Re: [IRCA] IBOC power

["David Gleason" <david@xxxxxxxxxxxxxxxx>]

I was one of the culprits by missing a zero. I was trying to avoid going
into db, and should have said "100" instead of "10" in the fraction. The HD
signal on FM is about 20 db down from the main carrier, which would be about
1/100th the power. Example: KLVE in LA, with roughly 28 kw ERP, has 285
watts of HD signal (that station uses a separate HD transmitter and
antenna).

With AM, the difference is a bit more complicated due to the core and
outside carriers, but is also way down in power. On AM, the power is off
frequency... if you look at the spectrum, it looks like someone is giving
you the finger, with the analog signal in the middle and the individual
digital channels on each side, separated by a guard band, like tiny folded
fingers. The digital energy is constant, while on AM, the analog varies, as
we know, in amplitude. 

Of course, the AM can carry one roughly 24 kbs audio band, with limited and
relatively slow data add/ons, while the 96 kbs FM signal can carry, with the
current algorithm, two stereo HD channels and quite a bit of data. 

Hope that makes sense... from the home of HD KSCA, KLVE and KTNQ. 


-----Original Message-----
From: irca-bounces@xxxxxxxxxxxxxxxx [mailto:irca-bounces@xxxxxxxxxxxxxxxx]
On Behalf Of Dan Strassberg
Sent: Friday, September 23, 2005 2:54 PM
To: irca@xxxxxxxxxxxxxxxx
Subject: [IRCA] IBOC power

Several posters have mentioned incorrect powers for the MW HD Radio digital
sidebands. My understanding is that the combined power in the two digital
sidebands is not supposed to exceed 1/16 (6.25%) of the nominal power of the
analog carrier. Each digital sideband is allocated half of the total
digital-sideband power or 1/32 (3.125%) of the nominal analog-carrier power.
As someone else stated, each sideband contains multiple carriers, each
carrying 16-QAM modulation. The envelope detectors used in most AM receivers
apparently make this form of modulation sound a lot louder than you would
expect based on the relatively low digital-sideband power. I think the
exaggerated loudness occurs because the modulation index of the digital
sidebands is much higher than the average modulation index of the amplitude
modulation on the main carrier. In analog AM, the sideband power is variable
and more power goes into the analog sidebands as the signal gets louder. I
believe that QAM (like FM) is a form of modulation in which the sideband
power is constant (and therefore always maximum). If anything I've said in
this message is incorrect, please set me straight.

--
Dan Strassberg, dan.strassberg@xxxxxxx
eFax 707-215-6367






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