Your old radios will still work; the high-definition (HD) signal
that's currently in use is compatible with good old-fashioned AM
receivers. This is because the digital information is placed (in
frequency) above and below the analog signal. There are couple of HD
stations here in the Washington DC area, and on any AM receiver I've
used so far, including my antique tube radios, this digital "hash" has
not bleed through to the audio stages. When accurately tuned to an HD
station, the voice and music sound just as it always has.

The Ibiquity website has information on how this all works, and at one
time, you could download a couple of conference papers describing how
the HD signal is formatted. Go to the following link and start
fishing around:

http://www.ibiquity.com/hd_radio/hdr...w_does_it_work

There's concern that HD signals cause interference because the digital
information is placed in what are often referred to as "guard bands,"
essentially the blank space on the dial between local stations.
Indeed, when you tune across an HD signal you hear hash, then clean
audio, then hash again. Consequently, to date the FCC has only
allowed HD signal to be broadcast during daytime when AM radio signals
travel shorter distances. However, this restriction may have been
lifted recently. Perhaps other posters can comment on this.

Personally, I'm indifferent as whether HD radio is here to stay or
ultimately proves to be a wash. If it does gain a foothold, and the
public enthusiastically buys new receivers to hear it, then it could
breathe new life back into AM radio, which has been moribund for
years. Right now, I'm not inclined to cough up $200 for a new AM
radio, but if the price were a third of that, I might be interested.

Finally, if HD radio becomes a permanent presence on the AM band, I
imagine it will be decades before a completely digital version is
adopted, if at all. AM broadcasting has been with us since 1922 and
there are millions of analog-only receivers out there. I therefore
imagine broadcasters much favor the current backward-compatible HD
format, because adopting it won't mean shutting out entire audiences.

-Dave Drumheller