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So I have access to a Neumann RSM 191. I was planning on taking it out and seeing what I could capture, but realized that I have no way to decode the MS while recording.
See, I have just a MixPre and an M10 neither of which offers MS decoding or MS linking. So my question is, do I go for the flexibility of MS and just get by with panning Mid = Left, Side = Right and monitor that way, crossing my fingers on mic placement, or do I switch to XY and know that what I'm monitoring is the stereo image I'm recording?
You don't really need M/S decoding unless you feel you need to monitor in stereo. I'd just go for it, because you can adjust the stereo field to the width you like when you bring it into your DAW later. I did this when I was recording for the DOORS with Tim, and I was pretty happy with the results. I did all of my mic placement based on the Mid signal.
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Just my 2 cents, I prefer to use the 191 in XY (100 or 120 degrees). Unless it is a source that I'm not sure if I want to keep stereo. MS gives you the option of trashing the S later and go with mono, which I often do when I use MS. If I'm recording something that moves, I don't tend to use XY. I don't like having sounds moving around the stereo field, unless I mean them to. But for stereo purposes, I'm just happier with the XY vs the MS stuff I've recorded with that mic.