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Topeka TV Reception |
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The purpose of this document is to help the
typical local TV viewer in the Topeka area in obtaining good television
reception. The author of this paper is a broadcast engineer,
unaffiliated directly with any of the Topeka or K.C. TV stations, but
has been a TV engineer for many years and lives in the Topeka area. The
views herein are not necessarily the views of any of the broadcast
entities mentioned. Topeka and the surrounding area is in a somewhat
unique situation, being a separate TV market but still being within
reception range of a major metropolitan market. As such the reception
problems this co-location generates are likewise unique. So some
education of the general TV viewer is in order so that they can get the
most out of their TV’s in this transition to DTV. First, some
basics. Even though the channel numbers on a TV are
continuous, the actual frequencies used are not all together in one
frequency band. There are actually three bands, Lo VHF, Hi VHF, and UHF.
Lo VHF consists of channels 2 thru 6 and are from 54 to 88 MHz just
below the FM radio band. As such these channels are the best at bending
over the horizon, but are also the most susceptible to noise from
lightning, power lines and the like. Hi VHF band contains channel 7 thru
13 cover 174 thru 214 MHz, and are less susceptible to noise, but are
more line of sight dependent. The UHF band for TV contains the lion’s
share of channels 14 thru 69 cover 470 thru 804 MHz. This is the least
noise susceptible band but is also much more dependent on line of sight
to work well. As a side note here, as part of the FCC’s master plan
for conversion to DTV, channels 2 thru 6 and 52 thru 69 will be vacated
and used for other services. Analog vs. DTV…. In addition, there is the issue of analog or
digital. Analog TV that was developed in the late 1930’s, actually has
two carriers. A visual which is AM type modulation and aural (sound)
which is FM modulation. AM relies on the change in power level to convey
the picture information. Anything which can change this level, like
lightning or power line noise, shows up as sparkles in the picture. This
is why analog TV looks poor at times, but the picture may still be watch
able if not great. Most of the power transmitted, energy if you will, is
clustered within 1 megahertz of the visual carrier frequency and as such
not all frequencies in the channel are as important in receiving the
picture as others. Digital on the other hand, relies on converting the
picture to numbers, lots of zeros and ones. This is the same process in
many ways as is used in digital cell phones or WiFi connections. And to
clarify, HDTV (high definition TV) and SDTV (standard definition TV)
also known at DTV use the same transmission scheme. The only difference
is how many bits of information are shoved into the pipe. This digital
technique means the power used to carry the signal is spaced out equally
all across the band. On an analog TV this energy looks like snow….This
is something to remember and will become clear later in this
article. To put a picture to this, below is a spectrum
analyzer photograph showing KSNT and KSNT-DT. This is really a graph
with the horizontal line showing frequency whereas the vertical scale
shows power. The left is the lowest frequency whereas the right is the
highest frequency. The photo clearly shows the visual and aural carriers
of the analog signal whereas the digital channel is the large
rectangular bump on the left with energy shared equally all across the
6MHz channel band.
So
now on to specific channels…. Below is a chart showing all the channels and their locations in the bands. The red channels are analog channels that will go away at the end of the FCC transition period. The Green are local, Topeka analog stations whereas the Yellow are the local DTV stations. The blue bars are the KC analog stations where the purple are the KC DTV channels. I will cover this in more detail later but for now the problems areas are where big bars lie next to small bars.
For those of you wanting more information here is a
table showing the channels, affiliation, whether they’re digital or
analog and some other pertinent facts.
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