Problema copertura: pregi e difetti del digitale

Craig Healy, DXer americano ha pubblicato un breve editoriale con il suo personale giudizio sulle potenziali conseguenze della controversa decisione della FCC, che il mese scorso ha autorizzato l'uso di IBOC sulle onde medie anche dopo il tramonto locale (una eventualità che, si tema, porterà a un inaccettabile aumento di interferenze alle stazioni lontane che operano sulle frequenze adiacenti). Attiro la vostra attenzione sul passaggio in cui Craig racconta di aver utilizzato almeno due apparecchi HD Radio e in base alla sua personale esperienza conclude che il "segnale digitale è molto fragile, la copertura possibile è pari ad appena un quarto di quella che un buon segnale riesce ad assicurare." Questa è una tematica che non mi stancherò di dibattere, per cercare di risolvere una volta per tutte i dubbi che riguardano la presunta superiorità dell'una o dell'altra forma di modulazione, analogica o digitale che sia.
Intanto, buona lettura delle interessanti osservazioni di Healy:

Editorial on the recent approval of HD Radio (IBOC) in the USA

On Thursday the 22nd of March, 2007 the Federal Communications Commission approved all facets of digital broadcasting in the USA. In particular, AM stations will soon be authorized to begin 24/7 use of their first adjacent frequencies for digital sidebands.

What this does is open the door to significant interference to all AM stations. A 50kw clear channel station can run a digital transmitter at far more power than would be authorized for any conventional station in that location on those two adjacent frequencies. It will be interesting from a rather perverse sense to see what happens at night..

I have had two HD Radios, and found that the digital signals are very fragile, and cover about half the radius (1/4 the area) of a good analog signal. Electrical noise from many sources like car ignition, light dimmers and bad wiring can easily disrupt things.

The programming is the real draw to a station, and HD Radio on AM will provide no change at all. A talk show in digital "high quality" is still a talk show. In the 35+ years I've been a broadcast engineer, I have seen a small handful of complaints about audio quality. These have always been when there is some correctable flaw, like a hum. Quality is not the issue and never has been.

I saw an interesting comment. Name and location deleted to protect the writer:
"The latest e mail from xxxxxx in xxxxxx said that the reason the big boys in the big markets are so pro IBOC is because they like the hash as it wipes out distant signals getting into their market. There is no way to stop skip, but if the IBOC hash wipes the signal out, then the locals will have to listen to their local station. Kind of like legal jamming. Considering that, then even if the public does not buy the radios, keeping the IBOC signal might be worth their while."

Jamming is illegal in many ways. If some entity has manipulated this process to allow it, then appropriate action should be taken to shut it off.

One possible result is many smaller non-IBOC stations refusing to drop power at night in an attempt to keep their current coverage area. Or, installing bigger transmitters to outright cheat to keep the coverage radius they had pre-IBOC. That would make a very interesting case, should it ever go to court. Two wrongs don't make a right, but when one of the wrongs is legalized, all bets are off.

Shouldn't the digital sideband powers be calculated by the same rules as used for analog signals? Just plopping in a new signal on a frequency without any regard for it's interfering effects is a recipe for chaos. No new station could be allocated today without a full allocation study. Why are these new digital allocations exempt? In effect, they are really two new stations in themselves.

What if there are unequal powers allocated for the upper and lower sidebands? For technical reasons, IBOC requires the two sidebands to be of equal strength to minimize interference to the analog signal. Didn't they see all this in the formulation stage? Or did they simply decide to ignore the obvious and forge ahead, oblivious?

There are some stations that have directional arrays that may never be able to be compliant with the specs for IBOC transmission. As a result, they will not be able to run it. Is it fair that some can and some cannot be digital? If digital-only operations are mandated, will these stations be forced to go dark? Is that fair to their communities? It would seem that many small town stations cannot afford the license nor the technical upgrades. Again, if digital is mandated, do these small towns simply lose their station, even if it is the only one? That seems grossly unfair.

Customers are staying away from these radios. Market penetration is very much below the radar. They hope to sell two million HD Radios by the year 2010. That's about how many iPods sell in a month.

Many years ago WLW had a project for HiFi radio. They made sure their transmitting plant was flat out to +/- 20KHz. There were receivers in that time that were up to that task as well. I would dearly love to hear that setup today, and compare it to the HD signal.

Instead of inventing this intrusive technology, they could have spent their efforts on DSP-based receivers to minimize impulse noise and other interference. A good DSP decoder can null out a coherent tone such as a 10KHz heterodyne without a negative effect on the audio. It could also compare the upper and lower sidebands to determine what would give the better sound. It could narrow the bandwidth to the most efficient point for best reception. And, the DSP decoder could look over a wide swath of spectrum to detect wideband noise and remove it. The technology is there, but they chose to ignore it.

Heck, if they could get such a good analog signal decades ago, what happened? What if a station today decided to use the mask designed for IBOC and implement a wideband analog signal? Would it be legal? How would that sound compared to the HD version,. assuming a good wideband receiver could be found? What if it even sounds better than HD Radio?

One of the significant problems of AM these days is the shielding effects of newer steel and concrete building construction. AM simply doesn't penetrate. Why did they think that overlaying a digital signal on the same band would penetrate these buildings any better?

A better overall plan would have been to embrace streaming technology. WiFi access is exploding, and WiFi-enabled iPods will soon be introduced. An iPod can do many things and at less cost than most single-purpose HD Radios. Streaming audio can have much better quality than even the best HD signal. Instead of HD1 and HD2, it is possible to have unlimited streams. The formats that can be streamed by a station are limited only by their resources and imagination, not some technical limit of their transmission medium. Reception of a stream is possible anywhere there's an internet connection, not limited to the coverage radius of a single transmitter. It literally is global. Power costs to run a stream are insignificant compared to a transmitter. No large towers, or arrays of towers are needed. Streaming is far more "green" than IBOC, if you care to look from that perspective. In terms of quality, variety and environment, HD Radio is obsolete right out of the box.

From a business angle, having the FCC mandate a proprietary system is unprecedented. All previous systems for anything were all open source. The technology to create a piece of equipment was there, every parameter. With HD Radio, none of these parameters are public, at least not enough that some clever engineer could roll their own. This is very unfair, and smacks of a monopoly. The whole HD Radio specifications should have been in the public domain. Were someone to reverse engineer this method, no doubt they would be hauled before a judge and significantly fined. All because some company has convinced the federal government that it has the only way to do the job. The parallel would be if the government decided that to drive on the interstate you had to buy a new Buick.

Recently it was announced that there may be pay-to-listen encrypted HD broadcasts. Could this be why they want the system proprietary? So nobody can legally write a decoder to bypass their pay scheme?

In conclusion, I'll relate a conversation I had with someone who has an extensive non-technical radio background. A local Clear Channel GM gave this person an HD Radio. It was tried, and shut off. Decoding the signal was very problematic even within the city grade contour of several stations. This person related to me that HD Radio was a bad joke and a complete waste of time. This was said without me even bringing up the subject. It seems to be a common "joe average" result when a non-radio person tries HD Radio. Too much effort to get a signal. They want to turn it on and get flawless audio, just like their iPod. It doesn't happen, so they simply return the radio for a refund saying it "doesn't work".