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Phasey sound with singer/guitarist

I'm recording a singing guitarist into my computer. I have a CM-200A on the vocal and a CM-700 on the guitar. The singer's voice sounds okay when I monitor just the vocal mic, but it sounds filtered or phasey when I mix in the guitar mic. What's going on?
Duane Livnel

Reply: The guitar mic is picking up some vocal leakage with a short delay. This causes phase interference (comb filtering or flanging) when the vocal mic and guitar mic are mixed to the same channel.

To get the vocal track in phase with its leakage on the guitar track, I've found this helpful:

1. Mix the two tracks at equal vocal levels (that is turn up the guitar track until the vocal leakage is as loud as the vocal track).

2. Reverse the polarity of the vocal track.

3. Nudge-delay the vocal track (slide it to the right) by less than a millisecond at a time until the vocal cancels out as much as possible. A 1 millisecond delay is typical.

4. Put the vocal track back in normal polarity. Set the guitar-track fader for a good mix. You should hear the vocal/guitar mix without much phase interference.

Other ways to reduce phase interference are:

* Mike closer and use EQ to compensate for the tonal balance you get with close miking.

*Use a pair of bidirectional mics touching side-by-side. Aim the front of one mic at the mouth, and aim the front of the other mic at the guitar. Bidirectional mics have deep nulls of reduced pickup at 90 degrees off-axis, and you can use that to achieve good isolation between the voice and guitar.

*Record the guitar first, then overdub the vocal if the performer is comfortable doing that.

*record the singer and guitar with one mic midway between the mouth and guitar. Adjust the mic height up or down to get a good balance between voice and guitar.

*Record the singer and guitar at the same time. Play the mix for the performer, and have them overdub just a guitar part, then overdub just a vocal. Delete the original vocal/guitar tracks.

* Mike the vocal with aCrown CM-200A aiming up at your mouth about 2 inches away. Be sure to put the foam windscreen on the mic to reduce breath pops. If this sounds too bassy, roll off the excess bass with your mixer (if you're using one).
Mike the acoustic guitar with a Crown CM-700 aiming down at the spot midway between the sound hole and the point where the fingerboard joins the guitar body. If the guitar sounds too bassy, set the bass-tilt switch on the CM-700 to reduce the bass. Also try the CM-700 aiming straight toward the guitar in the same position, but a little lower down about 3 inches away.

* Another option for the acoustic guitar is to use a good pickup, or use a Crown GLM-100 miniature omni mic taped near the sound hole. Turn down the excess bass with your mixer if you use this method.

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