Since the HDC codec provides 15 kHz bandwidth in stereo, while analog AM
stereo is limited to perhaps 9.5 kHz (given an ideal receiver and
understanding the absolute need for a 10 kHz carrier-beat filter), the
digital signal will sound far better to anyone whose hearing acuity
extends to 15 kHz, which is the bandwidth commonly accepted as being
necessary for an impression of high fidelity.

The digital signal will also have a noise floor in excess of 90 dB below
100% modulation, making it immune to the static and manmade noise that
has caused so many problems on the band in the last few decades.

Even disregarding the subtleties (of which there are many), these two
factors alone make HD AM dramatically superior to analog AM stereo.

In article , says...


After universal complaints about horrendously poor audio quality,
iBiquity has revamped their IBOC digital AM system by switching to the
"HDC" codec -- a slightly warmed-over version of AAC Plus, as used by
Digital Radio Mondial (DRM).

iBiquity claims a large improvement in quality -- but can IBOC, now
with HDC, even match up to the quality that analog AM Stereo has
offered for decades? Listen, compare, and judge for yourself:

http://www.geocities.com/kevtronics