have to wait for all of them to arrive
before you can do anything with any
of them. With streaming, you start to
use the packets as soon as enough of
them have arrived. That’s the
You can think of streaming as
playing during downloading, but
technically the two things are
different in many ways. Real time
shortwave radio depends on
There are at least three popular
streaming architectures: Real Player,
Apple Quick Time, and MS Windows
Media Player. Streaming files are
downloaded a chunk at a time and
deleted as soon as they are played.
The goal is for nothing to remain on
your computer. This is commonly
done to avoid copyright issues.
Since the audio goes straight to
the soundcard, it is not available to
use in another software application.
We’ll discuss how to deal with that
later on, but first, what are Internet
Internet radio suffers from
intermittent stops because of the way
that information travels across the
Internet. Sometimes packets are lost
along the way, or some packets
arrive sooner than others (in the
wrong order) and must be resent
from the source.
All of this can cause a break,
which is why there are “pauses”
when listening to Internet radio.
Intermittent audio is most often
associated with a slow or inconsistent
Internet connection. So, having a
faster connection may help. The
good news is that because of the low
audio bandwidth of shortwave radio,
data can be received fast enough and
decoded properly with normal or
slightly slow connections.
reaks may occur, but often are
not serious enough to affect the data
A big exception to this is
decoding FAX images. Since they are
synchronous transmissions, a break in
audio associated with a lost packet
can cause a breakup in the picture.
An example is shown later.
Nonetheless, it is still fun to
experiment with this mode. We are
still left with one problem. How do
we get the audio to our application
decoding software? We’ll take a look
at this next.
Piping the Audio
Your application software needs
to have access to the incoming
audio. In the good old days of
Windows XP, there was a provision
for recording all audio sounds heard
on your computer. This included
Internet sounds. Built into the sound
card architecture was a feature called
Stereo Mix. This could be used to
transfer or pipe the audio to other
applications. Since then, most OS
versions have either eliminated this
feature or have hidden it so well that
most users can’t find it.
In the past, to get around this
problem, hardware solutions were
used to patch from the speakers to
the microphone input.
This solution did work, but
sometimes had unintended
consequences such as being unable
to monitor the audio. Fortunately,
virtual audio cables (VAC) appeared
on the scene. VACs are basically
software implementations of this
same idea of patching hardware —
but using software. There are
advantages to using VACs.
A real adapter usually has a
digital-to-analog converter (DAC) and
an analog-to-digital converter (ADC)
in its signal path, which causes
unneeded DAC and ADC
conversions if they are physically
connected together. VACs do the
conversion without these extra
conversions, and also allow you to
monitor the signals.
The concept of virtual audio
cables has been around for several
years, and there are a number of
commercial products available that
will do a good job. However, they
are not free and may be complicated
to implement. Perhaps the most
popular commercial software is
Virtual Audio Cable 4. The trial
version of this product has been used
with good results.
Luckily for us, there is a free
product available that also uses the
concept of virtual audio cables. This
virtual audio mixer is known as
It is able to manage every audio
related item in your computer,
including the speakers, microphone,
soundcard, and Internet audio. So, if
you want to patch the Internet audio
to your decoding software, it is easy
to do. It’s available from http://vb-
42 June 2017
FIGURE 3. Block diagram of shortwave data receiver.