NAVTEX Decoder App For The Macintosh

Connecting Your Radio

To decode NAVTEX transmissions, you need a radio capable of receiving 518 kHz in USB (SSB) or RTTY mode.

If your radio has RTTY, tune directly to 518 kHz. You will need to check your radio's manual to see what the center audio frequency is, and enter that as the center frequency in the app, in Hz. For example, if the center audio frequency is 2200 Hz, enter 2200 as the Center frequency.

If your radio only has USB mode, tune down 2 kHz. Then enter 2000 as the center frequency into the app (2000 Hz is the same as 2 kHz).

Connecting your radio to your Mac:

You simply need to connect the audio output of your radio to your Mac's audio input (this is the jack on the back, usually with a little microphone icon). A standard 1/8" phono plug will work fine. Monophonic is fine. You can pick up a patch cable from any consumer electronics store. Get a cable with the correct plug to fit into your radio.

You may wish to place an isolation transformer in the cable, to reduce hum caused by ground loops.

If you have a Mac without sound input, you can use a USB microphone. We've successfully used the iMic by Griffin, as well as many other USB sound input/output devices.

You can either tap the audio from your radio's headphone or external speaker jack, or a line level or "Record" output. You'll need to experiment to see what works, and what the signal levels are. There are too many possible combinations and Mac models to give you a definitive answer for each particular setup.

Starting Up

Double click on the app icon. The first step is to select the sound input device. There is a popup menu near the right side of the window, which selects the sound input channel. Usually the options are Built-In Microphone and Built-In Input, select the latter to use the cable you plugged into the Mac.

If the PLAYTHRU button is turned on, the app will play sound from the radio through the default sound output channel on your Mac. This can be handy to verify your Mac is getting audio from your radio.

Directly above the sound input popup menu and PLAYTHRU button is a display of the received audio spectrum. This display will often have one or more red vertical lines, used as a guide to help tune in signals correctly.

Directly to the right is a volume indicator. To the right of this is a slider which adjusts the audio input gain. This slider is grayed out if the input device you selected does not support gain adjustment.

Above this is the Filter button. For many modes, this button can be used to apply an audio filter centered on the signals of interest, improving decoding accuracy.

Assuming you still have your radio connected and tuned into a signal, you should see activity in the volume indicator. This verifies that the app is getting the audio input, and can be used to help set the audio volume level from your radio. You want to avoid it staying pegged too high, this would mean the audio is being clipped, which will cause problems when trying to decode the signal.

NAVTEX

NAVTEX is transmitted using SITOR-B mode.

SITOR-B is an error-correcting mode of text transmission. It is commonly used in the maritime services, and is most well known as the method used to send NAVTEX transmissions. NAVTEX is a worldwide network of transmitters, typically using 518 kHz, although 490 and 424 kHz are also used. Information of interest to mariners is sent, including weather.

There are only a few things to set to use this mode. First, the center frequency of the received audio. You may also need to click on the INV button to set inverse mode, if the mark/space tones coming out of your radio are reversed from what the app expects.

When a SITOR-B/NAVTEX transmission is detected, the app will automatically synchronize to the transmission, and start printing decoded text.

While this is an error-correcting mode, it is still possible to get receive errors, especially under poor conditions. When a garbled character is received, the app displays an asterisk (*) in place of that character.

Should severe noise cause a loss of synchronization, the app will attempt to re-sync. The SYNC button may also be clicked to force re-synchronization.

There are three text indicators that appear:

PHASE: The app is receiving phasing/sync characters

GOOD: The app is receiving correctly

BAD: The app is receiving garbled characters, has lost sync, or there is no transmission detected, just static. When Parse NAVTEX is checked, the app will parse out correctly received NAVTEX messages, usually transmitted on 518 kHz.

You can save decoded text to a text file by selecting New Log File from the File menu. Select Close Log File when you are some saving text.

Email your comments and questions to support@blackcatsystems.com

Last modified February 11, 2015