Nuts and Volts - June 2017

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 essential difference.

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 streaming audio.

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 stops?

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 decoding.

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 to Your Application

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 Voicemeeter.

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-