Originally Posted by substance12
is the reasoning behind this because a diplexer also acts as a bandpass filter and attenuator? the signal to my tivo passes through 2 splitters and I still have issues on some channels.
I think there are two separate but related problems:
1) Some ONTs put out a very *hot* signal, much higher level than the TiVo RF front end can cope with. Using attenuators (or splitters) will reduce this signal level to the TiVo. The front-end electronics in the DVRs supplied by Verizon or the cable company can tolerate a wider range of input levels, and so don't need attenuators.
So, let's say the ONT puts out a signal at 30 dBmV. A Tivo might only work at a max signal level of 16 dBmV. So you need four 2:1 splitters in series (each reducing signal by 3.5 dB) to get down to a range where the TiVo will function correctly: 30 dBmV - 3.5 dB - 3.5 dB - 3.5 dB - 3.5 dB = 30 - 14 = 16 dBmV.
A 4:1 splitter reduces the signal by 7 dB, so you would only need two of those in series (assuming you needed to drop 14 dB).
Of course this is all very approximate. The splitters and the coax and the RF front end will all behave somewhat differently at different frequencies throughout the range of 54 MHz (channel 2) to 860 MHz (channel 135). That's probably why people have so many problems making this all work with 100% reliability.
2) Some (all?) FiOS installations use something called MOCA. This is a scheme that utilizes a 1 GHz+ frequency on the coax in your house for box-to-box communication and for things like Internet connectivity from a router in your house to the ONT outside the house. Apparently the TiVo RF front-end is adversely affected by this high frequency, but it shouldn't be. Cable channels don't go above about 860 MHz.
The diplexer is a solution to the second problem. It takes a single (combined) coax input and splits into low frequency (below 860 MHz) and high frequency (above). It does this with lower signal losses than a traditional splitter would add. That's a great win for people whose house coax is carrying low-signal-level OTA signals at those low frequencies.
The diplexer is (relatively) cheap and available because satellite companies like DirecTV play the same game as MOCA. I.e. they use 1 GHz+ to send satellite signals to receivers in your house. They chose this scheme so they could co-exist with existing antenna and/or cable signals already going to TVs in your house, without needing to run a second coax.
Some people say that you should add a 75-ohm terminator to the high frequency output of the diplexer if you don't use it. I'm not knowledgeable enough of the inner workings of those devices to know if that's really necessary.