A Closer Look At Mid/Side Recording

A number of stereo recording techniques have been developed over the years including Blumlein, ORTF, spaced pair, and coincident pair, of which X–Y and M–S are a subset. They each have their advantages, depending on the application. Let's look at what the M–S method has to offer.

M–S (Mid–Side) requires the use of two microphones, one with a cardioid pickup and one with a fi gure–8 (bidirectional) pattern. The cardioid microphone (mid) faces straight forward toward the center of the sound source, while the fi gure–8 (side) microphone's two elements face sideways, perpendicular to the mid microphone. It is important that the null of the figure 8 mic aligns with the zero degree line of the cardioid.

Some microphone manufacturers make stereo mics designed specifi cally for M–S recording, with all three elements properly aligned in one housing. There's nothing wrong with using two separate microphones, however, as long as the capsules are as coincident (occupying the same space) as possible.

The M–S technique requires a special decoder that produces a left and right stereo signal. The left channel is the sum of the mid and side microphones (M+S), and the right channel is the difference between the mid and side microphones (M–S).

The Onyx 800R has a decoder built–in, which is activated by pushing in the MID/SIDE DECODE button on the rear panel. Connect the MID microphone to channel 1 and the SIDE microphone to channel 2. The signals pass through the decoder just prior to the A/D converters and the analog line outputs.

Stereo recording reproduces the instruments in the same relative locations within the stereo image as they were in the original performance. The side to side stereo image and, to some extent, the front to back depth of image are preserved.

The M–S stereo recording technique allows you to adjust the width of the stereo image by varying the ratio between the mid and side signals. With the 800R, this is accomplished by adjusting the gain controls on channels 1 and 2.

Notice that the adjustments are made prior to the decoder. Adjusting the ratio of the left and right signals after the decoder will not have the same effect. If you want to adjust the stereo image later during mixdown, record the raw MID and SIDE signals to tape with the decoder turned off. Then, when you are doing the final mix, run the recorded MID and SIDE signals through channels 1 and 2 with the decoder turned on and experiment with the gain controls on channels 1 and 2 to adjust the stereo image to your liking.

Another benefi t of this recording technique is that the stereo recording is mono compatible. This is especially important for broadcast and film production applications.