[IRCA] Filtering out IBOC sidebands

["Dan Strassberg" <dan.strassberg@xxxxxxx>]

Patrick Martin wrote:
In this ever changing digital world we are entering, has anyone thought
of a complex digital noise blanker to take the IBOC noise out?

-----
Patrick: YOU just thought of it! It sounds quite practical to me, but I've
never designed anything with a DSP. I have no idea how good a job such a
dual-notch filter would do, but I suspect that it could take out the digital
sidebands pretty well--as long as you weren't trying to "reach down through"
them and pick up an analog signal buried underneath! I don't know how many
subcarriers live within each 7-kHz sideband, but I'd be surprised if it were
any fewer than eight. That means that the subcarriers are less than 1 kHz
apart. And the sidebands around each subcarrier probably extend outward from
the subcarrier by more than 1 kHz. In other words, the entire 7-kHz sideband
is densely packed with modulation. Offhand, picking out an analog carrier
(of a station first-adjacent to the IBOC station) from all of those digital
subcarriers on top of it sounds impossible, but eliminating most of the hiss
(and producing just silence as you tune through the sidebands) sounds pretty
easy. The question is whether designing and building a dual-notch DSP filter
just to eliminate the hiss would be worth the effort or the cost. Unless you
are close to an IBOC station and you want to listen to an analog station on
a second-adjacent channel, there doesn't seem to be much reason to try to
build such a gadget.

I get a 2.5 mV/m daytime signal from WMKI, which runs IBOC. If I want to
listen to second-adjacent WEIM, which delivers somewhere between 0.5 and 1
mV/m here, I have to put up with a little hiss on my Super Radio III. The
sound isn't great but it isn't terrible either.

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




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