that on this board?? If so, it appears that both WJR and WOWO have 'throttled back' the modulation to reduce the symptoms of the IBOC flu!!

One wonders what the engineers of WJR and WOWO are hearing in their own cars and on their own clock radios (and, oh yes, on the mod monitor in the rack). You'd think they'd certainly be pointing out the degradation to management! I guess a paycheck beats being labeled as a non-team-player.

 

[ve3jf]

One wonders what the engineers of WJR and WOWO are hearing in their own cars and on their own clock radios (and, oh yes, on the mod monitor in the rack). You'd think they'd certainly be pointing out the degradation to management! I guess a paycheck beats being labeled as a non-team-player.

"It is difficult to get a man to understand something when his salary depends on his not understanding it".
- Upton Sinclair

 

[Tom Wells]

Positive modulation is supposedly tolerated to some degree. I believe we have heard WOR runs 105 or 120% with problem, but
no doubt all the punch is gone from the former AM powerhouses who are doing this.
ANY overmodulation to the point of cutoff and splatter would drop every single digital radio out, as I believe the
sidebands reference the carrier for detection. So they cannot run the risk of that as much.
All of them here have lost the punch such that formerly lackluster smaller signals now sound "bigger" as as big by audio comparison.
When first implemented, all the 50kws here sounded like 250 w daytimers on a winter day from 20 miles out.
With audio fed on old unbalanced telephone lines.
Thank goodness ibquity has applied some upgrades, but the results have not been equally helpful.

WJR is one child in a poster of crippled children, all of them having chosen to carry a disfiguring parasite.

In Chicago, WLS has done the best job of sounding like themselves despite iBOC - kudos to Warren and crew.

WBBM has been improving the sibilance almost to the point of equalling WLS, but they never were as crisp.

WGN is the most heartbreaking. Their pre-iBOC signal was as clear as opening a window if you side-tuned a bit.
They weren't the brightest, but they were the purest. Now, they sound like a 1kw county seat station somewhere
in the hinterlands, from 25 miles out. They need to get over what they used to sound like and realize they'll have to crisp
up to be heard over the hissing, still obscuring many important speech details. And their audience is OLD, as we all know, and
doesn't need anything making it harder to understand their radio station.
Pull 2db out from 400 to 1000hz and give us some dynamic-peaked gain from 4khz up. Please?

 

[JohnnyElectron]

Tom begged:
Pull 2db out from 400 to 1000hz and give us some dynamic-peaked gain from 4khz up. Please?

Yep, I agree there; but, analog is gone from 5KHz up with the crappy HD analog filtering, and IBOC HD digital audio is only 'real' up to 4KHz - isn't it all faked replication above 4KHz? That's why it sounds so nasty, in my opinion. I wish they had the AM bandwidth to 'make it work' - if it could even sound as good as XM satrad - but it doesn't yet.

 

[Kelly]

Tom begged:
Pull 2db out from 400 to 1000hz and give us some dynamic-peaked gain from 4khz up. Please?

Yep, I agree there; but, analog is gone from 5KHz up with the crappy HD analog filtering, and IBOC HD digital audio is only 'real' up to 4KHz - isn't it all faked replication above 4KHz? That's why it sounds so nasty, in my opinion. I wish they had the AM bandwidth to 'make it work' - if it could even sound as good as XM satrad - but it doesn't yet.

In my experience the difference is the analog audio sound can be how one performs the HF roll-off. If using a Optimod 9200 digital audio processor, one should not use the 5kHz roll off filter at the output as the sole attenuation. What happens is the filter algorithm creates a great deal of harmonics which then become distortion in the pass-band when highly processed high frequencies are attenuated. Besides, why have the processor, (and clippers), limit audio that won't be used at the output? The best way to perform the roll-off is before the processing, no matter what processor you choose, then use the 5Khz filter as a "guard" for ringing filters, overshoots, harmonics, ect. A simple method is to use a good true parametric EQ, (not a graphic), ahead of the processing. Then apply the proper preemphasis at the processor, with a good 5Khz LPF.

 

[Mike Walker]

The frequency at which spectral replication begins can be as low as 4khz, or it can be octaves higher, depending upon bitrate. THERE ARE NO MUSICAL NOTES ABOVE 2-3khz...no fundamental tones. NO instruments have tuned fundamental "notes" that high. What is up there is harmonics, on even and odd multiples of the fundamental, and they are quite predictable...which is why spectral replication has been shown to sound better AT LOW BITRATES than trying to actually encode very high frequencies, allocating bits better left to the midrange where the ear is most sensitive.

Thousands of hours of LISTENING TESTS have confirmed that artifacts are LESS audible at low bitrates when bits are allocated in this way. It's based upon science...the science of psychoacoustics.

 

[Kelly]

The frequency at which spectral replication begins can be as low as 4khz, or it can be octaves higher, depending upon bitrate. THERE ARE NO MUSICAL NOTES ABOVE 2-3khz...no fundamental tones. NO instruments have tuned fundamental "notes" that high. What is up there is harmonics, on even and odd multiples of the fundamental, and they are quite predictable...which is why spectral replication has been shown to sound better AT LOW BITRATES than trying to actually encode very high frequencies, allocating bits better left to the midrange where the ear is most sensitive.

Thousands of hours of LISTENING TESTS have confirmed that artifacts are LESS audible at low bitrates when bits are allocated in this way. It's based upon science...the science of psychoacoustics.

Honestly, I'm not sure what science you're referring to there Mr. Wizard, but your statements are confused and misplaced.

No musical notes above 2-3kHz? Really? Even string instruments? How about piano? If the results of your "listening test" only extend to frequencies below 4kHz, then I suspect you may have severe hearing damage or loss.

Bitrate has nothing to do with frequency reproduction, but resolution. I'm not sure about you there, but I'm 49 years old, and in a recent hearing test, my range extended to hearing frequencies up to 17kHz. Perhaps your hearing aid is spectrum limited?

 

[Chuck]

The frequency at which spectral replication begins can be as low as 4khz, or it can be octaves higher, depending upon bitrate. THERE ARE NO MUSICAL NOTES ABOVE 2-3khz...no fundamental tones. NO instruments have tuned fundamental "notes" that high. What is up there is harmonics, on even and odd multiples of the fundamental, and they are quite predictable...which is why spectral replication has been shown to sound better AT LOW BITRATES than trying to actually encode very high frequencies, allocating bits better left to the midrange where the ear is most sensitive.

Thousands of hours of LISTENING TESTS have confirmed that artifacts are LESS audible at low bitrates when bits are allocated in this way. It's based upon science...the science of psychoacoustics.

Honestly, I'm not sure what science you're referring to there Mr. Wizard, but your statements are confused and misplaced.

No musical notes above 2-3kHz? Really? Even string instruments? How about piano? If the results of your "listening test" only extend to frequencies below 4kHz, then I suspect you may have severe hearing damage or loss.

Bitrate has nothing to do with frequency reproduction, but resolution. I'm not sure about you there, but I'm 49 years old, and in a recent hearing test, my range extended to hearing frequencies up to 17kHz. Perhaps your hearing aid is spectrum limited?

The highest fundamental frequency of a violin is C-7, which is 2093 Hz; C-8 is the top end of a piano, which is 4186.01 Hz. Some pipe organs have fundamentals ranging to twice that frequency and synthesizers can easily have fundamentals out to 20 KHz (or "DC to light," for that matter).

Note that I said these are fundamental frequencies. If you took away the harmonics, these instruments would sound absolutely dreadful. 10-12 KHz is what most people consider to be "good sounding," but just about anyone can tell the difference in a system that extends well past that point. 16 KHz is pretty well "bare minimum" if you want to discuss fidelity. Even though you may not be able to hear 20 KHz, these harmonic frequencies really do make a huge difference in the way we perceive how things sound.

Have you ever looked at a Mackie mixer and noticed the "Air" control? This peaking EQ circuit, centered at 16kHz, enhances guitars, vocals, percussion, whatever, giving a gentle "lift" to the extreme high end. The result is extra detail and fidelity in the high end. It may be good marketing, but it also works. You can actually notice the difference even if your hearing does not go up to dog frequencies.

While we are talking frequency response, let's not forget the low end. Bass guitar is generally considered to start at 42 Hz, but a Bosendorfer Piano starts at 27.5 Hz. Synthesizers are usually limited to 20 HZ, but some can go lower. There are pipe organs that go to 16 Hz and maybe lower.

Even these bass frequencies have harmonic content that is very important to how they sound. Anytime you discard party of the information on the encoding end, and try to reconstruct it on the decoding end, something is lost. That's the nature of the beast. There is no getting around it. The good news is some codecs are very good at fooling our minds into thinking we are hearing what really isn't there.

 

[ropertex]

1. It's still not strictly "on-channel", since it occupies nearly 20 kHz of bandwidth. So, it will still cause (and suffer from) nighttime 1st adjacent interference.

Analog AM bandwidth is 20 Khz : each sideband is up to 10 Khz from the carrier frequency.

 

[ve3jf]

1. It's still not strictly "on-channel", since it occupies nearly 20 kHz of bandwidth. So, it will still cause (and suffer from) nighttime 1st adjacent interference.

Analog AM bandwidth is 20 Khz : each sideband is up to 10 Khz from the carrier frequency.

This is a facile and simplistic view of bandwidth that is not useful from an engineering point of view. What is important is the "occupied bandwidth", and the standard definition of that is the bandwidth that encompasses 99% of the average power of the emitted signal. For a standard AM signal, even one having the full NRSC bandwidth, the occupied bandwidth is remarkably small. I've measured it to be in the 1 to 2 kHz range. But when IBOC is added to the AM signal, the occupied bandwidth balloons out to 28 kHz. At a little under 20 kHz, the occupied bandwidth of the iBiquity all-digital system is still much greater than that of the analog AM system.

If you look at the international agreements covering AM broadcasting in ITU Region 2 (the Americas), you'll find that channels in the AM band are defined to be 10 kHz wide, though there is a note to the effect that some degree of emissions beyond that limit are permitted by some administrations. In the USA, that bit of latitude has now been pushed to unbelievable extremes. In the rest of the world, 9 kHz wide channels are the norm, and they are much more fastidious about confining emissions to be within those channels.