Il produttore e presentatore dello "show radiofonico digitale" Kim Andrew Elliot ha dedicato una parte del programma a questa ricorrenza trasmettendo un testo che sarà pubblicato sulla rivista della NASWA (North American ShortWave Association).
Lo pubblico integralmente.
(Decodifica da trasmissione in MFSK32 sulla frequenza di 15670 kHz, software Fldigi)
"The following is my International Broadcasting column for the
March 2015 Journal of the North American Shortwave Association
(NASWA) ...
VOA Radiogram: Two years of strange noises on shortwave
Kim Andrew Elliott
March 2015 will mark two years of VOA Radiogram, the weekly Voice
of America program in which digital text and image modes are, as
illogical as it may seem, broadcast by an analog shortwave
broadcast transmitter.
A few years ago I was aware that more and more countries were
devising more and more ways to block Internet content. At the
same time, as a radio amateur, I was becoming active in the
digital modes. I was amazed at how well the digital modes, many
with built-in error correction, could cope with difficult HF
conditions.
In a rare flicker of inspiration, I thought of using digital
modes on shortwave to transmit text and images, the building
blocks of web pages, into countries where the Internet is
censored.
However, at the time, I thought that the digital modes had to be
transmitted and received in single sideband, because that's how
the hams do it. An engineering colleague set me straight: the
digital modes are conveyed via audio, and audio can be
transmitted by AM as well as SSB.
That revelation meant that the digital modes could be transmitted
on any existing analog AM shortwave broadcast transmitter. And it
could be received even on inexpensive portable shortwave radios
with no SSB capability. Digital Radio Mondiale (DRM), on the
other hand, needs a new transmitter, or at least a new exciter,
and a special (and these days hard to find) receiver.
How to decode the modes
To decode the VOA Radiogram content, audio from the plain or
fancy receiver must be patched into a personal computer. (In a
pinch, the radio's speaker can be placed next to the built-in mic
of a laptop PC.) Software does the decoding. Most VOA Radiogram
listeners use Fldigi from w1hkj.com, but other decoding software,
such as MultiPSK and DM780, is available.
The concept was first tested on WBCQ and WRMI. In March 2013, the
new VOA Radiogram program went on the air, transmitted from a
50-year-old GE transmitter, using 80 kilowatts, at the IBB
Greenville, NC, site.
Thousands of reception reports have so far been received from
shortwave listeners and radio amateurs. Most are located in
Europe and North America, but some are in Latin America and Asia.
Most weekends, VOA Radiogram is successfully decoded by a
listener in New Zealand, 14000 km from the transmitter.
MFSK is the best, so far
In the first weeks of VOA Radiogram, the various digital modes
available to amateur radio were tested side by side. These modes
included the various flavors and speeds of BPSK, QPSK, MT63,
Olivia, and Thor. It was, however, MFSK that provided the most
successful text decodes. And, as a bonus, MFSK can also be used
to transmit images, in a manner similar to slow scan television
(SSTV).
In addition to the decision about the mode was the question of
the speed of the mode. As a general rule, the faster the mode,
the less able it is to cope with difficult shortwave conditions.
MFSK16 (as in 16 baud) is amazingly robust in dreadful
conditions, but slow at 55 words per minute. MFSK64 is 240 wpm
but works well only in favorable conditions. MFSK128, at 480 wpm,
is blisteringly fast but really best suited for local VHF and
UHF. Ultimately, MFSK32 demonstrated the best combination of
speed (120 wpm) and performance in typical, i.e. usually not
brilliant, shortwave conditions.
Extends the range of (what's left of) shortwave broadcasts
Reception reports and audio files received from listeners
indicate that MFSK32 not only works well, it works better than
voice on analog shortwave. In conditions where a typical voice
broadcast is difficult to comprehend, e.g. my voice introduction
to each VOA Radiogram, the text is often copied 100%. Thus, the
introduction of software has extended the communications
capability of shortwave broadcast transmitters, just at a time
when those transmitters are being dismantled at an alarming rate.
If a country blocks our Internet content, it will probably also
jam our shortwave broadcasts. The text modes have been tested
against Chinese jamming of VOA and Radio Free Asia, and Cuban
jamming of Radio Martí. Remote receivers in or near the target
countries show the text modes slicing through some intense
jamming.
MFSK32 occupies only about 500 Hz of the 2000 Hz bandwidth
available in each sideband of an AM shortwave signal. New modes
in development use all of that available 2000 Hz, to some extent
for additional speed, but even more for additional error
correction. This would result in a text delivery technology
especially well- suited to shortwave broadcasting, and perhaps
even more resistant to jamming.
Now we need hardware and software solutions
It would be helpful if more of the surviving shortwave broadcast
stations would transmit text and images, even just a few minutes
per week. This would encourage software developers and receiver
manufacturers to facilitate the reception of these modes. The
Elecraft K3 amateur transceiver decodes the popular PSK31 mode
and shows the text on the rig's display, so receivers could also
do this with other modes.
Fldigi and the other decoding software now used by VOA Radiogram
listeners are really designed for amateur radio use. They include
several features, such as encoding, not needed for receive-only
use and intimidating to the non-technical. Therefore, a vital
requirement is the development of a software app that would
simplify the decoding process and make it possible on mobile
devices as well as PCs.
For more information about the North American Shortwave
Association, visit http://www.naswa.net .
Sending Pic:171x51C;
Please send reception reports to radiogram@voanews.com.
And visit voaradiogram.net.
Twitter: @VOARadiogram
Thanks to colleagues at the Edward R. Murrow shortwave
transmitting station in North Carolina.
I'm Kim Elliott. Please join us for the next VOA Radiogram.
This is VOA, the Voice of America.
Sending Pic:380x30C;
Before RSID: <<2015-03-01T19:58Z MFSK-32 @ 9046000+1500>>
Thank you for your support during these first 100 programs of VOA
Radiogram.
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