Re: [IRCA] Another perspective on AM IBOC, from the broadcasters' mailing list

["CHARLES HUTTON" <charlesh3@xxxxxxx>]

This seems like almost total nonsense. The IBOC energy from an adjacent 
channel lands directly in the frequencies of the channel you are tuned to.

Example: if you are tuned to 750, a station on 760 with  IBOC will produce 
IBOC signals at 750, and everwhere else from 745 to 775.

If you're tuned to 750, you have no way to differentiate between the signal 
from the 750 station and the IBOC from the 760 station - they are both in 
the same frequency range.


The only way around this is to know something about the pattern of the IBOC 
energy.
Unfortunately, it's "scrambled". The IBOC signal(s) also depends on the 
audio data being transmitted which we don't know. Combine those two and we 
have a fairly random bunch of IBOC energy. (Hence the almost white noise 
sound.)

Non-techies skip this: If you were to know the exact decoded bits, you could 
(at least in some cases) deduce the state of all the little IBOC 
subcarriers. You could then subtract that out of the total signal you 
received and clean things up. That's a lot of work, and also impossible 
unless iBiquity suddenly goes egalitarian and decides to give us access to 
the decoded bits.

Forget the magic IBOC filter. The Cubs will win the World Series first.



Chuck

>From: Scott Fybush <scott@xxxxxxxxxx>
>Reply-To: Mailing list for the International Radio Club of 
>America<irca@xxxxxxxxxxxxxxxx>
>To: ABDX@xxxxxxxxxxxxxxx,        Mailing list for the International Radio 
>Club of America<irca@xxxxxxxxxxxxxxxx>,        nRC-AM <am@xxxxxxxxxxx>, 
>dxhub@xxxxxxxxxxxxxxx
>Subject: [IRCA] Another perspective on AM IBOC,from the broadcasters' 
>mailing list
>Date: Tue, 27 Mar 2007 11:25:40 -0400
>
>This caught my attention over on the [BC] mailing list, and I suspect it
>may stir up some discussion on the DX lists:
>
> > I have been investigating some of what has been said on this
> > list about IBOC on AM. It appears as though it is really a
> > receiver problem. Don't get me wrong. I'm not advocating
> > trashing 150 bazillion conventional AM radios so that IBOC
> > will work. Instead, as I am beginning to understand the
> > technology, if an AM radio was designed with more modern
> > technology, digital demodulation of AM with a wall filter,
> > there would be no buzz. There really is plenty of bandwith
> > available for IBOC on AM. The feds were not bamboozled.
> > The problem is that AM radios of conventional design,
> > superheterodyne with envelope detectors, cannot handle the
> > adjacent channel IBOC interference at reasonable cost.
> >
> > What the industry really needs is a decent AM radio using
> > modern technology. Such a radio would directly digitize
> > incoming spectra. AM would be demodulated by multiplication
> > of the digitized data with the selected frequency. The
> > resulting 'garbage' would be subjected to a FFT, the bins
> > outside 10 kHz zeroed out, then the inverse FFT would be
> > taken. The result would be a very clean AM signal with
> > no response beyond the 10 kHz passband (9 kHz for some
> > foreign countries), and no intermodulation from out-of-band
> > responses. The same kind of preselection would be used for
> > IBOC digital. You just don't demodulate the AM signal to
> > baseband before digital processing. When I say
> > "no intermodulation," I am talking 96dB down. Since a
> > typical S/N of off-the air AM signals seldom exceeds
> > 60dB, the intermodulation is 30 dB below the noise so,
> > in fact, it is truly "no." Sixteen bits is 65536:1 =
> > 20 log (65536) = 96.32 dB
> >
> > In the past, such a receiver was incredibly expensive.
> > Nowadays, we have inexpensive components that can handle
> > these low bandwidth signals at very low cost. Remember
> > that "real-time" for AM is 10,000 changes per second --
> > truly trivial. For accurate replication of the RF
> > component, you would need to sample something like 10
> > times over-sampling. This means you need a 20 MHz 16-bit
> > ADC. These things are now cheap, $35.00/1000. You can
> > even get a whole development system for $158.00!
> > Goodle "16-bit DAC 20 MHz" and see for yourself. Once
> > somebody starts producing a million radios per month
> > the price would likely drop to the $5.00 range.
> >
> > So, I think that instead of complaining that the new
> > technology is not compatible with 85 year-old radio
> > design, some entrepreneur(s) should take the bull by the
> > horns and develop a decent radio. They don't actually
> > need to get such a radio into production. Leave that
> > for the Pacific rim. What they need to do is generate
> > the "IP" intellectual property with as much as possible
> > embedded into a single chip. Then they license this
> > technology and, perhaps, the chip design. There are
> > lots of "radio" engineers who are now quite versed
> > in software. A development kit and some (sometimes
> > not too) pleasant software debugging time, would
> > establish the viability of the digital radio approach
> > for conventional AM. Then you could attract some
> > investors. All words and ideas presented here are
> > placed into the Public Domain. Get to work!
> >
> > In the meantime, read my book:
> > http://www.AbominableFirebug.com
> >
> > Cheers,
> > Dick Johnson
>
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