AM HD TURNOFF PACE ACCELERATES

[Zach]

Since my last post I got an Insignia radio for FM IBOC and on FM it sounds good. I tried it out on one of the Mississippi Public Broadcasting FM stations on Mississippi. I wish that more stations in Mississippi would go the FM IBOC route. Here in Mississippi there wasn't a station here that went the AM IBOC route. Not even Clear Channel in Jackson had their  AM stations doing IBOC on AM.  I am also glad that WWL-AM in New Orleans did not go the AM IBOC route either.

I do have a question about Memphis. Why is WREC still IBOC and WDIA is no longer IBOC? They are both Clear Channel stations but WREC is Talk and WDIA is urban.

Clear Channel skipped over Jackson entirely with HD, or at least that was the case when I lived in the state a few years ago.  The only HD in Jackson is MPB and Jackson State's jazz and news public radio station.  MPB's statewide HD network is really, really good and has some strong HD coverage.

As for Memphis, my understanding is WDIA has gone through some issues at their transmitter site.  Either tower trouble or flooding trouble or both, I forget exactly what, so they have been running lower power on an STA for a long time.

But my guess would be that WDIA, being an actual icon of broadcast radio in Memphis and still very heavily listened to by a VERY loyal core audience, got complaints about the sound quality so they turned it off.  Or maybe it just broke.   The WREC site never flooded during the recent incidents and never went off the air during the worst of it.  In fact I think one of the other Memphis stations, maybe WDIA, was using WREC's site for a while as emergency facilities.

[iyiyi]

One more time... WGN radiates a 50,000 watt carrier at all times.  If WGN 100% modulates,  720 will radiate 75,000 watt signal.  50kw for the carrier plus 25kw in sidebands.  In an analog AM signal at 100% modulation, 1/3 of the power is in the sidebands.

Should WGN decide to add IBOC, at 100% modulation the analog carrier will be 50,000 watts.  The analog sidebands will add 25,000 watts to the signal -- exactly the same as if they didn't run IBOC.  The IBOC signal is a completely different animal.  It currently runs at -20dBc.  This means the IBOC would radiate a 500watt QAM signal using OFDM.  This OFDM basically provides a "constellation" of data.  The orthogonal carriers provide a Y axis component to produce a lattice that maintains a means of coordinating this data constellation into a useable QAM output.  Unlike analog, which produces a constant carrier output augmented by modulation percentage, the QAM signal produces a steady RF output regardless of modulation.  IBOC has no effect on the analog carrier or it's modulation levels.  Analog has no effect on IBOC's signal.  When an analog signal is unmodulated, only the carrier is emitted.  IBOC emits a constellation of data regardless of modulation percentage.

One more thing:  Take a sweep generator and check the IF response of as many different AM receivers as you wish.  Get back to me with how many of them have -3dB down points much above 3,500Hz.

-   

[Tom Wells]

Perhaps I am misleading by not specifying "Instantaneous power output."

Agreed that full modulation of a 50kw is 75kw, the carrier plus the audio, but that's the positive side of the modulation.
At the instaneous opposite half of the sine wave, the audio is not adding the amplitude, it's subtracting.

If the audio modulation becomes too high, the waveform becomes discontinuous, and we hear splatter across a MUCH
wider bandwidth.  The RF envelope is then "pinched off" and the square cornered waveform produces many products
that only sound bad.  Either on that frequency, or adjacents.

I agree that the iboc "constellation" is, unchanging, 500 watts on a 50 kw.
The terminolgy is much like calling a migrane headache a "halo".
Five hundred watts of screeching interference which would have been available for the "human-intended" modulation.

Analog modulation certainly has an effect on a iboc signal.  If it's too high, and the carrier pinches off, there's no carrier left
for the constelaltion to be relative TO, and then decode drops.

Which is why ibiquity states that neagtive modualtion peaks MUST be limited at -94% in order to insure
the carrier is always still somewhat there.

IF response, width and skirt shape vary all over the place.
IF response also varies by what frequency is chosen. 
I have a high preference for AM using 262.5  khz.
Then, too there "hi-fidelity" flat-top IF "stagger tuning" that permits center tuning while still keeping high frequencies
in balance.

There's coils with overcoupling, unity coupling and undercoupling.
These all "behave" differently.
The whole point of "tuning" is to select the user's desired sound.
The "average" radio, up until the point at which fixed tuning steps were introduced, was about 6 db down at 10 khz.
This would permit side tuning of local stations to achieve a balance between low and high frequencies
that was as crisp or dull as desired. 
In cases of listening to strong distants, the frequency balance was usually pretty good center tuned,
and the 10 khz whistle was still audible, but center tuning allowed the user find a minimum point.

I might have a few radios areound here as dull as 3 db down at 3500.
Any of the radios with magnetic "can" headphones, for sure.
1927 Atwater Kent model 35
1934 Philco cheapie cathedral
1930 Majestic Super screen grid
A few really cheap 5 tube AMs

Last would be my wife's Kenwood HD car radio. 
I really must make an aircheck of how weird it sounds on AM.
The sensitivity is fine now that it has a real whip antenna, but even a telephone landline never sounded so bad.
It can't be tuned at all, it only has fixed preset "steps".
Maybe the IF reponse is wide enough to sound OK, but we'll never know because there's no way to tune it.
Even the Collins R390 A in the 4khz filter mode has enough crispness to enjoy most signals, if not center tuned.
Once the IF bandwidth gets that narrow, the ability to tune continuously becomes more important.

[iyiyi]

You have analog QAM and digital QAM confused.  CQUAM limited negative peaks at 96% modulation because at maximum modulation, the carrier is phase shifted relative to the linear sidebands.  This difference results in an effective carrier that instantaneously appears to be 96% of what it should be to the sidebands.  Negative modulation peaks are limited to 96% for THAT reason.  There are no such constraints with an IBOC analog OR digital signal.

[Tom Wells]

You have analog QAM and digital QAM confused.  CQUAM limited negative peaks at 96% modulation because at maximum modulation, the carrier is phase shifted relative to the linear sidebands.  This difference results in an effective carrier that instantaneously appears to be 96% of what it should be to the sidebands.  Negative modulation peaks are limited to 96% for THAT reason.  There are no such constraints with an IBOC analog OR digital signal.

No, there are no such constraints, but running the modulation past the point where the carrier is so weak or neutralized that there is
no reference for the constellation, the "bits" in the constellation do not "exist".
Sufficent data redundancy, buffering, and high/low data modes permit some loss of data.

If you have a suggestion for how the constellation can exist without reference to a "somewhat" continuous rate function such as a sine wave or even a square wave, please elaborate.

Even though the power available in the "carrier" varies as modulation subdivides the power into sdiebands, enough remains
(if  not cut off completely) to provide reference for the 500 watts of data modulation at the frequencies above 5 khz.

I've never had occasion to find that my mind has generated "false facts", so I can generally consider that when my mind tells
me I've read something factworthy AND it fits in with reality as I have experienced and been taught in radio engineering school,
then it's  a fact, amd I retain it for reference at times such as this.

Please cite ibiquity documentation that would suggest uncontrolled, over 100% negative modualtion peaks are "just fine".
Even the digital-only ibiquity AM method REQUIRES a small "vestigal" carrier for the bits to be referenced to.

[muiscmike]

Since my last post I got an Insignia radio for FM IBOC and on FM it sounds good. I tried it out on one of the Mississippi Public Broadcasting FM stations on Mississippi. I wish that more stations in Mississippi would go the FM IBOC route. Here in Mississippi there wasn't a station here that went the AM IBOC route. Not even Clear Channel in Jackson had their  AM stations doing IBOC on AM.  I am also glad that WWL-AM in New Orleans did not go the AM IBOC route either.

I do have a question about Memphis. Why is WREC still IBOC and WDIA is no longer IBOC? They are both Clear Channel stations but WREC is Talk and WDIA is urban.

I was in Mississippi and Alabama a few weeks ago.  The MPB station in Meridian, MS  had consistent signal in HD in Union, Mississippi and Silas, Alabama.  I also heard some HD stations in Mobile, Alabama.  I was listening to gospel quartet music on WKSJ 94.9 and the HD was a few seconds behind the analog.

[Zach]

I was in Mississippi and Alabama a few weeks ago.  The MPB station in Meridian, MS  had consistent signal in HD in Union, Mississippi and Silas, Alabama.  I also heard some HD stations in Mobile, Alabama.  I was listening to gospel quartet music on WKSJ 94.9 and the HD was a few seconds behind the analog.

Yeah there's been a mismatch for a month or two now.  The HD went off a while back, and when it came back on, the audio was all screwed up, sounding really warbly and tinny in HD on the main channel but basically OK on the HD-2.  Been like that, with the bad delay, ever since.

[satech]

You have analog QAM and digital QAM confused.  CQUAM limited negative peaks at 96% modulation because at maximum modulation, the carrier is phase shifted relative to the linear sidebands.  This difference results in an effective carrier that instantaneously appears to be 96% of what it should be to the sidebands.  Negative modulation peaks are limited to 96% for THAT reason.  There are no such constraints with an IBOC analog OR digital signal.

You're thinking of the defunt Magnavox PMX AM Stereo system, which required negative modulation to be limited to -95% to prevent the receiver from "popping". C-Quam does not have this problem, and can be fully modulated to -100%.

The cosine correction in the receiver's C-QUAM decoder chip does reach its maximum level at this point, so you may hear some distortion in a stereo receiver as the carrier approaches pinchoff, but there is no objectionable popping noise, and no technical requirement -- either by Motorola or by the FCC -- that the negative modulation must be limited below -100%.

[Geographer]

WLW has had their IBOC off for a few days. I hope it stays off and they return to their 10-15 khz bandwidth. Their audio used to sound delightful on a wide band radio. If fact, most cheap radios have a wide bandwidth and AM can sound fairly good on them. I'll gladly give up receiving WOR at night in return for a better sounding WLW. I hope their engineers read these posts. Leave IBOC on the FM band where it works better.

[MarioMania]

Just turn it off and go back to stereo