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Re: [IRCA] WHP 580 IBOC
Thanks for the clarification. I don't know if anyone has ever asked this
or not, but why do some IBOC stations have such a hiss behind their
audio and others do not? I have noticed that for a long time. Does the
night skip have something to do with it? It is very irritating to listen
to these stations at times. One of the worst is KFBK 1530.
Hiss behind the audio... Yes, does happen. Some stations have a more
correct setup that others. The digital sidebands are sometimes less
confined to the correct bandwidths. For example, the 550 station had extra
sidebands at 27kHz either side of the 550 spot. While it was within the FCC
requirements, they were still there. The upper and lower sidebands are
supposed to be phased. One is -90° and the other is +90°. That puts them
at 180° and they null out in the analog band. If the antenna system isn't
perfectly flat on the load across the whole spectrum, then some digital hiss
would be heard as the null isn't as exact.
And night skip does affect it, if there's a signal from an adjacent IBOC
station. WBZ-1030 is 30+ miles from Providence. KDKA-1020 IBOC interferes
with them at night. It affected one of my clients receiving EAS at night.
I had to use one of the large ferrite loops set to null KDKA to fix that.
Other than a noninductive resistor load, there really is no flat transmit
load anywhere. Antennas are different loads across a frequency band, some
worse than others. That's probably the main reason why some stations are
worse than others, and some don't run it at night.
There is a limitation requirement for spurious signals. It's not expected
to be zero, but under a specific value depending how far off the center
frequency it is. Ever notice a second or third adjacent station has some
noise? Happens on FM as well. I have a local 93.3 with IBOC. When they
run that, I cannot hear a 92.7 station about 40 miles away.
Given the quick addition of internet access in new vehicles, IBOC will
probably be overrun by streaming audio. My personal guesstimate is that
when the streams are accessable by a significant percentage of people, even
transmitters may be less important. Think of the change from horses to
automobiles a hundred years ago...
Craig Healy
Providence, RI
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