Hd Radio basics
HD Radio uses a variety of technologies to enable it to carry digital audio in an acceptable bandwidth and with the new high quality that is required. The transmission uses COFDM combined with specialized codec to compress the audio.
Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects.
One of the requirements for HD Radio was that it would maintain compatibility with existing stations. To achieve this there are two versions; one HD Radio system for AM, and the other for FM.
In what is termed hybrid mode, the AM version has a data rate of 36 kbps for the main audio channel and the version of HD radio for the FM bands carries 96 kbps. In addition to this HD radio can also be used to carry multiple audio channels, and in addition to this secondary channels for services such as weather, traffic and the like may be added. However adding additional channels will reduce the available bandwidth for the primary channel and audio quality may be impaired.
In hybrid mode a radio receiver will first lock onto an analogue signal. If this is possible, then it will try to find a stereo component (FM only) and finally it will endeavor to decode a digital signal. If the digital signal is lost then it will fall back to the analogue signal. The success of this process depends upon the transmitting station being able to synchronize the digital and analogue signals. Often the digitization process takes a noticeable amount of time and the digital and analogue signals may not be transmitted in time with each other.
Once HD Radio is fully established, the hybrid mode may be removed and at this point no analogue information will be transmitted. However it is envisaged that this will take some time as this can only be viable when very few analogue radios are in use.
References
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