On Sat, 28 May 2016 22:13:54 -0700 (PDT), wrote:

"SCA sub-carriers never went above
about 80Khz, so there was no increase in occupied bandwidth. IBOC
changed all that by grabbing all the spectrum between +/-100kHz and
+/-200KHz: "

I don't know how familiar you are with high end audio but there
are tuners out there that can switch between narrow and wide band IF,
and I think a few, very few, variable. The wider bandwidth results
in much lower distortion. However then there is more noise and
you need a stronger signal.

What little I've done with broadcast FM was at various radio stations
and designing some low end receivers. That was also long ago, so I'm
fairly well behind on the technology. However, if "high end" means
"high price", I've fixed a few of these systems:
http://www.elanhomesystems.com

If you've ever swept a decent analog FM receiver, you may have noticed
that the IF bandwidth is much wider than the occupied bandwidth
(200KHz) of the signal. In order to keep the group delay fairly
constant within the IF bandwidth, this was necessary. That meant that
while the audio was very good quality, the receiver would pickup junk
from adjacent channels. Sensitivity is limited by the receiver front
end and is unaffected by the wider bandwidth.

Roll forward a few decades and we are now blessed with all digital
receivers. The best example is the Sony XDR-F1HD tuner:
http://ham-radio.com/k6sti/xdr-f1hd.htm
http://www.amazon.com/Sony-XDR-F1HD-Radio-Discontinued-Manufacturer/dp/B00168Q248
These typically sell on eBay for $150 to $250 or $500 with some
necessary modifications. Seems to be a demand:
http://www.ebay.com/sch/i.html?_from=R40&_nkw=XDR-F1HD&LH_Complete=1&LH_Sold=1
The difference is that this receiver has a "brick wall" digital IF
filter that doesn't need to be excessively wide in order to get low
group delay and low distortion. The narrower bandwidth is also good
for reducing much of the alternate channel noise produced by IBOC (HD
Radio) signals.

Meanwhile, the lower end FM receivers are also deriving benefits from
digital IF and demodulators. The Sony tuner uses an NXP/Philips
SAF7730 chip while most of the low end stuff uses various Silicon Labs
AM/FM chips:
http://www.silabs.com/products/audio/fm-am-receiver/
http://www.silabs.com/products/audio/fmreceivers/
Basically, it's almost an SDR (software defined radio) with most
everything in a single chip. Tecsun uses these chips in their various
radios. Plugged into a proper audio amp and speakers, they sound very
good, with little IBOC junk from the adjacent channels. (Note: Not
all Tecsun radios use digital receiver chips).

Actually it is all going digital soon, a couple of countries are in
the process of that right now.

No, it's not, at least in the USA. The FCC, in its infinite wisdom,
has endorse exactly one proprietary and expensive standard (IBOC)
totally owned and licensed by iBiquity. As usual, half measures don't
work. In this case, the broadcast industry wasn't quite sure if they
were going to commit to doing IBOC, so instead of 100% digital FM,
iBiquity was forced to provide the half ass analog/digital compatible
system that we're cursed with today. Incidentally, the failed AM
stereo system following a similar life cycle demonstrating that the
FCC and the industry doesn't learn from their expensive mistakes.

In 2012, a made a graph of the number of HD Radio stations in the SF
Bay area and in the USA.
http://802.11junk.com/jeffl/crud/hd-radio-stations-calif.jpg
http://802.11junk.com/jeffl/crud/hd_radio_stations.jpg
http://www.stateofthemedia.org/2013/audio-digital-drives-listener-experience/9-number-of-stations-dropping-hd-outnumber-those-adopting-in-2012/
There's actually been a decrease in stations, that have unplugged
their HD Radio equipment since 2012 but I don't have numbers handy.

I know little (or nothing) of what's going on with DAB (digital audio
broadcasting) as it's not used in the USA:
https://en.wikipedia.org/wiki/Digital_audio_broadcasting

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558