Weird Stereo Audio Phasing incident

[Steve House]

The problem is very common when adapter cables are used. An 'off-the-shelf' XLR->TRS cable is normally intended to be used to connect a balanced, mono XLR to a balanced, mono TRS. As such it's wired XLR pin 2 (signal +) to TRS tip, XLR pin 3 (signal -) to TRS ring, and XLR pin1 (ground) to TRS sleeve. As pointed out earlier, when you use such a cable to connect an XLR microphone to a TRS stereo external mic input, you end up with the positive phase signal on the left channel and the 180-degree out-of-phase minus signal on the right. It will sound 'phasey; as described above, when played in stereo on speakers or headphones, but if left and right are mixed down to mono at any point in the chain they will cancel. The proper way to connect a mono XLR mic (or line source) to a stereo TRS input on a recorder or camera is to wire the cable XLR pin 2 to TRS tip AND ring shorted together, XLR pin 3 to TRS sleeve, XLR pin 1 to cable shield which is left floating at the TRS end (OR XLR pin 3 short to pin 1, pin 1 to cable shield to TRS sleeve). This puts identical in-phase signals on both left and right channels which will mix to mono without problems.

[Greg Miller]

I agree with Steve's info about pinouts, but with one caveat.

If the unbalanced mic input supplies DC "plug in power" (which will be on both tip and ring), then the DC will appear on XLR pin 2. XLR pin 3 will be grounded (at the TRS jack). So the "plug in power" will be connected across the mic terminals, and some DC current will flow through the mic's audio circuit. Depending on the mic, this audio circuit might be one of three possibilities. It might be the secondary of an audio transformer, in which case you probably won't damage the mic; however, the current might cause magnetic saturation of the transformer core which will affect the frequency response. Or it might be a dynamic mic element itself, in which case you will have the issue of diaphragm displacement (which will affect the audio) and you might cause some permanent damage. Or it might be the active output of the mic's electronics (in the case of a battery-powered condenser mic) in which case the DC might possibly cause some damage or malfunction, depending on the specific circuit design.

So if you use an adapter wired as Steve described, be sure the camera's "plug in power" is turned off.

Otherwise, you need an adapter with an added DC blocking capacitor.

The other option, of course, is to continue using the "wrong" adapter, but just invert one of the tracks, and then add the two tracks to get one good mono track, before you do anything else in post.

[Brian P. Reynolds]

Originally Posted by Greg Miller

The other option, of course, is to continue using the "wrong" adapter, but just invert one of the tracks, and then add the two tracks to get one good mono track, before you do anything else in post.

Many video editing programs are incredibly difficult to do audio stuff on, some will not allow to Mono tracks let alone phase reverse. Its better to get the recording correct at the start rather than trying to fix it later.