III. Phase Correction

You may notice a drop in high-end response and/or a dead or phasy tone when you first combine the cabs. The dual cab combination you chose is likely experiencing a comb filter. Anytime you record a single audio source with two mics that are at different distances, there will be negative interference occurring at regular intervals throughout the frequency spectrum. In short, this means the tone has dead spots. This is due to a slight delay in one of the signals. For certain frequencies, the two signals perfectly cancel out. Others are just out-of-phase to various degrees, while some perfectly reinforce each other.

Higher-pitched frequencies get the worst of it. Because they have a shorter wavelength, even a slight delay can cause perfect cancellation. This is probably what you notice. The tone sounds like a blanket was thrown over the amp. Even though the Pod is using different cabs, the signals aren't as different as they sound. Even the dark cab has some treble in there, and it can wash out the bright cab's high-end. Even if you use the same mic on both cabs, it doesn't mean the signal isn't delayed more in one than the other. This could be a result of how the cabs themselves behave, differences in the distances Line 6 used to mic them, or differences in how the DSP processing implements them.

I have discovered a method to phase correct the two cabs. Some effects can be set to be transparent to the signal; however, they do require processing time, which introduces a very slight delay to the signal. The most common effects I use are EQ's and Compressors. By using a combination of such effects, we can acheive phase correction. The difference is night and day.

Author's note: the information below gets fairly technical and might be more work than necessary. The easier way to do this is to simply throw a parametric EQ with default settings behind one of the amps before the mixer. Toggle it on/off and see if the tone improves with it on. If not, do the same test on Channel B. If the tone sounds muffled with no EQ, EQ on Channel A, and EQ on Channel B, or if it just sounds "off" and "phasy", give up on the combination you've picked - you'll probably need to use 2 or more effects to phase correct the cab/mic choices, which starts to become a hindrance. I list combinations below and in Appendix C, this spreadsheet/alternate Google Cloud version/pdf version BROKEN LINK that work well using 0 or 1 EQ's. This setlist BROKEN LINK CANNOT DL???also contains examples.

I have done some pretty thorough research on this, and am continuing to update it. The link below is a spreadsheet with a matrix of every possible combination of cab/mic with another, for the Tread V-30, Hiway, XXL, Greenbacks, Uber, Brit, and Blackbacks 4x12 cabs with all available mics with the delay required to make them in-phase represented in a number of samples assuming a 96 kHZ sample rate. It contains tabs on how to use it, a list of relatively tone-transparent effects with the amount of delay they introduce, and the combination of effects necessary to get any particular amount of delay.