Why a DC (direct current) component is undesirable?

[janu203]

When choosing Line Codes (e.g. Bipolar or Polar formats) for transmission of bits over a communication channel such as twisted pairs, it is always desirable that no DC component is present for a particular choice.
According to William Stallings (Digital Communication Book)
"With a dc component to the signal, there must be direct physical attachment of transmission components.
With no dc component, ac coupling via transformer is possible; this provides excellent electrical isolation, reducing interference"

I cannot understand what does he mean in above paragraph. I don't know much about AC/DC circuits and their workings

 

[berkeman]

When choosing Line Codes (e.g. Bipolar or Polar formats) for transmission of bits over a communication channel such as twisted pairs, it is always desirable that no DC component is present for a particular choice.
According to William Stallings (Digital Communication Book)
"With a dc component to the signal, there must be direct physical attachment of transmission components.
With no dc component, ac coupling via transformer is possible; this provides excellent electrical isolation, reducing interference"

I cannot understand what does he mean in above paragraph. I don't know much about AC/DC circuits and their workings

A DC component is undesirable for transformer-coupled communications, because it can cause saturation issues in the transformer cores. You would rather use all of the B-H curve of the transformer for coupling the AC communication information, and not have an offset B-H curve start to limit the AC components.

You want to use as small a transformer as you can for cost reasons, but as you make it smaller, the AC component of the communication signal starts to get close to the tips of the B-H curve. Adding a DC component to the waveform just pushes you that much farther toward one of the tips of the B-H curve, which is bad. In other words, to accommodate a DC component, you have to make the transformer physically bigger than you would like, which makes it more expensive.

Does that help?