[mythtv-users] HDHomeRun Vs. PCIe tuner card?

[Brian Wood]

On Monday 17 August 2009 11:15:12 drew einhorn wrote:

> What's a directional coupler?  Sounds like it is not bi-directional, and
> the opposite of what is being recommended.

I can see where the nomenclature might be confusing, but DCs can be 
bi-directional just as splitters can be.

Most hybrid splitters have the same loss from the input port to each of the 
output ports (there are some exceptions).

A directional coupler has an input port, and output port and a "tap" port. The 
loss from the input to the tap can range from about 8 db. to over 40 db. The 
loss from the input to the output is typically 1 or 2 db. 

Since the amount of signal is finite, the more signal to the tap, the more 
loss to the output. This allows you to use higher values of tap when the 
signal level is high (close to an amplifier) and lower tap loss values as the 
signal level in the main feed drops. 

This is in fact how the cable system does things. Directional couplers with 
multiple tap ports are mounted on the pole or in a pedestal, the values 
chosen to achieve the correct level to the subscriber. DCs are always useed 
to split off subscriber signals, unless you are on a system old enough to be 
using pressure taps, but you would have to be as old as me to remember those.

Most modern CATV DCs have replaceable faceplates, so the level to the 
subscriber can be increased by replacing the faceplate with one of a lower 
tap loss value, at the cost of reducing the level for everyone all the way 
down the line.

But the main advantage of a DC over a splitter has to do with isolation loss 
and matching (return loss). A splitter depends on having every port properly 
terminated to achieve the 30 db. isolation from port to port that is required 
to meet spec. A DC will have its rated isolation whether the output ports are 
open, shorted or properly terminated (75 ohms). So no matter how badly the 
subscriber screws things up, the system will still meet the required 
isolation spec.

Because some signal is lost in the matching components of a DC, they are less 
efficient than a splitter, some signal is "thrown away" to achieve a good 
match.

A "perfect" 2-way splitter would have a 3 db. loss to each output port, but in 
reality they have about 3.5 db. loss, since nothing is perfect in this world.

Poorly terminated splitters can cause reflections, similar to multipath, which 
can cause problems with digital signals. This is why cable companies use them 
to feed cable modems.

Cable companies would not use DCs instead of splitters if they didn't have to, 
since DC are more expensive than splitters.

I've tried to simplify this explanaiton a bit, a more detailed explanation is 
here:

http://en.wikipedia.org/wiki/Power_dividers_and_directional_couplers

Though even that has been somewhat simplified.

-- 
Brian Wood
beww at beww.org