Complex sentence: It must be noted that all real world audio signals are continuous time analog signals and hence an Analog-to-digital converter followed by a quantizer must be used to convert the continuous time analog signal to a discrete time digital representation.
Split into simpler sentences: It must be noted that all real world audio signals are continuous-time analog signals. Therefore, sampling and quantization must be applied to convert the continuous-time analog signal to a discrete-time digital representation.