AW: HD Radio is a farce!
The FM bandwidth in Europe is only half of what it is over there (100 kHz instead of 200 kHz), and as 40 kHz of this bandwidth is needed for the analog service, HD Radio can only use 60 kHz for the digital service (if not less because of clearance distances) instead of 160 kHz in America.
It's a bit different. 200 vs. 100 kHz is the channel spacing, not the bandwith. The digital component of an IBOC/HD system extends up to +/- 200 kHz, thus an IBOC signal has a bandwith of no less than 400 kHz.
The crucial difference is that in the USA no FM stations serving the same place are closer than 800 kHz to each other. Contrary at Berlin (as an example) a number of stations operate within 500 kHz of each other, and in rural areas it is common to have stations with overlapping coverage areas within 300 kHz of each other. There is just no room for the +/- 200 kHz digital component of IBOC, even when accepting that reception of distant stations is no longer possible.
It´s a different story in AM anyway, as the US have 10 kHz bandwith and Europe only 9, afik there is no solution for that problem yet.
Again, 10 vs. 9 kHz are just the channel spacings, that's not what makes the real difference. With the NRSC standard in the USA audio up to 9.5 kHz can be sent into the transmitters, which means that the complete spectrum of carrier plus two sidebands occupies 20 kHz. The ITU-R (formerly CCIR) specifications for Europe actually call for the infamous 4.5 kHz cut-off, but with a careful set-up one can stretch the ITU-R limits to 6 kHz of transmit audio (and quite a number of European AM stations indeed do that).
The AM system of IBOC / HD Radio (taken aside the digital-only variant) uses two digital sidebands, extending up to 14.7 kHz from the carrier. This means that such a signal also occupies the two neighbouring channels. One could say that American AM stations always did this with their analogue sidebands, but it makes a difference if it is just sideband splash or a digital signal (which causes a continuous hiss in analogue radios, as can be studied on any DRM signal). The latter ruins a station on the next channel that previously was still listenable, and basically this concept ruins skywave reception of distant stations at night.
I think Brazil experiments with the AM IBOC system, too. But even Canada does not and considers DRM instead (although most likely AM will simply die out there). So there is no point in even discussing it for Europe.
