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Guys do you now why line coding also called digital base band modulation named "line coding" where does it come? and how is the name related to modulation

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    $\begingroup$ It's mostly a historical thing -- networking people were coming up with bipolar, Manchester, 2B1Q, etc, trying to minimize DC and help with synchronization, while digital comm people were doing M-PAM with pulse shaping. Later it was realized that the two are essentially the same thing, but prioritizing different system properties. $\endgroup$
    – MBaz
    Jan 21 at 20:34

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Line coding is the process of how one can choose to map, i.e. encode, information (e.g. bits) into signals (voltage/current) that can be sent over a transmission line. There are different encodings that can for example help with ambiguity resolution, bit clock detection, error resilience/detection/correction, and can be specifically tailored to the transmission medium's properties. Note that line coding can also be further filtered to reduce its spectral bw usage (or other line codes may be chosen for their spectral properties). In general line coding is a baseband transmission technique.

Now days, with many comms systems running wirelessly and using digital communication techniques and transmitting at RF, we tend to use line codes a little differently there. For example, we might line code the bits prior to using a BPSK modulation - why would we do this (instead of just mapping a 0 to one symbol and a 1 to the opposite symbol, without coding)? Well - one reason might be to use a differential line code so that on the receiver you can resolve a 180 degree phase ambiguity that a Costas loop in the receiver often exhibits.

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