Remote control – 3: Editing waveform
== 3 == EDIT WAVEFORMS
Now you need to rebuild the original signal of the remote. As explained above, it’s just a matter of carrier present/absent. So:
a – Add 2 new mono tracks into Audacity project
b – Copy the “squared” signal into one track
c – On the other track, create a “tone” (the carrier): 38000 Hz 19000 Hz frequency, 0.9 amplitude
d – Select and delete unneeded parts of the tones; to do this, you have to zoom in: select a part of the signal, and click on the “zoom to fit” button:
You’ll obtain something like this:
e – Now click on an edge of the signal and drag the mouse down to the second track, until the selection reachs the edge of the next pulse in the signal:
f – Once thw WHOLE low-part of the signal is selected (see picture below), click on the “silence” button:
g – You should obtain “silence” on the lower track too:
h – Repeat steps e-f-g for all silence parts of the signal, until you obtain something like this:
Ok, now you have replicated the original signal.
Unfortunately, you can’t just “replay” it: soundcards can usually reproduce only signals up to about 20 KHz, but a remote control use 38 KHz modulation.
But some guys had a great idea some years ago (look here for their patent, now free available on internet; free registration required): simply connect two IR led to left and right channel, one the opposite way of the other, discarding GND, and play a stereo signal made of same signal with opposite phases: this will result in each LED producing a 19 KHz signal; their opposite phases allow signal “sum”, and the result is a 38 KHz signal.
How to accomplish all of this?!?
i – After deleting original signal track (you don’t need it anymore), create a new mono track in Audacity (menu can vary amond different versions):
j – Copy into it the just obtained (reconstructed) signal:
k – Select the whole track
l – Select “invert” from “effects” menu: this will inverti waveform phase:
m – select the first track and choose its own menu item to join it to the track below into a single stereo track:
You should now see a single stereo track:
Ok, now you eventually have the needed signal, which you can use to control your device.