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Re: [IRCA] IBOC Hash AM vs FM OT question
Scott,
Thanks for the detailed explanation. Now I understand a bit more of what is going on. During E Skip, it should be interesting. KHNC is directional nulling the South, so that is the reason. I am now getting KBCS 91.3. I read they are on Cougar Mountain. I have had bits of Dance pop weakly on 89.5, but very weak. A strong Tropo into the Puget Sound may bring it in. I uses to get it on occasion back in the 80s. I think they were 320w ERP, but long before they were on Cougar. Another weak Puget Sound station I used to get, was 89.3 KAOS Olympia. They are solid as a rock now, so their tower must be up on a mountain too. One thing about FM IBOC/HD, it is sure a different animal. hi.
Patrick Martin
Seaside OR
KGED QSL Manager
> Date: Wed, 20 May 2015 21:04:25 -0400
> From: scott@xxxxxxxxxx
> To: irca@xxxxxxxxxxxxxxxx
> Subject: 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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