To send digital information, we must devise analog signals to represent bits. The process of converting between bits and signals that represent them is called digital modulation. With digital modulation:

A. the most straightforward form of digital modulation is to use a positive voltage to represent a 1 and a negative voltage to represent a 0, a scheme called NRZ (Non-Return-to-Zero)

B. schemes that directly convert bits into signals (occupying frequencies from 0 up to a maximum) result in passband transmission

C. it is possible to share a channel by multiple signals, this scheme is called baseband transmission

D. all of the above are true

A

Explanation:

Digital modulation is defined as modulation of carrier signal using discrete signals. The digital signal to be transmitted is first converted to analog signal using Digital-to-Analog converter. At the receiving end, the analog signal is converted back to digital signal using Analog-to-Digital converter.

Options:

B. schemes that directly convert bits into signals (occupying frequencies from 0 up to a maximum) result in passband transmission. (False)

Schemes that directly convert bits into signals result in baseband transmission, occupying frequencies from 0 up to a maximum.

C. it is possible to share a channel by multiple signals, this scheme is called baseband transmission (False)

It is possible to share a channel by multiple signals, sharing signals is called multiplexing.

A. NRZ (Non-Return-to-Zero) is a simple and straightforward digital modulation scheme. In NRZ, a positive voltage is represented by 1 and a negative voltage represents a 0. (True)

Therefore, A is the only correct answer.