# Broadband transmission, signal is sent by modulating a carrier

beckett
Hi folks,

quick question, i have just read a small snippet on data transfer that has sparked some thoughts. Basically it was this:

Baseband transmission, one carrier frequency is used and the electrical signal is applied directly to the wires.

Broadband transmission, signal is sent by modulating a carrier (eg amplitude or frequency etc).

Now, if for example i am connected to an earthed wire and i raise and lower the voltage on the point of connection to represent a sequence of 0's and 1's what exactly is going on? Would this be considerered as amplitude modulation of some sort, what frequencies are involved, what is the signal exactly. I can understand modulating a certain frequency and sending this over the air where it may then be demodulated and useful info is extracted but i can't grasp what is going on in a wire using voltage changes as a means for information transmission.

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Guybrush Threepwood

Originally posted by beckett
Now, if for example i am connected to an earthed wire and i raise and lower the voltage on the point of connection to represent a sequence of 0's and 1's what exactly is going on?

well, it's not very clear what are you trying to do, but if you're trying to output something on a grounded wire all that's going on is burning the transmitter.

I can understand modulating a certain frequency and sending this over the air where it may then be demodulated and useful info is extracted but i can't grasp what is going on in a wire using voltage changes as a means for information transmission.

well, before sending something over the air there has to be an antenna and before that it's just electrical signal traveling on a wire.
basically let's say you want to transmit digital info on a wire. then you use a wire and a voltage reference (that's the ground). When you want to send the bit '1' you rise the voltage on the wire and the other party detects this voltage by coparing it to the reference. When you want to send the bit '0' you put the wire at the same voltage as the ground and the other party detects the lack of voltage on the wire.
You could say that this is one basic type of modulation. You could also do that with two wires and no ground. So that is why your question with the grounded wire is a bit confusing...