App Note:
GPS Triggered Audio with BGM
Interrupting and Resuming a Background Audio Track
A single Digital Audio Machine 4 (AM4) equipped with a gps antenna can play a background music track, interrupt it with different audio at certain geographical points, and then resume playing the background track where it left off.
Design Engineer
Audio AM4
GPS Builder

First, you need to create a gps playlist which will “tell” the AM4 what points geographical points to change the audio.

Begin by running GPS Builder. Open the sample file ply00000.gps. Important Note: This file is named ply00000.gps so that it will load the gps points into the machine on boot. If this file is any other name, the gps file will not be loaded.

The first line of this file sets the background music using the command “Set Background”. The “2” in the “Data1” column represents the file snd00002.mp3 or snd00002.wav. Any sound file that begins with “snd” and is followed by a five digit number, can be referenced by the number alone.

The Data2 column represents the amount of fade-in or fade-out that will occur between the background music and the sound that may be interrupting it. (For the “Play” command and “Set Background” command only)

If you’re ever unsure of what to put into Data1 or Data2, simply press the “…” button to view the wizard.

The “Set Background” command, in combination with the ply00000.gps filename, means that as soon as the AM4 powers up, it will play and loop the sound snd00002.mp3. The AM4 will also begin comparing the current gps point found by the gps antenna to each of the points in this gps playlist.

Each point is defined by three parameters: Latitude, Longitude, and Direction. Direction is an optional parameter. As soon as the AM4 matches these parameters, the action set by the Command column will occur.

The AM4’s gps antenna point does not have to match the gps playlist point exactly for the sound to trigger. It looks at the “Tolerance” field to determine how far from the actual point the sound can trigger. The tolerance only has to be set once, and it is retained throughout the playlist.

Tolerance is set in meters, but it can also be set by speed. Since the AM4 receives gps points from the antenna at a rate of 1 per second, it is possible to miss a point completely. The Tolerance can be set in terms of speed by clicking the “…” button in the Tolerance column.

If there is no latitude, longitude or direction set for a command, that command will occur immediately following the previously gps triggered command in the list. For example, line 2 of  ply00000.gps shows snd00001.mp3 being triggered at latitude 28.4 and longitude –81.32. If this trigger occurs, snd000001.mp3 will be played followed immediately by snd00003.mp3.

The easiest method to determine gps points is to physically ride the route with a laptop running gps builder, and with a gps antenna connected to the laptop. Press the space bar to retrieve the incoming gps point into the current line. If riding the route is not possible, gps points can be found using the “map” tab in gps builder, or using any other map program. Then, type the latitude and longitude into the appropriate columns.

Once your gps playlist is complete, load it and all of the sound files onto the AM4. Use a compact flash card reader or use standard ftp to transfer the files. (Note: See Digital Media Manager for ftp) 

That’s it! Plug the gps antenna into the AM4 and go for a ride!



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