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Sec. 8–10 Cable Data Modems 653
• Co-channel ATSC signals interfere less with each other than co-channel NTSC analog
signals.
• Several SDTV programs may be transmitted simultaneously with a HDTV program
over each channel.
The U.S. DTV system was implemented by assigning a new DTV channel for each
existing NTSC station. During the transition period (before June 2009), each station simultaneously
broadcast an NTSC analog signal on its existing channel and an ATSC DTV signal on
its new channel. DTV stations are only assigned to be on channels 2 through 51. (Channels 52
through 69 are not used for TV any more, but are now assigned for cellular phone use, creating
the new 700MHz cellular band, e.g., compare Table 8–8 with Table 8–13.)
On June 12, 2009, the analog NTSC signals were permanently turned off (except for
some very low-power, low-antenna-height stations). Now that the NTSC signals are off,
the FCC has allowed some of the DTV stations to change back to their old NTSC channel,
provided they can meet requirements for transmitting with enough power to get the desired
coverage but yet not interfere with the other DTV stations in their area. (This channel
change is very costly for the DTV stations, since they have to have antennas and DTV
transmitters for both the old NTSC channel and the originally assigned DTV channel.)
The new ATSC DTV system uses the concept of virtual channels. That is, the FCC
assigns a DTV station to one of the RF (radio frequency) channels 2 through 51 (see Table 8–13
for the RF channel number frequency bands), but the DTV broadcaster may insert data indicating
another channel number, called the virtual channel number, into the the digital bit stream
that it broadcasts. When the DTV signal is received, the HDTV receiver will detect this virtual
channel number sent out by the broadcaster and display that virtual channel number on the TV
screen. Most broadcasters choose to use their old NTSC RF channel number as their virtual
channel number. In this way the consumer sees the old NTSC channel number on their HDTV
set, even though the DTV station is broadcasting on a different RF channel (the DTV channel
assigned to it by the FCC). For example, WCJB, a TV station in Gainesville, Florida, was on RF
channel 20 for almost 40 years transmitting their analog NTSC signal, and the TV audience
know them as the station called TV20. For DTV transmission, the FCC assigned them RF channel
16, and, consequently, they now broadcast on RF Channel 16. However, when one tunes in
their on-the-air DTV signal, the HDTV set displays channel 20, their virtual channel number,
even though the HDTV set is actually tuned to RF channel 16. Thus, as far as the viewer knows,
TV20 still appears to be on channel 20.
For a list of DTV stations in your zip code area with their corresponding RF channel
numbers, virtual channel numbers, and their signal strength in your U.S. zip code area, go to
www.antennaweb.org
8–10 CABLE DATA MODEMS
Older CATV systems use an all-coaxial distribution system and operate up to about
450 MHz. Modern CATV systems that provide a high-speed internet connection (via the use
of a cable modem) use fiber to distribute the TV and data signals from the CATV head end to
the neighborhood of the customer. In each neighborhood the signals are converted from light