Date: Tue, 19 May 2015 14:19:15 -0400
From: scott@xxxxxxxxxx
To: irca@xxxxxxxxxxxxxxxx
Subject: Re: [IRCA] IBOC Hash AM vs FM OT question
For those who are interested in the technical reasons why Patrick
doesn't get the same kind of adjacent-channel hash that those of us in
urban areas do, read on. For those who don't...another thread will be
along shortly with convention news ;)
--
As far as propagation is concerned, there is nothing at all magical
about the digital signals that flank the analog FM signal to create HD
radio on FM. The ionosphere doesn't care whether it's analog or digital
modulation. If a signal is (a) strong enough to be received and (b)
doesn't have something stronger sitting over it, you'll get it. If
either of those factors doesn't exist, you won't.
So: it helps to think of the digital signal as something separate from
the analog. If you're getting KUOW in HD, for instance, you're really
getting:
--94.6 ---
KUOW lower digital carriers (on the same piece of spectrum used by an
analog 94.7 signal)
--94.8 ---
KUOW analog signal (centered at 94.9)
--95.0 ---
KUOW upper digital carriers (on the same spectrum used by an analog 95.1
signal)
--95.2 ---
Depending on the station's technical setup, the digital signal may
literally be completely separate from the analog. At WXXI-FM, for
instance, our digital signal uses a separate transmitter, separate
transmission line and separate antenna bays interleaved with our analog.
It is possible (albeit not currently legal) for us to completely turn
off our analog transmission chain and run only the digital carriers. If
you were listening on an analog radio, you'd hear nothing on 91.5 (or
maybe even be able to DX something else on that frequency), but an HD
radio would detect the digital carriers and still give you WXXI-FM when
tuned to "91.5."
The digital carriers operate at much lower power levels than the analog.
Initially, digital operated at just 1% of analog, or 20 dB below carrier
(-20 dBc). More recently, the FCC has started allowing stations to use
higher power levels of 4% (-14 dBc) or even 10% (-20 dBc) of analog.
So using KUOW as an example, let's say it's still -20 dBc. That's 100kW
in analog and 1 kW in digital. If you're in metro Seattle, that 1 kW
digital is plenty to still ride right over anything else that might be
coming on the adjacent channels of 94.7 and 95.1. If you're way down in
Seaside, though, those 1 kW digital signals are DX: point a good antenna
right at Seattle and you might get them strongly enough to decode, IF
there's nothing else in the way on those frequencies. Point the antenna
away from Seattle or disconnect it and you won't hear much of anything,
as would be the case with ANY signal of 1000 watts from 100+ miles away.
This leads to a bunch of interesting DX scenarios when you start to
break it all down:
For instance - let's say that you were a little closer to your
semi-local on 94.9, enough so for it to be an un-nullable pest. But
let's also say that your local 94.9 is analog-only. So you might have a
spectrum that looks like this:
94.6 ---
KUOW lower digital sideband, weak but with nothing else in the way
94.8 ---
Your local analog 94.9, loud enough to overwhelm KUOW's analog
95.0 ---
KUOW upper digital sideband, weak but with nothing else in the way
95.2 ---
On an analog radio, all you'd hear is the local when you tune to 94.9.
But when you tune an HD radio to "94.9," if it can hear those upper and
lower sidebands, it will ignore the analog in-between...and so you might
hear your local 94.9 in analog for a few seconds and then, when the HD
decodes, you'll hear KUOW instead, because while your radio says "94.9,"
it's really looking for signals above and below 94.9 to decode.
There are all sorts of permutations on this that can happen when the
dial is more crowded. It's easy, for instance, to think of scenarios
where the spectrum is clear for a distant analog signal but its HD
sidebands are overwhelmed by locals. For instance, I can easily hear 105
kW WTSS 102.5 Buffalo in analog from about 70 miles away - but its
1050-watt digital carriers are completely obscured by locals WVOR 102.3
and WLGZ on 102.7. If either of those locals goes off, there's WTSS in
digital, because that bit of spectrum is suddenly open.
Does that help make some sense of all of this?
s
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