Mid/side Matrix (LR ▸ MS and MS ▸ LR)

1. Left Input Left channel of a stereo signal to be converted to mid/side
2. Right Input Right channel of a stereo signal to be converted to mid/side
3. Mid Output Mid component of the stereo signal
4. Side Output Side component of the stereo signal
5. Mid Input Mid component of a signal to be converted to stereo
6. Side Input Side component of a signal to be converted to stereo
7. Left Output Left channel of the stereo signal
8. Right Output Right channel of the stereo signal

Overview ⚓︎

This pair of mid/side matrix modules converts a stereo audio signal into its mid and side components (LR ▸ MS) and back to stereo (MS ▸ LR).

Normally, a LR ▸ MS module will come first in the processing chain. Then, the mid and side signals will be processed independently and will be recombined into a stereo signal by a MS ▸ LR module before going to the output.

This technique makes it possible to apply different effects to the mid and side components of the stereo signal.

In Depth ⚓︎

Mid/side processing comes from the mid/side recording technique where the mid signal is recorded with a cardioid microphone pointing at the source while the side signal is recorded with a figure-8 microphone pointed at 90° from the source.

This can be converted into left/right by summing the mid and side signals for the left channel and subtracting them for the right channel. This is what the MS ▸ LR module does.

Conversely, a left/right stereo signal can be converted to mid/side by summing the left and right channels for the mid, and subtracting them for the side. This is what the LR ▸ MS module does.

Pitfalls ⚓︎

Effect on Hard-panned Sounds ⚓︎

Since the mid signal is the sum of the left and right channels, it follows that a signal that is hard-panned to the left or right will appear in the mid output. This is counter-intuitive, as we would expect a hard-panned signal to only appear on the side output. Bear that in mind when applying effects to the mid signal: it will actually be applied to the whole sound, but it will be more prominent on elements that are centered in the stereo image.

Phase Considerations ⚓︎

Mid/side processing is very sensitive to phase. Applying any effect that has an effect on phase (like delays and filters) may cause phase cancellation and may have an impact on stereo imaging. Not to say that you cannot use a filter on the mid or side signal, but the outcome may not be exactly what you expect.