IBOC notturno: la fine delle onde medie USA?

Per la vasta maggioranza dei DXer americani sarà un "AMmageddon", il definitivo, biblico disastro delle banda delle onde medie. L'antefatto, di cui non ho parlato prima perché non aveva troppo senso, è la definitiva pubblicazione, avvenuta il 15 agosto, della nuova norma FCC che autorizza le stazioni americane che operano con tecnologia digitale HD Radio in banda AM, a tenere acceso il trasmettitore anche dopo il tramonto (questo ovviamente per le stazioni che sono autorizzate al nighttime, che non sono tutte). La norma entra automaticamente in vigore il 14 settembre prossimo venturo. Per quella data, dicono i DXer, il segnale IBOC sguinzagliato nell'onda di cielo provocherà un disastro, sommergendo con le sue intereferenze anche le stazioni adiacenti più lontane.
E' difficile dire se la visione pessimistica è anche realistica. Personalmente ritengo che le interferenze saranno pesanti e che i miei colleghi nord-americani dovranno dire addio a quei segnalini da poche centinaia di watt ascoltati a migliaia di miglia di distanza. Per noi ascoltatori d'oltre-atlantico potrebbero esserci effetti riscontrabili, soprattutto per quelli che hanno la fortuna di ascoltare dal Nord Europa. Ma le nostre orecchie sono quelle del DXer, è facile che i segnali che ci interessano risultino "inutilizzabili": spesso sono già in condizioni estreme.
Il partito dei pessimisti sostiene che la vita sarà difficile anche per gli ascoltatori normali, magari dislocati in città minori non troppo lontane da centri metropolitani. Se le loro stazioni locali si trovano su canali adiacenti a qualche altra emittente da 10 o 50 kW, l'ascolto risulterà assai degradato. Gli ottiimisti, fautori da sempre di HD Radio dicono di no. Chi ha ragione? Lo dirà il tempo, tra quattro giorni si comincia e bisognerà anche vedere quante stazioni sceglieranno di tenere acceso IBOC anche di notte.
Nel frattempo la solita newsletter di Gonsett che mi piace citare spesso, riporta il link a un'altra newsletter, The Local Oscillator (bellissimo titolo, diciamolo), scritta dal capo ingegnere del gruppo CBC (Crawford Broadcasting), che non sta in Canada ma raggruppa alcune stazioni americane anche loro già attive con HD Radio. Il numero di settembre di The Local Oscillator contiene un articolo di Cris Alexander che mi pare molto pacato e fattivo. Dai primi conti fatti da Alexander, non sembra che l'effetto di IBOC nel nighttime sarà poi così devastante. L'Armageddon delle onde medie è già in atto da tempo, grazie ai pazzeschi rumori elettrici delle nostre aree abitative. Un po' di rumore digitale in più potrebbe anche passare inosservato. Il che non è affatto consolatorio, maledizione.

Apocalypse Now?

On August 15, the new terrestrial digital radio rules were at long last published in the Federal Register. The industry has been waiting since March for this. The new rules that were enacted almost six months before will go into effect on September 14.
The digital radio rules contain quite a number of provisions, most of them operational in nature, but one of the provisions – AM nighttime digital authorization – is the one that seems to have the attention of a great many in our industry. Most of you have no doubt followed the arguments for and against in the trade press and online message boards.
Two distinct camps have emerged in the months since the rules were first enacted. The “pro” group is headed by WOR chief engineer Tom Ray. Whether or not he volunteered for the job, Tom’s positive outlook and enthusiastic support of the AM HD Radio platform has put him at the forefront of the group in favor of and looking forward to AM nighttime digital operation. This group maintains that the platform is effective and causes little or no additional interference. Of all people, Tom should know. WOR was involved in nighttime HD Radio tests early on.
The other group, the one that opposes AM nighttime digital operation, is by default headed by Canadian radio engineer Barry McLarnon. This group insists that AM nighttime digital transmission will be the undoing of the AM band. They seem to think that massive amounts of interference will be caused, wiping out the nighttime coverage areas of many stations, especially smaller stations. This group seems to believe that an “apocalypse” will occur on the September 14.
It has been interesting following some of the rhetoric. I saw one prediction that the night limit of WJR in Detroit would be raised to something like 14 mV/m from the interference caused by WBBM. You may recall that some time ago, I had an email dialogue with the manager of WYSL in Avon, NY who was concerned that his nighttime coverage would be wiped out by adjacent-channel WBZ when the nighttime digital prohibition was lifted. This station, WYSL, was used as an example in one of the recent argument rounds.
As with most issues, the truth lies somewhere in between the two extremes, but I believe it is much closer to that of the Tom Ray camp than McLarnon’s version. I’ve done some study on the issue and made some calculations and found that in most cases, the night limit is not raised at all by the addition of adjacent-channel digital carriers.
Take the WYSL example. The station operates with 500 watts night on 1040 kHz, a U.S. clear channel. Its four-tower directional pattern puts a main lobe north toward Rochester. The station has a 50% RSS night limit of 13.87 mV/m with that limit entirely set by co-channel class A station WHO in Des Moines, Iowa. WBZ operates with 50 kW on 1030 kHz, first adjacent to the WYSL frequency. That means that the WBZ’s digital carriers will be right in the audio passband of WYSL. I ran the numbers and found that WBZ currently produces a night limit of 6.25 mV/m at the WYSL site. RSS in the –28 dBc upper digital carriers as a co-channel signal and WBZ’s contribution at the WYSL site becomes 6.727 mV/m, a 7.6% increase. But because 6.727 is still less than 50% of the WHO limit of 13.87, it does not raise the limit. It does, however, increase the interference level slightly. But is it enough to make a real difference? I don’t think so.
The WYSL example is typical of what you’ll find on the AM band. In most cases, the added interference from the –28 dBc digital signals of adjacent channel stations is a drop in the bucket compared to the existing interference from co- channel stations.
There will certainly be exceptions. It seems to me that the skywave service areas of the class A stations will be considerably eroded. Those contours are not protected from adjacent-channel interference, and there are typically many first-adjacent channels within and around the contour. When those stations fire up their digital carriers, the interference they produce will appear as co-channel interference to the already weak skywave signals, making them impossible to listen to in many locations. Some might argue that this has been the case for a long time now anyway. I would tend to agree.
I mentioned WJR above. Someone in the naysayer camp predicted a night limit rise to 14 mV/m from digital interference from WBBM in Chicago. I ran the numbers and came up with something like 2.5 mV/m. That’s nowhere near 14 mV/m, but it is five times the current 0.5 mV/m “limit” for WJR and it does represent real interference. But the truth is that the WJR 0.5 mV/m skywave contour is not listenable in most locations anyway because of atmospheric and manmade electrical noise plus interference from adjacent- channel stations. So again, the truth is somewhere in between the two extremes.
So, will the earth stop spinning on September 14? Will we all fly off into space? Will the AM band as we know it come crashing in? Hardly. In addition to the simple mathematics of the situation as illustrated above, the reality is that there are very few stations ready to go on the air with nighttime digital signals. There are less than 250 AM stations currently transmitting digital signals. Some of those are daytimers. Those stations won’t be firing up digital signals at night. Of the rest, many have different day and night antenna systems. A good number of those night antennas are not ready for digital operation. My guess is that less than half will be on the air with digital signals the night of September 14. If the percentage of the AM stations within our own company that are ready for nighttime digital operation holds true across the board, about 90 stations will be transmitting digital signals that night.
90 stations is a small enough number that we can evaluate the effect of the added interference. The next few months will be interesting as we see what the real effect of nighttime digital is. And if we have to alter the game plan a bit when we have more data, no doubt that is what we will do.
CBC Digital Nighttime

As we roll into September, we are making preparations at several of our stations to commence nighttime HD Radio operations. At three locations, this process is simple: we just leave the digital on at sunset. KLZ, KKPZ and WDCD are all DA-U stations (operate with the same parameters day and night). That makes things easy.
We have some work to do at several other stations. WEXL in Detroit is directional day and non- directional at night. We had to construct and install a phase rotator network between the transmitter and phasor input when we first did the HD conversion to get the digital to work properly. Our sweep of the nighttime antenna input showed that it was acceptable with a good orientation already, so our phase rotator network included an RF contactor to bypass the network at night. Tom Gardull has got to make sure this contactor is properly wired in and working and he has to set up the night parameters in the NE-IBOC before the 14th.
WLGZ in Rochester is another station that will need a bypass relay for nighttime digital operation. The parts have been shipped to Brian Cunningham and he is working on getting the change implemented. We expect good night digital performance from this station.
We have experimented with the 50 kW daytime transmitter at KLTT and found that it works just fine in the digital mode at the 1.4 kW night power. Ed may have some fine tuning to do yet and he will undoubtedly have to set up a different parameter set in the HD generator for nighttime operation. We suspect that much the same situation exists at KCBC. We’ll be looking into that shortly.
We will likely try some of our low nighttime power stations in digital at night to see what kind of performance we get. I suspect that in most cases, it won’t be worth operating the HD after dark, but we’ll see. At all other stations, we will wait and see how things settle out. All of the remaining stations will require significant additional investment to make them work in digital at night. Before we make that investment, we’ll see how the interference picture shapes up. Call me a “cautious optimist” in this regard.