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- 406 is a poem by John Boyle O'Reilly. It was believed to have been the number of one of O'Reilly's prison cells, and was the number of his first hotel room after he arrived in the United States. Hence the number had a mystical significance to him, as intimated in the poem.
I do not know the meaning of the sign,
But bend before its power, as a reed bends
When the black tornado fills the valley to the lips.
Three times in twenty years its shape has come
On lines of fire on the black veil of mystery;
At first, tho’ strange, it seemed familiar,
And lingered on the mind as if at rest;
The second time if flashed a thrill came, too,
For supernature spoke, or tried to speak;
The third time, like a blow upon the eyes,
It stood before me, as a page might say:
“Read, read,—and do not call for other warning.”
I do not know,—O Mystery, the word
Is lost on senses too impure. I stand
And shrink subdued before the voice that speaks,
And know not that its words is light or gloom.
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An ITU conference in Geneva, Switzerland, on December 7, 1984, resolved to discontinue the use of 50 kHz channel spacings throughout Europe.[2]
- Most countries have used 100 kHz or 200 kHz channel spacings for FM broadcasting since this ITU conference in 1984.
- Some digitally-tuned FM radios are unable to tune using 50 kHz or even 100 kHz increments. Therefore, when traveling abroad or importing receivers, stations that broadcast on certain frequencies using such increments may not be heard clearly. This problem will not affect reception on an analog-tuned radio.
- A few countries, such as Italy, which have heavily congested FM bands, still allow a station on any multiple of 50 kHz wherever one can be squeezed in.
- The 50 kHz channel spacings help prevent co-channel interference, and these take advantage of FM's capture effect and receiver selectivity.
In the medium frequency portion of the radio spectrum where most AM broadcasting is allocated, signals propagate full-time via groundwave and, at nighttime, via skywave as well. This means that during the nighttime hours, co-channel interference exists on many AM radio frequencies due to the medium waves reflecting off the ionosphere and being bounced back down to earth.
In the United States, Canada, Mexico, and the Bahamas, there are international agreements on certain frequencies which allocate "clear-channel" broadcasting for certain stations to either have their respective frequencies to themselves at night, or to share their respective frequencies with other stations located over hundreds or even thousands of miles away. On other frequencies, there are "Regional Channels" where most stations on these frequencies either reduce power or change to a directional antenna system at nighttime to help reduce co-channel interference to each other's signals.
In the United States, there are six "Local Channel" frequencies, also known as "graveyarders" where nearly every station on those frequencies has the same power and antenna pattern both day and night and, as a result of skywave propagation, there is normally massive co-channel interference in rural areas on these frequencies, often making it difficult, if not impossible, to understand what's being said on the nearest local station on the respective channel, or the other distant stations which are bouncing on the same channel, during the nighttime hours. Skywave has been used for long distance AM radio reception since radio's inception and should not be construed as a negative aspect of AM radio.
FCC deregulation allowed many new AM radio stations on the former clear and regional channel designations; this is the principal cause of overcrowding on the AM band at night. A new source of interference on the AM broadcast band is the new digital broadcast system called HD, any AM station that broadcasts HD superimposes digital "hash" on its adjacent channels. This is especially apparent at night as some stations, for example WBZ transmits its 30 kHz wide signal for hundreds of miles at night causing documented interference and covering another station on an adjoining frequency (WYSL 1040) as far as 400 miles away.