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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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