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Re: [IRCA] IBOC Hash AM vs FM OT question
On 5/20/2015 12:49 PM, Paul B. Walker, Jr. wrote:
I have to ask a stupid question.. you say you get the analog station, but
the RDS from the HD station.. do you know for sure you've locked into the
HD signal of a station?
Because some analog stations have RDS and it has nothing to do with HD.. I
worked for a station that had RDS and was not operating in HD.
There are no stupid questions, so long as you're willing to pay
attention for a complex answer.
"RDS" is both a specific and a generic term. As DXers, we use it
generically to describe any text info that appears on the display of a
radio. Specifically, though, "RDS" (or "RBDS" in the rest of the world)
is one distinct way of transmitting that text: as low-speed digital data
over a 57 kHz subcarrier that's part of an FM station's analog
transmission.
HD Radio (and other digital transmission schemes such as DAB) have their
own data systems that operate completely independently of the 57 kHz RDS
signal. For HD Radio on FM, that's called "PAD" (program associated data).
Where it gets confusing is that most (but not all) HD receivers can also
decode RDS as well as PAD, and often display both sets of data in
similar ways. Let's go back to Patrick's 88.1 situation and see what's
happening:
If you put a spectrum analyzer on the low end of the FM dial at
Patrick's QTH, you'd see this, roughly:
87.8 ---
KQOC lower HD sideband
88.0 ---
KQOC's analog, overridden by stronger KWAO analog
88.2 ---
KQOC upper HD sideband
88.4 ---
So what does a radio do when confronted with this mix of RF?
Typically, an HD receiver locks in on analog first, so KWAO's analog
audio will be the first thing you hear. If KWAO had RDS, you'd probably
see its RDS data decoding fairly quickly, too. While you're hearing
analog KWAO (and maybe seeing the associated RDS), the radio tuned to
"88.1" is also looking at those upper and lower sidebands (87.8-88.0 and
88.2-88.4 MHz, roughly) to see if there's something digital to decode.
If it finds data there, the radio then starts decoding it, but that
takes a few seconds, in part because HD includes a time delay. (On the
Sony receivers, it will say "Linking" if it's found HD but hasn't
started decoding it yet.)
Once the radio locks into those digital sidebands, you'll hear the HD
audio and the data that it displays will come from the HD PAD stream,
not the analog signal's RDS subcarrier. In effect, while the radio may
say it's tuned to "88.1," it's actually *ignoring* that analog stuff
between 88.0-88.2 and tuning into the sidebands above and below. That's
why KWAO appears to "turn into" KQOC. If KWAO ever turned on HD, its HD
would be much stronger at Patrick's QTH than KQOC's, and KQOC would go
away completely.
This also explains what happens on 96.5 - the PAD data is designed to be
more robust and to decode faster than the more processing-intensive
digital audio. Most HD radios will always try to default to whatever
they can get from the digital signal, even if it doesn't match the
analog. So if it can pull just enough KJAQ signal to decode the PAD
data, it will display that instead of KCYS' RDS...even if there's not
enough KJAQ signal to decode digital audio and so the KCYS analog audio
stays on.
If I were designing an HD receiver specifically for DX'er use, I'd
probably set it up with two sets of displays and maybe even two sets of
audio outputs, one showing the RDS from the analog signal, one the PAD
from digital. Because really, what's happening here is that you have two
completely separate transmission systems going on at once on the same FM
band.
Oh, and KNHC? It is indeed on Cougar and has been since 2002. But it
uses a directional antenna with a deep null to the south, which helps to
explain why it's not getting down to Seaside.
s
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