FM Radio Signal
![Picture](/uploads/9/8/2/0/9820651/614148552.jpg?353)
Figure 1.
Image used under GNU Free Documentation License
FM radio stations transmit analog signals between 88.1 MHz and 107.9 MHz in the United States. Each signal is separated by 200 kHz to allow more information to be sent in each signal then AM signals (10 kHz for separation). This increases the clarity of the sound at the end of the receiver. Upper and lower sidebands (as shown in Figure 1) are transmitted with analog carrier signals and they result from the modulation of the carrier wave. This is where the information is actually stored. Since they are mirror images of each other, they contain the same information. Broadcast stations wanted to increase the quality of sound but only had a certain bandwidth to work with. This eventually led to the development of HD Radio.
FM HD Radio Signal
![Picture](/uploads/9/8/2/0/9820651/101694239.jpg?353)
Figure 2. Image used under GNU Free Documentation License
By compressing digital data, FM broadcasting has been able to increase the quality of sound and finally bring radio music very close to CD-quality. The compression method that is now being used is a "lossy" method and is called HDC (High-Definition Coding). This means that some of the information in the sidebands are discarded from the original signal which brings down the quality of sound. This is why they say that HD Radio is very close to CD-quality but not entirely. Figure 2 allows you to see that the digital data has replaced the upper and lower sidebands. The digital signal is piggybacked onto the analog carrier wave. It is an in-band on-channel system because of the piggybacked digital signal. IBOC allows a transmitted signal to use the same amount of bandwidth as a normal, non-HD Radio signal.