ANALOG AND DIGITAL SIGNALS

In many cases, the signal of interest is initially in the form of an analog electrical voltage or current, produced for example by a microphone or some other type of transducer.
In some situations, such as the output from the readout system of a CD (compact disc) player, the data is already in digital form. An analog signal must be converted into digital form before DSP techniques can be applied. An analog electrical voltage signal, for example, can be digitised using an electronic circuit called an analog-to-digital converter or ADC.
This generates a digital output as a stream of binary numbers whose values represent the electrical voltage input to the device at each sampling instant.