Mixing matrix 2: Phase inversion

Mixing matrix article described how matrix mixing works and some channel swapping magic. This article focuses on phase inverting. It describes what is phase inversion, how to test and fix incorrect phase and how to make pseudo-stereo effect from mono track.

Phase inversion

Phase inversion is simply flipping signal upside down:

That's all. Simple, isn't it? Now it's time to add some mess...

Imagine that 2 speakers reproduce the same sound. In this case sound should be heard from the center between speakers.

But what if one of speakers has plus and minus wires swapped?

As you can see, ears cannot locate the sound. Obviously, it is very bad for sound reproduction because it completely breaks original stereo picture.

Another negative effect of phase inversion is signal cancellation. When two signals with opposite phase meet they cancel each other:

Simply, this happens because two waves push air molecules in opposite direction, thus producing no sound at all. (Of course, in fact it is much more complex process) This effect is most remarkable at low frequencies and almost unnoticeable at high frequencies, thus total effect is like turning basses off...

How to check

Well, inverted phase is bad. How to ensure that we have our speakers connected right? This can be done with a simple stereo test file (similar test is widely used on test disks).

This file contains two sounds: one comes from the center (in-phase) and other from surround (inverted phase). If you hear surround message from the center, your speakers are connected incorrectly.

Similar multi-channel test file for multi-channel system. It consists of several tests of speaker pairs: (1) left and right, (2) left and left surround, (3) right and right surround. If all tests passed your speakers connected right. If some tests failed, you can determine which channels connected wrong with following table:

How to fix wiring

The best way is to re-wire your speakers. But if it is hard to do, matrixing is possible!

First of all, determine speakers with incorrect phase (see above). For each inverted channel channel change the sign of each element of the matrix row for that channel.

Example: we have a quadro setup and test result is following: -++ (right & right surround inverted). By default (with Auto matrix enabled) we have following matrix:

There are two rows marked, for R and SR output channels which we need to invert. After matrix correction it will look like:

That's all. Now you can do the test again to ensure that everything is ok.

How to fix audio track

Sometimes audio track in a movie may have incorrect phase. If you have listen the test file above you can hear this simply. And sometimes it may be very annoying. The only way to fix it is matrixing!

Usually files with incorrect phase are stereo, that simplifies our job: we need to invert only one input channel, either left or right (it does not matter because only phase difference is heard).

Example. We have quadro setup and copy front channels to rear channels:

To fix phase of input track we need to invert one input channel. Here we will invert right channel. Unlike everything we have done so far, now we need to work with columns (not rows!):

Right input goes to right and right surround channels. Now both are inverted, that is exactly what we need.

Pseudo-stereo from mono

Is phase inverting so bad? Generally, yes... But sometimes it can make a good result!

As was said before, phase inversion takes sound out of center. We can use this to make simple pseudo-stereo effect on mono tracks. Extremely simple matrix, that makes mono track to spread around:

If you have more than 2 speakers you may play with different signs for channels.