1, the figure explains frequency division multiplexing, which has nothing to do with digital or analog; only frequency reuse.

2. Analog speech frequency is usually 4khz. This is common sense, so there is no reason why it is so low, because the voice has such a large bandwidth.

so the figure may be the frequency multiplexing of the analog signal of each 4khz.

3. The digitization of analog speech signal needs at least 4000 sampling times per second. If the 8-bit digital signal is transmitted, then the bandwidth requires the bandwidth transmission of 64kbps.

the digital signal is sampled by the analog signal, and the higher the sampling rate is, the closer it is to the original signal.

so it is also possible to use the frequency bandwidth of 4khz to transmit digital signals.

as long as your algorithm is good enough, you can transmit more digital signals at lower frequencies.

3. Digital signals can be transmitted by high and low voltage values, both high and low digital signals. When it comes back to it, it's a sequence of numbers like 1001100.

4, the reason why the analog signal is an analog signal, there is no value at all, ah, just transmit the analog pulse signal directly. Is to show the waveform of the original voice through the waveform of the voltage.