GMDSS FAQ

GMDSS App

To use this app, you need to feed the audio output from a your radio into the device. You can do this via one of several ways:

Set the device near the radio's speaker to pick up the audio. While easy to do, this is not ideal, as background noise can interfere with the audio, causing garbled decodes.
Plug headphones into the radio, and set one earphone near the mic. I find that with good quality headphones, this can work extremely well.
Use the microphone input jack of your device, by obtaining an appopriate cable and connecting the radio's line level output to the microphone input.
With the Camera Connection Kit, you can plug a USB sound input device into the iPad, and use a patch cable between it and the radio's audio output. This is by far the best solution.

You can use the volume indicator in the upper right corner to verify that audio is getting into your device, and that the volume level is appropriate.

If you are running iOS 7 or later, be sure to give the app permission to use the microphone. If you denied permission, you can always give it again by following these steps:

Open Settings app and go to "Privacy"
Select "Microphone" to get a list of all apps that have requested access to the microphone, and to control which apps have access to the microphone

If you're looking for a radio to use with the app, there are many different types to choose from. Take a look at our listing of radios to use with your app, to see several, along with some suggestions on how to choose a radio.

You can use the volume indicator at the top to verify that audio is getting into your device, and that the volume level is appropriate.

Tune your radio to one of the GMDSS-DSC frequencies: 2187.5, 4207.5, 6312, 8414.5, 12577 or 16804.5 kHz, using RTTY mode if your radio has that mode. Set the frequency in GMDSS to the audio center frequency of your radio in RTTY mode, usually around 2200 Hz. Check your radio's manual to be sure.

If your radio does not have RTTY mode, you will need to tune in SSB mode, to an offset frequency. For example, to receive on 6312 kHz, you can tune to 6310 KHz USB, and then set the center frequency in GMDSS to 2000 Hz (518 kHz - 516 kHz = 2 kHz, which is 2000 Hz).

You can use the Invert audio signal setting in Setup to invert the mark and space tones, if needed because your radio reverses them.

The spectrum display at the top shows the audio spectrum, with red lines drawn over the mark and space frequencies. This can be used as a tuning aid.With a properly tuned signal, there are two broad peaks in the spectrum, near the two red marker lines.

You can listen to and decode this GMDSS-DSC recording as a test. Set your app to a frequency of 2220 Hz with Invert OFF.

The spectrum display in the upper left corner shows the audio spectrum, with red lines drawn over the mark and space frequencies. This can be used as a tuning aid.

Usually you will leave Invert off, turn it on if your radio inverts the audio spectrum.

That's it!

Here are some details on directly connecting an audio line source to your iPad/iPhone/iPod Touch

Email your comments and questions to support@blackcatsystems.com

Last modified November 23, 2012