TheEC: Better HD Radio for 102.7FM

Nov 15, 2014

Spectrum analyzer of WRNI-FM's signal, with adjustments in progress.
Credit Aaron Read RIPR

Friday night and mid-day Saturday (Nov.14 & 15) ended up being a bit of a saga for what was supposed to be a routine upgrade for our 102.7FM signal in South County.   So first off, an apology to our RIPR listeners on 102.7FM, and to MVYradio's listeners to the 96.5FM signal in Newport.  There were several dropouts, periods when RIPR was on backup transmitter (and thus 96.5 was off entirely), or when both transmitters were down.

Exactly what happened could be described as an avalanche of minutiae, but I'll lay it out as best I can.

You may recall that RIPR's 102.7 signal (WRNI-FM) broadcasts in HD Radio.   This is a digital transmission system that operates in conjunction with traditional analog FM broadcasting...also known as an "In-Band, On-Channel" system, or IBOC.  (pronounced "EYE-bock")   In addition to being a more robust transmission method than analog, and thus requiring a fraction of the wattage to achieve the same coverage (more on that in a minute), HD Radio allows for more than program channel to be broadcast to any HD Radio-equipped receiver.   Since HD Radio is the official NRSC standard for digital audio broadcast in America, more and more radio receivers in the market are HD Radio-capable, especially in cars.

Now, you may also recall that MVYradio leases the HD2 digital subchannel of our 102.7FM signal for the express purpose of providing content to their 96.5FM signal in Newport.   But as HD Radio receivers become more commonplace, there have been listeners who report that they like being able to listen to MVYradio by tuning in directly to the 102.7-HD2 broadcast.   Even though it's only audible to HD Radio receivers, the 102.7-HD2 signal covers a lot more area than 96.5 does.

Graphic representation of an FM HD Radio transmission, showing the digital HD carriers on the left/right of the main analog carrier.
Credit Chriss Scherer, Radio Magazine

  It's here in our tale that I have to really delve into engineering arcana!  :)   I mentioned before that since HD Radio is a digital medium, it has advantages in signal robustness, and a much lower-wattage signal can cover the same area than a higher-wattage analog signal.   In fact, the original NRSC standard for HD Radio specified that the digital carriers be a whole -20dBc below the analog carrier.  (dB is short for "decibels" - and dBc means "decibles relative to carrier")  That's a mere 1%!  So to duplicate the coverage our analog signal on 102.7 achieved with 1950 watts, you only needed 19.5 watts of digital.

Well, that was the theory, anyways.

As HD Radio transmitters were rolled out into the real world, it quickly became apparent that, for most stations, -20dBc for digital wasn't enough power to really duplicate analog coverage.   These things happen, and when you're talking about over 11,000 AM & FM stations across the USA, it's hard to predict things in the lab.

RIPR's Nautel V7.5 transmitter

 So broadcasters petitioned, and the FCC agreed, to allow more digital power.  Everyone got blanket permission to increase to -14dBc digital power (4%) and some were allowed to -10dBc (10%) on a case-by-case basis.  

Unfortunately, our 102.7FM signal would not qualify for more than -14dBc...but even so, going from -20 to -14dBc is a hefty increase!  That's +6dB, or effectively doubling the power, then doubling it again!  (from 19.5 watts to 78 watts)

That's enough of an increase to both make 102.7-HD2's listening area noticeably larger, and noticeably improve the reliability of reception within that area, too.  

Better still, technically we have the hardware in-place to do it.  

Best of all, this weekend I had a need to hire a local signal measurement expert who owns a spectrum analyzer.  These are pricey beasts; usually several thousand dollars, if not several tens of thousands.   RIPR doesn't have a big enough need to own our own, so we contract out jobs like that to someone who does a couple times a year.   Completing an HD power increase for 102.7 would also require a spectrum analyzer for the calibration, so I can kill two birds with one stone!

Seems like a no-brainer, right?  Well, not so fast...

Our Nautel V7.5 transmitter for 102.7 is fairly new, but it's still of a very early-generation of HD Radio transmitters.   As such, it was designed to operate with with the digital carriers at -20dBc injection levels...and that's it.   You CAN modify it to transmit at higher injection, but it's not a simple process.  It requires following a long procedure involving deep-level programming using a hex editor and a serial interface, not unlike what computer programmers did back in the 1970's and 80's.   Your intrepid engineer actually does have some experience in that realm, so I waited until it was "after hours" to give it a try Friday night.

He knows what's coming.
Credit Bill Watterson

Here's where things started to go wrong.  

Unfortunately, your intrepid engineer has experience, but also has butterfingers.  I hit one wrong key in the sequence...JUST ONE KEY...and suddenly erased several key calibration values.  The Nautel M50 exciter suddenly couldn't interpret the delicate balance of digital HD created by the exciter against the digital carriers the larger V7.5 transmitter was actually putting out; known as the Reverse Path.   No Reverse Path, no HD Radio.


A frantic call to the after-hours tech support revealed that while our V7.5 isn't exactly old (less than a decade) it's from a first-generation of HD Radio equipment and most transmitter manufacturers retired first-gen stuff real fast when the second generation rolled out about seven or eight years ago; the improvements were huge.  All well and good, but for me it meant that the only guys who might know what the proper calibration path values are weren't on call and wouldn't be reachable until Saturday at the earliest, but maybe not until Monday.


And it's entirely possible that the calibration can't be done in the field, and I'll have to send the whole exciter (some 30-odd pounds of large, expensive, and hard-to-ship electronics) back to the Nautel factory in Nova Scotia, Canada.


By now I've got RIPR back on the air using our backup transmitter, an old Harris HT3.5 transmitter, but the backup can't do HD Radio, which left MVYradio dead in the water on their 96.5FM signal.  I call up the gentleman who owns and runs the 96.5FM to go over things with him.  He's not actually an MVYradio employee, although he knows their owner.  And by fortuituous coincidence, he's also a signal measurement expert who taught me a good 40-50% of everything I know about radio engineering...and also happens to be the expert I was hiring to use his spectrum analyzer this weekend.

Radio broadcasting, like Rhode Island itself, is a very small world.  :)

Yes, engineering has moments like this.

 We brainstorm over ideas for a while about what we can do to get MVYradio's audio to 96.5FM first thing on Saturday morning.   Eventually we settle on using RIPR's Comrex ACCESS portable audio codec with a Verizon Wireless 4G hotspot card.  

And by now I've killed the power to the Nautel entirely and I'm making preparations to run on the backup transmitter all weekend.   Just before I leave (and I've been there for about two hours now) I fire up the Nautel to check a setting...and...



...and miraculously it works again.  (!!!!!)

I don't know how or why it did start working again, but I'll take it!   And the thing is, I did cycle the power on the transmitter repeatedly, trying to get it to reboot to defaults, and it didn't work.  I suspect there are are embedded default values in the firmware, but it takes an extended power outage to flush the values I entered (by mistake) from the volatile memory.

Now I can get back to continuing the procedures to increase the HD power levels.  I did seriously consider backing away slowly and abandoning the project, but I was already at a point where it'd take more work and introduce more risk to change things back than it would to plow ahead.

Fortunately, things mostly worked okay.  Took some six different control get the measured HD values into the ranges they needed to be.   But that's okay, it was all within the realm of what the instructions covered.   So that's good.  I've got things running stably into a dummy load, it's 11pm, and it's time to switch things back so MVYradio is back on the air and RIPR is back to normal.

I shut the transmitters down for 30 seconds (that's normal, you can't switch things when they're running or it'll scorch the antenna switch an electrical fire) turn off the dummy load to disengage the safety interlock, press the button to transfer the antenna switch annnnnd....


The darn antenna switch just refused to, well, switch.   Grrr.  So I can't put the main Nautel transmitter back on the main antenna!   After five minutes of frantic fiddling...while the transmitters are still off and it's dead air all around...I give up and manually force the antenna switch.   That was not easy; those suckers are meant to be motor-driven and it takes some hefty hand- and arm-strength to do it!    (now guess what my next project for next week is going to be!)


But it's done...

....and I'm crossing my fingers and hoping against hope that the antenna switch actually switched and that I'm not about to start an electrical fire,

...and I press RF ON on the Nautel,

...and it fires right up.  Works just fine.  


Now, fast-forward to Saturday afternoon, and I've got our signal expert and his spectrum analyzer set up, and we're further tweaking the settings to make sure the added HD Radio injection levels don't warp the spectrum signature of 102.7FM.   There's a standard, called a "mask" (you can see it in the pic at the top of this article), and as long as your signal levels are below it, you're good.

Of course, ours weren't below it.  Drat.

So it took a lot of fiddling, and eventually I had to reduce the HD injection level from -14dBc to -15dBc...a small price to pay.   But the rub was that I had to make a lot of small, incremental changes, and each time the transmitter had to be "off" while making the change.

That meant a bunch of 10 second long outages during the 3pm rebroadcast of Only a Game for our South County listeners.   I'm sorry about that, folks.  I really thought it would just take one or two small changes to get things where they needed to be,'t.  We had to tweak and tweak and tweak and tweak and tweak.   Each time one thing was changed, it changed how other things needed to be tweaked as well.   So it was a real case of sneaking up on the final settings that (just barely) met the "mask" and allowed me to slap a label on it and call it done.

But like I said, the good news is that 102.7-HD2 is now listenable over a wider area.  MVYradio fans even as far away as East Bay (Bristol County) with HD Radios should be able to pick things up.   And RIPR fans, also with HD Radios, can enjoy high-fidelity digital NPR over that same area, too!