Use a mono room mic
The Decca Tree technique uses a spaced stereo pair and centrally-panned mono room mic. The trick is to get the stereo pair in phase with the mono mic.1 I might be able to fake a mono mic by summing the left and right to one channel after processing them with phase correction.
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How to mitigate stereo recording issues
How to mitigate stereo recording issues [[Position spaced-stereo mics equidistant from the important sound source]] [[Use a mono room mic]]...