Channel Mono setting oddity

Discussions about the use and operation of SAC (Software Audio Console)

Channel Mono setting oddity

Postby jlklein » Thu Mar 10, 2016 4:19 pm

Hey folks,
I had a video that we played on the church projection PC this weekend and the voices were gone but the music was there (this was a video camera recording of people talking with music underneath).
The service before mine had no issues and the only difference was that in their mix file the playback submixer input did not have the mono switch engaged (this channel is fed from a submixer that carries all of our DVD, Tape, PC, etc sources to one stereo input).
We usually have it set to mono in our service for split track accompaniment tracks which have music on one channel and demo vocals on the other, so we pan the submixer to the music side and set the SAC channel to mono so it comes out both main speakers.

Later I did some testing and sure enough, having Mono engaged was the issue, as it caused the center-panned vocals to cancel out (and much of the music).
I tried all the varieties of Mono and everything worked backwards: both L+R options cancelled the vocals but L-R and L or R only worked fine.
(This was also a teaching experience in that we could have left split tracks panned center on the submixer and chosen the Mono option of L or R only and done the same thing...sigh).

Anyway, I was thinking this might be a wiring issue, although it's kinda hard to screw that up with plugging RCA output jacks into RCA input jacks on the submixer, and then using 1/4" TRS cables to plug the submixer outputs into the SAC MOTU 1/4" inputs.

Anyone know of a good way to test this wiring theory out other than pulling all the cables and continuity testing all tips to tips and rings to rings?
I'm also looking for any good audio files that might be available to test polarity of signals that I could also burn to a CD to check not only the PC input to the submixer, but also the 2 CD players and 2 DVD players.

Thanks,
Jeff
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Re: Channel Mono setting oddity

Postby RBIngraham » Mon Mar 14, 2016 4:27 pm

Sounds to me like it is either a wiring issue or could it be just a phase issue in the material being played back. The latter seems unlikely but you never know. If you're having issues the best bet will always be to actually test every cable for a polarity issue. And don't count out the possibility of an issue within the hardware. Remember the original Mackie 1202? Yeah, I've got one of those and it has severed me well for MANY years. But the 1/4" TRS outputs are wired out of polarity with one another. If you sum them you get cancellation. I don't use it as much as I used to but when it was one of my primary tools, I had a special adapter I carried around to correct that issue. Could be a similar issue with the camera or however you're feeding the audio into the camera.
Richard B. Ingraham
RBI Computers and Audio
http://www.rbicompaudio.20m.com/
SAC details and goodies at: http://www.rbicompaudio.20m.com/SAC.html
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Re: Channel Mono setting oddity

Postby jlklein » Wed Mar 16, 2016 12:26 pm

RBIngraham wrote:Sounds to me like it is either a wiring issue or could it be just a phase issue in the material being played back. The latter seems unlikely but you never know.


It was actually a polarity issue with the audio on the video that was recorded with an SLR camera, so no SAC or cabling issues after all...not surprised as we've never had this issue all the years we've been using SAC.

I got the original video and exported the audio, opened it up in Audacity and saw that the voice track was only on the left channel...except for the part that the voices suddenly cut out when viewed originally. You couldn't hear it cancel through the PC as a stereo channel, but if you split it into two mono channels it cancelled right out, same as when I played the video on my iPhone and my son's older Galaxy Android phone (mono external speakers). Inverting the waveform did fix it, but I ended up just copying the whole live voice portion and copied it to the right track and it fixed it.

The Youth Leader who made the video (but hadn't recorded the live testimony parts) sent me this week's installment which is all live testimony recorded from the SLR camera. The first week's video had started with a voiced over part that was done on the editing PC, which worked fine, but on this week's video the live voices make up the whole video and are on both channels, out of polarity and cancel the entire video. The background music is not, however, so when I inverted one of the channels the voices came back but the music lost a lot of fullness (expected). I asked him to send me just the raw vocal audio for me to fix and then he could add it back to his video editor.

I don't know if it's something he's doing with the voice track or something with the SLR camera, but he did say that the first day of shooting had no problems, only the second day. I surmise that the first day only had vocals recorded on the left track, whereas the second day was on both tracks and somehow out of polarity...whether due to the camera goofing it up, a video editing error or something else did is still an unknown...so much fun ;)

Thanks,
Jeff
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Location: Baltimore, Maryland

Re: Channel Mono setting oddity

Postby BrentEvans » Wed Mar 16, 2016 2:35 pm

if he's using an external mic or preamp on the DSLR, it could be the cable going to the DLSR has one channel reversed polarity.
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