This kind of phase cancelation can happen a couple of ways. Many mixers have a phase reversal switch. If this switch is activated on ONE SIDE of a stereo mixer, then the two channels will cancel when mixed mono. It’s important to realize that even though the audio sounds good playing back as stereo, if it’s played in mono the cancelation will occur. This will usually result in the QC dept. rejecting the program.
Another cause of phase reversal is miswired connectors (mic or line level). The more-or-less standard wiring of an XLR (TRS) is pin 1 shield (sleeve), pin 2 hot (tip), and pin 3 cold (ring). If pins 2 & 3 (T & R) are reversed you get a phase reversal compared to a properly wired connector. You can use multiple cables together without phase reversal as long as they’re wired properly. While Rafael is correct that different length cables will cause phase differences, you’d have to have a tremendous difference in length to cause a reversal.
Other causes of phase differences (not necessarily cancelation, but resulting in a “hollow” sound) are using two mics at different distances from talent, or reflections from a hard surface like a table or desk. One test every soundperson should make is to listen to mono playback. If cancellation is occuring that’s the way to catch it.
John
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