It is a fact that cables lose strength as soon as the signal leaves the emission point. First, the cable itself loses signal (“cable loss”). The smaller the diameter, the longer the run, the higher the channel, the more loss will result. Feed-through and isolation loss can also occur in larger sytems - offices, apartment buildings, etc. Sometimes, even high heat conditions can cause copper cable to expand and cause signal loss. But by far the most common reason for signal loss is the use (and misuse) of splitters.
In the home or office, the distribution of a cable/DSL signal is accomplished with a “splitter.” This device equally splits a single signal into two or more separate signals, which can then be directed to different locations in the building. The problem is that, each time a signal is split, it experiences signal loss of 3.5db (db = decibels, the measure of signal strength). So, for example, if you live in an older home with 4 splitters on the line, you will lose 14db of signal strength by the time you reach the last cable connection. This may not be sufficient to receive a high-definition TV or internet signal. You may have to add an electronic “booster” (a broadband amplifier) to strengthen the signal sufficiently, usually by 10Db. Moreover, you have to be aware of the “bandwidth rating” for the splitter, which is typically 1Ghz these days, but you can still find 300, 500 or 750 Mhz splitters in older installations. And a higher bandwidth signal will lose more db than a lower bandwidth signal when passing through a splitter. For example, you will lose 3.5db of signal passing through a two-way splitter at 750Mhz, but lose only 3db at 550Mhz. But the problem also is that if you have a 750Mhz signal passing through a low-loss splitter that won’t allow more than 300Mhz, you still won’t get a full signal or adequate reception. A type of splitter called a “combiner” actually works in both directions, i.e. two lines in can be split into 8 lines out and, in the reverse, eight lines in can be combined into two lines out. Splitters can also be considered either “active” or “passive” Active cable modem splitters run on power and serve the purpose of trying to reclaim some of the lost signal strength that results from the split. Passive cable modem splitters do not require power, and simply serve the purpose of splitting a signal.
A tap (a/k/a “DC” or “Directional Coupler”) is an entirely different and much more complex animal. It is a device used when a “trunk” cable is required to connect to a device, then continue “downstream” to one or more other connection locations, usually in a loop-through design. While a splitter, which is intended to be used at the dead end of a drop, always splits the signal in half, a tap is resistance rated, so that it attenuates unevenly, with the highest attenuation closest to the signal source, getting lower at each connection downline, spreading the resistance more evenly across the network. For this reason, taps can be engineered in different configurations, having multiple ports with multiple values, some with looped return paths which allow the use of PPV (pay-per-view) and VOD (video on demand) services. Taps are either wall taps (fit inside a standard electrical box) or drop taps (wall mount), but are always wired in an “in/out/tap” configuration.