Firstly, I take it that you mean that a coincident XY at 90degrees with figure-of-eight mics is pretty much the same as MS with two figure of eight mics? If so, what, if any, are the differences?
As I've been working down the thread picking up on the various comments, I see others have beaten me to it (sorry -- I've been away and am catching up ) so sorry if I'm repeatting what others have said.
But... XY fig-8s and MS fig-8s appropriately decoded theoretically provide identical sound stages.
The differences are in the practicalities. In an MS array, anything at the centre of the sound stage is directly on-axis to the Mid mic. With an XY array, it is inherently well off-axis to both mics. Most mics rae designed to sound best when used on axis, and as you move progressively off-axis the frequency response normally becomes less than opttimal. So, where you have critical mid-stage sources (such as when recording a solo instrument), then the MS array might well sound crisper and more natural than an XY array.
The other aspect is that by adjustingng the level of the Side mic relative to the Mid mic, the effective stereo acceptance angle can be adjusted in a way which isn't possible with XY mics.
Secondly, when MS was first explained to me I remember that I was given a very definite reason why it was considered technically advantageous for the mid mic to be omni that seemed to make good sense at the time.
The only advantage is the better LF response... but the inherent disadvantage is the 360 degree acceptance angle which is great if you're wanting to record huge atmos soundscapes, but of little benefit in the majority of applications!
Unfortunately, I can’t remember the reasoning now! Can you think of any reason why I might have been told this?
I can... but I'm too polite to say
Using a cardioid means that the left and right extremities of the stereo are created by combining the beautifully flat response of the figure-of-eight with an incredibly dull response of the off-axis cardioid.
This is true-ish -- in that most cardioid pattern mics do sound incrasingly dull as you move around to the extremes of the pickup at +/-60 to 70 degrees. Large diaphragm mics are far worse at this than small diaphgram mics -- and nmot all mics are equal in how bad they sound off axis.
However, this is only going to be relevant if you are trying to capture accurate tonality 70 degrees either side of the centre axis... and if you are, there is an argument that says an XY array of cardioids would be a better and more pracical choice.
What’s more, when you collapse this picture to mono you lose all the definition from the sides – the image will be thin and unrealistic.
Er.. yes... obviously -- because you're using a cardioid mic that is deliberately designed to reject sound from the sides and rear.
The original idea behind MS was that, in mono, you still got the whole picture
Not true. The original MS was developed with fig-8s, inherently rejecting any sound sources from the cardinal sides! Doh!
but if you use a cardioid microphone you only hear the centre when you go to mono.”
True... obviously. I can't helpo but think this guy is either being misquoted, or has a fairly poor understanding of the technique and its strengths/weaknesses!
An omni Mid mic will, indeed, provide much better (although still not perfect) tonality for side sound sources if that's what you want. But it will also mean that the stereo acceptance angle is 360 degrees (the equivalent of back-back cardioids) which gives a very narrow image of the source and a hugely spacious sounding environment. Clearly, in a nice room that may well give a lovely effect for a solo guitar, but this is quite a specific application. it would be completely terrible for an orchestra, for example!
Technical Editor, Sound On Sound