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A single-ended signal is a non-differential, in the last one one signal is referred to another as they are complementary, in sinlge, the ginal is referred to the return path (commonly, ground)
There are many grounds in signaling systems with several purposes.
A ground refrence is generaly assumed to be very low resistance and very inductance and to be associated internaly with a signal trace in a cable. They are generaly not designed to carry any kind of current so as to avoid disturbing the "refrence" by which the signaling levels on the signals are measured.
In general a non "refrence" ground is used for the return of current both for signaling and for power, due to the levels of current that might flow a ground return can be quite noisy and should not be used for the refrence for signaling levels or other analogue inputs.
Earths are for connection to primary mains power etc and earth is either a composit value from multiple phase power or a genuine local earth derived by a metal spike in the ground.
One of the problems with sending signals down a wire is that energy is lost to the environment due to changes in the field around the wire impinging on conductors and no conductors with different dielectric constants to "free space". Thus energy from one conductor can transfere into another conductor, this is often called "cross talk" now one thing that can be done to reduce this is have two conductors of the same length at very close proximity creating equal but oposit fields to each other.
That is for every unit of charge flowing up one wire an equivalent unit of charge flows down the other. However neither movment of charge is refrenced to anything other than the other. The down side of this is where you have other conductors with different charges flowing in them this can add extra charge to both the signal and return signal wires if one of them is used as the "ground refrence". they way around this is to make the ground refrence a voltage between the two voltages on the signal wires that is one is assumed to be V+ the other at V- by the same amount with respect to the ground refrence... Now doing this is known as differential signalling and one advantage is the ground refrence can be "assumed" not actual. That is you just define the signal ground as being half way between V+ and V- at both ends of the cable with out actually having a real wire or conductor. this has the benificial side effect that no current can flow in this non existant ground therefore it cannot generate any noise...
There are a number of other benifits (such as phantom signaling) to diferential signalling which makes it considerably more desirable than a simple signal and signal return with it's uncertain voltages that can lead to eath loops causing increased extranious signal pick up.
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