Communication Networks/Analog and Digital Radio
HD Radio 
In the United States, analog radio broadcasts has been operating for many years in two distinct service bands: AM and FM. AM signals were relatively low frequency, measured in Kilohertz, and used amplitude modulation. FM radio stations were higher frequencies, measured in megahertz, and used frequency modulation.
Recently, however, a new type of radio broadcast was introduced, a digital type known as High-Definition (HD) radio.
What is HD Radio? 
Spectrum of HD Radio 
HD radio stations, because they are digital and therefore suffer fewer of the nonlinearity effects of analog modulation occupy a smaller bandwidth then traditional radio stations. To save space in the spectrum, HD radio stations are broadcast in the buffer regions between analog radio stations. Originally, these buffer regions were created because there was bleed-through between radio stations because of nonlinearities in the transmitters. Modern transmitters, in an effort to save energy, have mostly reduced these errors. Because of this, the buffer regions between stations are mostly unused and free from noise and interference.
Existing radio receivers can detect these HD channels, but an addition of a digital demodulator and signal reader is necessary to read them.