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70D - Losing left audio channel


This is something I have never noticed as I don't record audio to the camera directly most of the time, and when I do, I typically send mono to it (which hides the issue I am about to point out).


However, this month we lost quite a bit of video causing us to have to go back and reshoot for some of the audio, because I sent stereo to the camera. In other hand, I have two mics plugged into an amp, and then sent the signal to the camera in stereo so mic one would be left and mic two would be right (making post edit more of a breeze if audio levels end up being different between the mics). When I did this, the mic in the right channel was crisp and loud as it should be. The mic in the left channel however sounded like ambient noise of someone talking clear down the hall and around the corner (about 5% of the audio of the other mic...if even that). 

Switch the external amp back to mono and both mics come in fine (because both are being fed to both sides, thus you never notice the issue as it is hidden.


Monitoring from the external amp since the 70D doesn't have an audio out jack, the amp is fine. To make things wierder, the camera shows both channels on the on-screen meter just fine.  So audio from mics to amp is fine, and audio coming into the camera up to the meters "looks" fine, but the final file that is is screwed. So I am guessing it is an issue with the recording process to the file (a codec/corruption issue?).


Just to make sure, we even tested the amp plugged directly to a PC and other equipment (instead of the camera) and both channels came out just fine. This made extra sure that the equipment going into the camera was indeed working 100%. ALSO, we plugged the same setup into a Canon SL1 to see if we can replicate it on a different model, and everything came out 100% (just fine). 


So if you split the two mics so each is in their own L or R channel, the camera fails and loses one of them in the record. Are we the only one experiencing this (is the camera bad)? Or is this a common thing with the 70D?