Channel Vision Stereo Receiver Understanding Digital CATV/CATV Modems, Systems With Digital CATV

Understanding Digital CATV/CATV Modems

Due to the launch of digital CATV across the United States there
is a need	for	clarification of digital CATV, which Channel
VisionTM offers a basic understanding of. Analog channels start
from channel 2 to around the 70’s, digital may start around mid
80’s to as high as 115, some as high as 135. Exclude 95 to 99,
(the old FM band now used for pay-perview or other
programming). Compression of channels will vary in ratio, one
cable company may use 8 to 1 ratio, compressing 6 to 12 or as
high as 16 channels every 6MHz or every channel. The
compressed channels, or forward feed of information is introduced
to the digital to anolog CATV converter box supplied by the
CATV company. This converter box creates hundreds of			Systems With Digital CATV
channels.	The	digital forward feed to the converter box must
remain Undisturbed. This means if an incorrect low pass filter is
installed or a modulated channel is placed on top of the digital			Now that we have somewhat of a understanding of digital, let’s
information, it will disable the digital converter box. We must			work around the digital converter. For years
understand frequency allocation to prepare for profitable			Channel VisionTM has always suggested 2-RG-6 coax to each TV
installations.		Satellite frequency starts at 950 MHz, CATV	location for good reason; you can dedicate the
systems are maxed at 750MHz and very few are at 860MHz. The			digital converter or modems and use the other coax for
dBmV signal characteristics are opposite from the typical analog			distribution/modulation/satellite/antenna/CATV cable ready.
signal, remember the FCC standard is 0dBmV minimum and			*Please see diagram A on the next page
15dBmV maximum at the TV. The digital converter’s optimum
level is 0dBmV to-10dBmV analog and drop out levels are -			If you only have one coax to each location, of course your options
18dBmV.			are not as open as having two coax to each location. We can still
Every system will vary, you would need a true digital meter to			modulate with a limited amount of space.	Channel VisionTM has
read digital, though an RF meter will tell you analog			created a 3202 low/high pass filter combo. This filter will pass all
measurements. The analog signal will vary anywhere from -			channels below 70 and 80 and above, allowing two channels to
30dBmV to -10dBmV.An RF amp may be required for			modulate.
multiple converter boxes due to the level from the cable			Channel Vision also has low/high pass in a single filter part #
company brought into a residence as low as –30 dbmv.			3205-60/69. This filter has a 4 channel slope on both sides
Modems also have a low analog dBmv requirement. The			allowing 2 channels to modulate 64-66.	The 3205-70/79 filter
			also allows two channels 74-76.
analog signal may be too hot for the modem. CATV
			*Please see diagrams B and C on the next page
companies may use taps in a line to pad down signal;
optimum levels will vary from 0dB to			Channel Vision has also created a 3102-118 low pass filter to add
- 10dB, +10dB may be to hot. Some CATV companies use
			to the existing line of low pass filters.
channel 106, and of course if the incorrect filter is used it			The 3102-118 will pass all forward digital information with a 4
will disable the modem as it would the digital CATV box.			channel slope allowing 2 channels of modulation or more
The modem is usually dedicated at the front end of the			depending on the TV’s tuner capability.
distribution system. Usually a two way splitter with one leg			*Please see diagram C on the next page
for modem and the other leg for distribution. CATV
companies are still testing the water with digital cable
converters and modems. A resident may have a modem, a
TV, and various splitters off of one line and be trouble free,
while the neighbor a block away has no equipment other
than a modem and a TV off of separate lines and has drop
out failure; return path seems to be the biggest issue. Some
CATV companies have found that high value hard line taps
are causing modem failure. System specifications of the
CATV system you are installing in is the key to a successful
installation.