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The shortwave transmissions are realized by using the amplitude modulation of
the audio signal. The transmissions are subjected mainly to variations in the
propagation conditions, fading, atmospheric noise, among others that affect
the reception quality.
The amplitude modulation, as well the frequency modulation, is an analogue
technique, where it is considered that voltages and currents of a circuit
are treated as varying in a continuous and linear way.
Yet in a digital technique, the voltages and currents are converted into one
of two possible states: "0" or "1". Generally, digital
techniques allow the processing of a big ammount of information at a high
velocity in a manner impracticable for analogue techniques.
Besides, digital techniques are much more efficient than analogue techniques.
Researches for the substitution of the actual analogue transmission system
by the "digital radio" are being realized since the 80's.
However, a definitive technology has been not reached, yet.
Below, some of the most relevant digital techniques in the present days:
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DAB - Digital Audio Broadcasting:
The work on this system began in 1981, as a replacement for the actual analogue
system for frequency modulation (FM), making use of sophisticated techniques as
coding and error correction, besides the so-called "data reduction".
However, DAB doesn't represent a considerable improvement with respect to FM,
as the fact that the data reduction technique degradates the audio quality,
resulting in a slightly inferior quality comparing with CD audio.
One of the main advantages is the improvement of the reception quality for
mobile systems, that with DAB no more are affected by multipath reception,
very common in FM; by contrast, DAB takes advantage of multipath reception.
Another difference between DAB and FM is the possibility of combining signals
from different broadcasters into an only data-stream, a "multiplex".
The frequency range used by DAB is not the
the same range used in analogue FM.
in analogue FM.
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DRM - Digital Radio Mondiale:
This system, whose development began in 1997, was projected as a replacement of
the amplitude modulation (AM), used in long wave, medium wave and shortwave.
It is used the so-called AAC (Advanced Audio Coding) technique, besides the
SBR (Spectral Band Replication) technique. The audio quality is rather like
monophonic FM with slightly less high-frequency information. The reception is
totally without fading and distortion. DRM uses the same channels and bands
utilized by analogue AM.
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WorldSpace:
The development of this system began in 1990, and its operation began in 1995.
In contrast with DAB and DRM, WorldSpace uses satellites for the transmission
of the audio signals; At the moment there are two fully-operational satellites:
AfriStar (covering Africa, Middle East, parts of Europe), and
AsiaStar (covering most of Asia, including China and Far East).
The third satellite to be launched, AmeriStar,
will cover Latin America and Caribbean.
It is used the L-band (1467-1492 MHz), with uplinks at 7 GHz.
The compression technology adopted is MPEG 2.5 Layer 3.
The data stream is divided into time-multiplexed sub-channels.
Each sub-channel uses a rate of 16 Kbits/s, and each emission
can use 1, 2, 4, or 8 sub-channels.
The transmissions in the highest rate (128 Kbits/s) have very good audio
quality; with only one channel (16 Kbits/s) the audio quality is similar
to a low-quality medium wave signal.
The digital systems above described are being used mainly in Europe, but
in Brazil there is no prevision of the implementation of digital radio.
Technical discussions are currently in progress.
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