I'm sure Dave's explanation is right on target. Two or three of the
Washington area stations are now running hybrid IBOC on AM, daytime
only.
I think the concept is that the "adjacent channel" (10 KHz away from
a station) was never well protected from interference. In existing
analog AM, I think they were allowed to have a bandwidth of 20 KHz,
that is plus or minus 10 KHz from the carrier. This means that, if
the content of a broadcast was as loud near 10Khz as it is near DC,
which it isn't, you could only expect to copy an adjacent channel
station on the upper sideband if it was 10 Khz above the interfering
station, or on the lower sideband if it was 10 kHz below the
interference source.
In hybrid IBOC, we have broadband noise at a low power level
occupying the spectrum from the carrier out to plus or minus 10 KHz,
plus a louder signal from plus or minus 10 to plus or minus 15 KHz
from the carrier. The adjacent channel loses what protection it had
during the daytime, and you can only expect to get some DX reception
on the alternate channel (20 KHz away) or further.
I did a Google search on "hybrid IBOC" and got several hits,
including a technical PDF file which I will read shortly.
A M will never be the same, especially if this signal is allowed to
proliferate at night.
On Sat, 26 Mar 2005 21:11:42 -0700, Phil Scovell wrote:
> A few months ago, I switched my Sony 20 10 on and pressed the
>button for a little 500 watt radio station up in the mountains I
>try and listen to some times just to see how the band is doing.
>The station is always just above the noise level. This particular
>day, a few months ago, some loud noise was covering the station
>but by tuning up 5 kHz, I was able to get passed the noise to hear
>the station once again.
>
>Tuning back down the band, I swept back and forth over the noise.
>It was just a loud hiss. Switching on the BFO, I tuned across the
>broad signal several times. I found, or rather could not detect
>at any rate, any carrier at all. So, it sounds like a jamming
>signal on single side band. In this high noise area I live in, I
>just figured it was another unexplained noise on the A M broadcast
>band and let it go.
>
> A few months later, I purchased a new i com R75 and after
>attending night school for six months, a little tongue in cheek
>there, I learned how to use the receiver. Hot dog! I was going
>to DX my brains out on the old broadcast band once and for all.
>
> Recently, while tuning around the broadcast band with my R75,
>I discover there wasn't just one of the side band sounding hissing
>signals but several and in different places throughout the
>broadcast band. Switching into a side band mode, I tuned around
>on this signal. It almost sounded as if there were a tiny weak
>carrier signal underneath the noise but I couldn't tell for sure.
>I punched in the CW mode and repeated my sweep slowly across the
>signal. I thought I still heard a very tiny carrier signal under
>the noise generated signal. Just for the fun of it, I punched in
>the RTTY mode since the filtering is very narrow. I thought maybe
>I could hear better. Tuning the hissing sound carefully with the
>RTTY filtering switched in, I suddenly stopped and listened. I
>swear, I am hearing a super high speed data signal. The tiny
>little carrier I thought I heard in the side band and CW modes is
>actually a signal that sounds very much like super high speed
>teletype. No fooling. I began tuning the band and finding these
>same signals in various places. They are always transmitting on
>off channel frequencies, that is, they are never on like 600 or
>610 or 620 but on 605 or 615 and they generally narrow out in
>width before they begin to interfere with broadcast station
>signals. But that's not all.
>
> As I began to locate more and more of these signals on the A
>M broadcast band, I swear they sound like high speed data
>transmitted signals, I discovered something else about them. They
>are not on the air 24 hours a day. I am now in the process of
>trying to narrow down the times they go off and on. I never seem,
>so far, to find them during evening hours or late at night but
>they are there during the day. There is still more to this story.
>
> As I tuned around, finding more and more of these signals, I
>realized, with my new R75, I could now read the S meter. Could
>these be transmitting from different locations? If so, wouldn't
>their signal strengths be different from one signal to another?
>Sure enough, they are. They are pretty loud, too. So far, they
>run from 10 over S 9 to 20 over S 9 but I have not located and
>logged their exact frequencies as of yet. Their signals are so
>loud in places, however, they do slop over on to the signals of
>weaker stations out of the area. I haven't checked above 1600 to
>see if some of these signals are up there yet but I will. We have
>two local Denver stations transmitting above in the expanded
>portion of the broadcast band.
>
> Does anybody know what these are? What frequencies do
>wireless keyboards and wireless computer connections transmit on?
>Surely, they can't be that low in the spectrum.
>
> I repeat, these signals have not always been here. It has
>been less than a year I have discover them because I check signal
>strengths of out of town stations, and little stations up in the
>mountains behind Denver, almost daily to see how the broadcast
>band conditions are doing at various times. I would have noticed
>these signals long ago if they had been there. Plus, as I said,
>last night, as I hunted for them, they were gone and not
>transmitting at 9 o'clock at night.
>
> Now I suppose somebody is going to tell me what they are and
>I'll feel like an idiot for not knowing the obvious. Yes, I have
>checked all sorts of things in my house to see if it is coming
>from another in house source. It's not and I have no wireless
>equipment of any kind. Anybody know what this is?
>
>Phil.
>K0NX
>
Some people think that the devil is in the details. Actually, God is in the details.
Lloyd Rasmussen, Kensington, Maryland
home: <http://lras.home.sprynet.com/>
Work: <http://www.loc.gov/nls/z3986>
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