Mark Roberts wrote:

If AM IBOC is such hot stuff, why is it restricted to daytime hours only?

Because the group delay that is the result of the uneven ionosphere prevents
it from being decoded properly when the signal is received on skip, and the
amount of stuff coming over the horizon on skip prevents it from being
properly received when it's being received on groundwave at night.

I can now hear the damage it does to the analog signal on KCBS. The
effect varies from radio to radio, but on almost all of them, the
noise floor goes up when IBOC is on.

I wouldn't be bothered by this so much if the digital system actually
sounded good. But the audio quality of the digital carrier is actually
a lot worse than good analogue AM. It's like listening to Cylon Warriors.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey had written:
| Mark Roberts wrote:
|
| If AM IBOC is such hot stuff, why is it restricted to daytime hours only?
|
| Because the group delay that is the result of the uneven ionosphere prevents
| it from being decoded properly when the signal is received on skip, and the
| amount of stuff coming over the horizon on skip prevents it from being
| properly received when it's being received on groundwave at night.

I didn't quite expect an answer to a rhetorical question -- but it does
point to a very fundamental flaw in the scheme.


--
Mark Roberts |"Bush campaign ads boast that 1.5 million jobs were added in the
Oakland, Cal.| last 10 months, as if that were a remarkable achievement. It
NO HTML MAIL | isn't. During the Clinton years, the economy added 236,000 jobs
in an average month." -- Paul Krugman, NY Times, 7-6-2004

In article ,
Mark Roberts wrote:
Scott Dorsey had written:
| Mark Roberts wrote:
|
| If AM IBOC is such hot stuff, why is it restricted to daytime hours only?
|
| Because the group delay that is the result of the uneven ionosphere prevents
| it from being decoded properly when the signal is received on skip, and the
| amount of stuff coming over the horizon on skip prevents it from being
| properly received when it's being received on groundwave at night.

I didn't quite expect an answer to a rhetorical question -- but it does
point to a very fundamental flaw in the scheme.

Sorry, I can't seem to find Bob Orban's reply to this. But I agree with
Bob that the encoding scheme is very robust about dealing with group delay
issues. Even so, I have found the actual skip performance poor to the
point of unusability.

Bob, do you have a citation on any actual measurements of this stuff?
Ionosonde data is easy to get, so it should be really easy to build a
simulation of ionospheric distortions in matlab or something, even if you
just take into account group delay and multiple reflections. Has anyone
actually done any simulations on the current encoder to see how it survives
under various simulated skip conditions? I'd be curious to see which of
the various skip characteristics is the most serious issue.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article , says...


Mark Roberts wrote:

If AM IBOC is such hot stuff, why is it restricted to daytime hours only?

Because the group delay that is the result of the uneven ionosphere prevents
it from being decoded properly when the signal is received on skip, and the
amount of stuff coming over the horizon on skip prevents it from being
properly received when it's being received on groundwave at night.


I do not believe that this is correct. The modulation scheme of iBiquity's
IBOC system is basically multiple carrier COFDM, and is quite similar to the
modulation scheme used in Digital Radio Mondiale (DRM), which is known to be
robust in the face of selective fading. It all depends on the symbol spacing
used in the COFDM carriers. If the multipath period is shorter than the
symbol spacing, then the various reflections will reinforce. Because there
are multiple narrowband carriers in COFDM, the data rate on each carrier can
be very low (allowing long symbol spacing), yet the presence of multiple
carriers still allows the overall system to deliver a high data rate.