| HD Radio |
HD Radio, High Definition Technology
- an overview or tutorial of the basics of the technology for HD Radio, High
Definition Radio, the digital radio system developed by iBiquity and chosen as
the digital radio system for broadcasting with the USA..
Digital technology is being applied to many areas of radio
communication including radio broadcasting as it offers some significant
advantages. While DAB digital radio is becoming established in some areas of the
globe, the system that has been chosen for use in the USA is known as HD, or
High Definition, Radio. Using HD Radio, will enable high quality audio to be
received along with the ability to incorporate many new features and facilities.
The HD Radio system has been developed by iBiquity, and has
now been selected by the FCC in the USA. It will take the place of both the
existing AM and FM transmissions, and offers many advantages for both listeners
and broadcasters alike:
- Improved audio quality - it is claimed that HD Radio broadcasts on the
AM bands will be as good as current FM services and those on the FM band
will offer CD quality audio.
- Reduced levels of interference. AM transmissions in particular are prone
to static pops and bangs as well as high levels of background noise. HD
Radio will almost eliminate this.
- Opportunity to use additional data services. By using digital
technology, HD Radio provides the opportunity to add data services such as
scrolling programme information, song titles, artist names, and much more.
- There is also the possibility of adding more advanced services such as
surround sound, multiple audio sources, on-demand audio services, etc.
- Easy transition for broadcasters and listeners. Although new HD Radio
receivers are required to receive the new transmissions in their digital
format there is considerable re-use of infrastructure and spectrum.
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 specialised codec to compress the audio.
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Note on OFDM:
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 another there is no mutual interference. This is
achieved by having the carrier spacing equal to the reciprocal of the
symbol period. This means that when the signals are demodulated they
will have a whole number of cycles in the symbol period and their
contribution will sum to zero - in other words there is no interference
contribution. The data to be transmitted is split across all the
carriers and this means that by using error correction techniques, if
some of the carriers are lost due to multi-path effects, then the data
can be reconstructed. Additionally having data carried at a low rate
across all the carriers means that the effects of reflections and
inter-symbol interference can be overcome. It also means that single
frequency networks, where all transmitters can transmit on the same
channel can be implemented. |
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 endeavour 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
synchronise the digital and analogue signals. Often the digitisation 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.
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