Up to this point, the transmitted signal has been shown as a sine wave, which is the simplest sound wave or audio wave. This is an analog (or analogue) signal, which describes the continually changing audio voltage that is converted into air pressure vibrations to produce sound.
The advent of the digital age has brought with it the technology of Digital Audio. When an audio signal is described as being "Digital", it means that its waveform has been sampled at very fast, regular intervals and measured to produce a number for each sample, which is then converted to binary data (i.e. ones and zeros). When the audio is needed to make sound, this stream of numbers is converted back into voltages, which are then used to create sound in the traditional manner.
Digital audio is now widely used as an efficient way to process, send and store audio data, delivering clean, high-quality audio. This technology can also be applied to wireless audio transmission, which maintains the audio in digital numerical format over the radio waves, eliminating most of the unwanted noise induced by the transmitting and receiving circuitry. The diagram below is purely to illustrate this effect. In reality, frequencies would be much higher and the FM shift would be far more subtle.