Videomaker – Learn video production and editing, camera reviews › Forums › Technique › Sound › Audio channels….cancelling each other out?
November 22, 2013 at 6:05 AM #71298arthouseParticipant
Recently I did a short video project for a client and provided a copy of the video as a digital file (in WMV format).
The project in Premiere was comprised of a stereo audio track for the dialogue, and a stereo audio track for the background music.
The client wanted to show this video for a presentation and play it through Windows Media Player, hooked up to a projector/PA system (or something similar – I'm not sure of the exact set up). When it came to testing the video playback, apparently the video was playing fine but there was no audio after the first 30 seconds. The client asked their engineer what the problem was and the engineer told him that the audio channels were cancelling each other out/interfering with one another/"audio was 180 degrees offset".
This is something I do not understand, and after some research it seems it could be something called audio phase cancellation(?). I've been working with video for a while now and have never encountered this problem and I'm unsure if this is to do with the way I've encoded/compressed the video on export to the WMV format (I rarely export this format) or whether this is a physical problem at the point of hooking it up for the presentation.
What makes it all the more confusing it that the video and audio both play fine on my PC in stereo. The client ended up importing the WMV file back into Premiere, duplicating the already mixed audio track, then assigned one to the left and one to the right and apparently this solved the problem.
Does anyone understand what's happening here?
November 22, 2013 at 7:25 AM #209188gldnearsMember
" What makes it all the more confusing it that the video and audio both play fine on my PC in stereo. "
Drop the other shoe: How does it sound if you play it in Mono??
A logical explanation would be that the " projector " sound is mono, and when your two channels are combined to mono they are cancelling each other.
November 23, 2013 at 3:46 PM #209209BrianParticipant
I have a client who did a short video on their own and ended up doing the same thing. The clip sounded fine on his computer but almost no sound when posted online. The reason it sounds fine on your PC is that you are listening on a stereo setup. Two discreet audio feeds are being sent to separate speakers. At the showing, the PA setup was mono. They summed the two audio channels into on signal path and the two channels canceled each other out.
Think of it this way… an audio signal is a wave. If both channels are phased correctly, the two channels will simply be added together with no decernable effect. If the two are out of sync, the equal but opposite signals will cancel each other out. 1-1=0. Capiche? The problem is figuring out where one of the signals got reversed. It could have been done when you edited the program. It could have been a impropperly wired cable at your client's presentation or the audio guy could have reversed the phase on one channel on the pa system. It's pretty easy to hear out of phase audio. Sit equally between the speakers, send the same signal to each and if the phase is reversed, it's almost like the air is being sucked out of the sound. It almost hurts a bit. There are plugins, meters and all kinds of gadgets to check phase but your ears work just fine once you learn. As for the PA guy at the event, he was an idiot. Anyone worth their salt knows that the quick fix is to unplug one channel.
December 7, 2013 at 2:16 PM #209316HichamParticipant
This is really strange!!!
Even if there is a phase cancelation, you must hear some audio. For the complete cacelation of the audio signal, the left channel and the right chanel have to be the same, this is the case only if your mix is mono, not stereo
If your mix is stereo, after a phase cancelation, you'll have a signal degradation (no bass, something like reverbe…)
I had some years ago a similar trouble, but finally i found that my mix was not stereo, but dolby 5.1
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