[opendtv] Re: FCC on revitalizing the AM band

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
  • Date: Tue, 27 Oct 2015 01:47:34 +0000

Cliff Benham wrote:

Bert, I completely disagree with all you say below.

Now you sound like Craig. :)

http://www.hd-radio.com/i/pdfs/Waveforms_AM.pdf

The above link is the Ibiquity white paper that describes IBOC (aka HD Radio)
as applies to the AM band. It provides the power level restrictions and channel
width, in hybrid and digital-only modes. I'll refer to it below.

HD radio is 24 hours a day.

I checked. You're right, although some AM stations do shut AM-band HD off at
night.

http://www.radioworld.com/article/am-hd-radio-has-stalled-now-what/3774

"Citadel Broadcasting turned off the nighttime digital on its 10 AMs in the
fall of 2007. At the time, Director of Corporate Engineering Martin Stabbert
cited complaints from listeners and stations on adjacent channels, including
both Citadel stations and others, in and outside the affected markets."

"Journal Broadcast Group has two out of six AMs airing IBOC signals full-time;
it has not turned any off."

HD radio is limited to 17% of the radiated power of standard AM.

Take a look at the white paper, Table 1. In hybrid mode, the "primary upper and
lower" digital sidebands, which are the strongest ones and stretch way out to
+/- 15 KHz from the AM carrier, use 64-QAM and must be 30 dB down in power
compared with the AM carrier. Closer in digital sidebands are 16-QAM, and must
be 37 or 43 dB down. So overall, that's way down. 30 dB down is 0.1% of the
analog carrier level, and that's only the strongest digital sidebands. That's
how compromised the strongest HD Radio digital signal is, in hybrid mode.
Pathetic.

If the station goes all digital, Table 2, those primary sidebands are now
adjacent to the center frequency, and 15 dB down (3% of the center carrier).
That's already 30 times as much digital power as they can transmit now. And
it's also likely that the FCC would rethink its power limits, in all digital
mode, as they did for the VHF DTV stations. You haven't even seen what AM-band
HD could do, especially if all other analog AM were off the air.

HD requires a 20 KHz channel bandwidth which encroches into the
two adjacent AM channels making their reception practically
impossible because of the digital noise that interferes with them.

Take a look at Figure 2. In hybrid mode, indeed, the HD Radio digital sidebands
extend to +/- 15 KHz (slightly less actually) from the center frequency. But at
quite reduced power, which makes hybrid mode very compromised. Yes, that's a
wide channel, and intrudes on adjacent AM stations, no doubt, but it's also
allowed, in daytime, for analog AM (in case anyone has a receiver that can
actually benefit from it!).

But now look at Figure 3. In all-digital mode, Cliff, the strong primary
sidebands extend only +/- 4.9 KHz of channel center, and the low power
secondary and tertiary sidebands go to +/- 10 KHz, but 30 dB down in power.
This beats the existing AM spectral emissions mask, Cliff, which calls for 25
dB down from 10 to 20 KHz of center. Table 3.

And potentially, if the FCC bans use of those secondary and tertiary digital
sidebands at night, you'd still be left with a very useful digital signal,
taking up the existing 10 KHz channel width only. (In daytime, even today,
analog AM stations can go out to +/- 15 KHz, right?)

You cannot pick up an AM HD signal much farther than the local
footprint, certainly not from a state or half the couotnry away.

You mean, when the signal is at best 30 dB down from the analog carrier, and
there are tons of other high-powered AM stations crowding the AM band? Not
surprising, right? Even that Radio World article suggests this improvement
would occur, in digital-only.

Bert THESE ARE NOT MINOR TWEEKS!

I meant, minor tweaks to AM band use, compared to eliminating analog
altogether. "Minor tweaks" in the sense that they make the AM band even less
useful than it already is, under the guise of helping revitalize AM.

Yes, the HD signals are intended only to be received within
the markets they are located, hence the lower power allotted
to the HD signals.

We went through this with DTV. The digital power is always down compared with
analog, not just because digital reception is less impaired by analog
artifacts, but also because digital power is continuous, as opposed to peak
power usually quoted for analog. In the FM band, the hybrid mode digital is 23
dB down from analog, but in all-digital mode, only 10 dB down. And again, I
wouldn't be surprised if the FCC increased that all-digital limit some.

You have to be careful with Wikipedia articles, Cliff. It does not tell the
whole story. Their description of FM, for example, is amazingly misleading.
They apply AM ideas to explain FM bandwidth. Ibiquity also wrote an FM-band HD
Radio white paper, separately.

Bert



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