29 November 2017

Avionics: Intercom output level for recording.

I picked up a fancy GoPro Fusion (marketing video below) to use with my aircraft.  It's a full 360 degree camera, in both xy- and xz-planes.  I also picked up the GoPro HERO 6 Black, GoPro HERO5 Session  and GoPro Session.  The Fusion and HERO6 Black will be placed under the canopy.  The HERO5 Session will be placed on the tail and the Session will be placed on the wing.

I would also like to record the intercom audio on the HERO6 Black.  This is why I wired in panel-mounted jacks, on the lower right of my panel, for audio input and output (Music 1 In and Out in the image below), figuring I would use the latter for a video camera.  For the Music 1 Out jack, I used the GMA-240's Passenger Headset Audio Out pins (40 and 41 on J2401). Note, the GMA 240 was replaced with a GMA 245 on 12-Aug-18.

Because the output level from the GMA-240 is for a headset and can exceed ±4V (depending on load impedance) it can't be connected directly to the GoPro.  This is because the GoPro's audio input level requires "line level" (i.e., about ±1V) and has a 8k-Ohm input impedance in line input mode (2.2k-Ohm for mic and 47k-Ohm for powered mic).

By placing a 47k-Ohm resistor in series with the left and right audio lines into the jacks, the GoPro can accept output from the GMA-240 through the jack.  This occurs since this simple voltage divider network reduces the magnitude of the audio voltage into the GoPro to about 14.5% of GMA-240's output.

When I originally put in the panel-mounted audio jacks, I used Molex connectors on them figuring that I would need to replace the jacks in the future due to potentially many connect/disconnect cycles.  Turns these Molex connectors were ideal for accommodating the 47k-Ohm resistors in a little jumper cable.

The image below shows the back of the panel on the right side (looking aft).  The two Music 1 In and Out jacks are shown on the left of the image.  The bottom jack is the Music 1 input.  The top jack is the Music 1 Out jack which is the audio from the GMA-240's Passenger Headset Audio Out.  You can see a small Molex-plugged jumper cable with the 47k-Ohm resistors placed between the Molex connectors from the jack and from the GMA-240. 

When plugged in to the GoPro, the audio from the intercom is clear and unsaturated.  Should I ever need to, I can simply remove the jumper cable with resistors and return the jack to its headset-level output.

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