Re: [IRCA] IBOC Hash AM vs FM OT question

[Scott Fybush <scott@xxxxxxxxxx>]

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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