Question

How can we convert digital signals into analog signals?

Answer 1

Analog signal is converted to a digital signal using a two step process.

Sampling
Quantization

The device used to do this is called as ADC (Analog to Digital Converter).

Step 1: Sampling converts a continuous time continuous amplitude (real valued) signal to discrete time continuous amplitude (still real valued) signal. Remember only time axis is discretized and not the amplitude axis.

Step 2: Quantization converts the discrete time continuous amplitude signal to discrete time and discrete valued (from a set of finite values, so that it can be represented by finite bits and can be stored on a computer).

Different sampling techniques and different quantization techniques is all together a big topic.