I am having an issue with Mid-Side recording that is really confusing me. I am using a Rode NTG3 as the mid, and a MKH30 as the side mic. I have been conducting tests using my voice and also a sine wave. The issue is that the stereo field is not properly translated after MS decoding. When I speak at 90 degrees off axis, this is translated after decoding as if my voice is more or less in the center. Whenever I speak at about 45 degrees off axis, the stereo positioning after decoding is correct. This issue happens on both left and right, so both 90 degrees L and R are positioned in the center.
I have done various tests in order to find the root of the problem. With regards to decoding, I have tried three different types: manual, using the freeware MSED plugin in my DAW, and using the MS decoder in my field recorder (Tascam DR680). The result is all the time the same.
I have also tried adjusting the width of the M/S decoding. The issue stays pretty much the same regardless of the width I set. The stereo field changes according to the width setting, but 90 degrees off axis is always centered.
I have done tests using my Rode NT1a and NT2a, with the NT2a set to figure of eight and used as the side mic. This configuration behaves pretty much normally. The wider I set the MS width, the more pronounced the 90 degrees sound is placed at either hard L or R.
I have done various adjustments of the recording gain of the NTG3 relative to the MKH30. As the NTG3 is more sensitive (31 mv/Pa) than the MKH30 (25mv/Pa), I'd assume that normally you would turn the gain of the NTG3 down a few dBs in order to get an even stereo field. Whatever adjustment I made to the gain, it never sounded like proper stereo in case of sounds coming at 90 degrees off axis.
I've also had briefly access to a Neumann 191, and in fact had exactly the same issue as with the NTG3/MKH30, so this confuses matters even more. Neither of the mics that I have used are having any issues of themselves, it's just that when doing this test I get these results. What I'm trying to say is that I'm pretty sure that none of the mics I am using is faulty.
I have also tried various cables, this is not the issue either. Neither is the issue with my recorder. I have used both my Tascam and my M-Audio soundcard, both with the same results. The MKH30 is mounted as close to the NTG3 as possible, and moving the position of the MKH30 did not help.
I have uploaded a recording to showcase the issue. For this test, I played back a sine wave (440 hz) at about a meter distance at both 90 and 45 degrees off axis and recorded it simultaneously with the NTG3/MKH40 setup and my Rode NT4 for reference.
I've added comments in the sound file to explain which is which. One can clearly hear that at 90 degrees, the MS setup captures the sine wave as if it's coming from the center, whereas at 45 degrees things are positioned correctly.
If anybody has any idea what is going on here, I would be very grateful because I am out of ideas how to solve this. I will be renting another MKH30 with MKH50 and MKH60 to conduct more tests. Perhaps the issue has to do with the Rode and Sennheiser not being properly matched, but looking at the specs of both mics, there is not too much of a difference - one would still expect these mics to behave normally in a MS setup, I would think?
-- EDIT: alt text http://daanhendriks.co.uk/wp-content/images/ms_setup.jpg This is how I had the mics positioned when I posted my question. After Ryans response, I have moved the MKH30 more backwards but the problem still persists.