On Thu, 24 Jan 2008 08:20:02 -0600, craigm wrote:
For example, if a station on 1510kHz runs IBOC, the digital signals are
transmitted in 1495-1505kHz (interfering with stations on 1490 and 1500)
and from 1515-1525kHz. (interfering with 1520 and 1530kHz)

That can be compared to an analog station on 1510. Its transmissions cover
from 1500-1520 kHz. This interferes with the stations on 1500 and 1520.
Adjacent channel interference is managed by keeping station apart both
geographically and frequency-wise. This may work during the day, but not at
night.

The big difference here is that the portion of the analog signal that
extends into the 1500/1520 channels is that which results from modulation
products above 5kHz.

This high-frequency material, in an analog signal, is relatively weak and
infrequent. You might hear an occasional "crash" or "splash" from the
adjacent channel station, but it's not enough to annoy most listeners.

The IBOC digital sidebands are there *all the time* and *at full strength*.

However, the FCC required a digital system to be compatible with analog.
Eureka-147 is not used in the AM band. This is not without issues, though.
Switching encoding from MPEG to AAC+ will obsolete many receivers.

Arguably, Eureka is *more* compatible with analog - because it operates in
its own spectrum and doesn't interfere with the analog transmissions.

The choice of IBOC over Eureka was not originated by the
government. If the industry had asked for an out-of-band solution like
Eureka, they would have received it. It was the industry that insisted on
the IBOC system, for two reasons:

1. IBOC uses existing transmitting antennas. No need to find tower space
for new VHF/UHF transmitting antennas; no need to fight zoning battles.

2. IBOC maintains the relative coverage areas of existing stations and the
relative fidelity improvement of "FM" over "AM". Eureka would potentially
equalize the coverage areas of all stations in a market - allowing 3kW
"92Q" to fully compete with 100kW "101.1 The Beat".

Eureka doesn't operate in the AM band, but it would open enough spectrum
to allow for VHF/UHF digital assignments for existing AM stations.

Receiver obselence (sp?) issues would not have been issues if the Eureka
system had been chosen from the outset. At this point I think it's too
late to put the genie back in the bottle - either IBOC is going to
succeed, or digital radio is going to fail altogether in the U.S.. I
strongly suspect the latter.

An interesting note, the AM analog signals received on my IBOC
receiver
sound better than that heard on almost any other AM receiver I own. The
FM on the receiver appears to have excellent selectivity also.

Very true. IBOC receivers use DSP and that feature seems to work VERY
well. Furthermore, I've found my IBOC receiver is able to
effectively reject interference from adjacent-channel IBOC stations when
tuned to analog stations. As you say, the FM selectivity is also quite
good.

For me, FM IBOC works. The IBOC signals are clearer. The second
program
offerred by some station does provide unique programming.

I find no noticable difference in audio quality here. Our local NPR
station does offer a worthwhile HD2 subchannel.

I wonder about the long-term viability of HD2 subchannels on commercial
stations?

- How long can stations afford to continue operating HD2 subchannels
without selling airtime? If the music licensing fees increase or the HD2
encoder requires a $2,000 repair, will the station be willing to spend the
money knowing it's not going to bring in any revenue?

- If stations do begin selling airtime on their HD2's, how long can they
continue to carry unique formats? Do they have to start dropping the
"deep cuts" in favor of plain ol' classic rock to pull that extra point or
two?

The FCC is managing the airwaves in a manner that does not
support
DXing. They are focused on radio working in the local markets, not long
distance reception. The broadcasters are also focused on the local
markets as that is where the advertising revenue comes from. On AM, IBOC
is attempting to improve audio fidelity for listeners in the local
markets. It may (or may not) be working.

- In some cases, IBOC *is* interfering with local reception. Admittedly
relatively few cases.

- Who's being ignored here (as is often the case in telecommunications
policy...) are rural and suburban residents. Listeners who due to a
relatively quiet environment get good service from relatively distant
stations - and due to a greater distance from major cities, get "city
grade" service from relatively few stations.

At my location, only ONE AM station delivers an interference-free
nighttime groundwave signal. Only ONE AM station would be receivable in a
world where all AM stations ran IBOC 24/7. I am fully dependent on
skywave for any choice in nighttime AM.

And I'm not in rural Montana or Alaska. I'm in central Tennessee only 30
miles from Nashville. At least a million people in Tennessee alone are in
the same situation.