Here is some info from the Sound Devices web site.
http://www.sounddevices.com/notes/general/ms-stereo-basics/
Which has this salient point "Because of the inherent mono
compatibility of MS stereo, when an MS stereo signal is folded to
mono, only the Mid microphone signal remains."
Phil
--- In "John Lundsten"
<> wrote:
>
> To clarify the narrator/forest example I gave earlier, let's say the
> narrator(or whatever subject of interest - a close-up frog at the
> edge of a pond full of croaking frogs in a field of chirring
> crickets?) is 12 inches in front of the M mic, and the S mic is
> capturing the forest sounds to provide the ambience to go with it.
>
> JL} Ok that is NOT a MS recording & is a great example of " a very
odd
> recording with the forest ambience in anti-phase". [and yes I do
mean phase
> not polarity]
>
> Let me explain, proper MS is a coincident technique where the main
chan "M"
> HAS ALL THE SOUND elements IE just like mono.
> The "S" chan contains difference information. It is basic / central
& vital
> any sound in the S is also to be found in the M.
> It is perfectly possible to have a sound in the M only - this will
be the
> case for a sound source placed dead centre - this is correct
because there
> is NO difference between L&R for a centrally placed/panned sound.
>
> As a source goes from the median plain to either L or R there will
be
> increasing amounts appearing in the S.
> Any info contained only in the S will cancel if mono'd, this is not
a fault
> width info is meant to disappear in mono it does using ANY Mic or
mixing
> technique it's just that in MS one has (1) a Totality (L+R) & (2) a
stereo
> placement / difference signal (L-R).
>
> There is an S component in any stereo recording that will disappear
in Mono.
>
>
> If your definition of 'mono compatible' means it sums to stereo with
> no comb filtering problems, then MS does this very well, as does XY
> (discounting minor phase differences between channels due to the
> inability to put both capsules in exactly the same physical point in
> space,
> JL} Agreed
>
> a problem suffered equally by *all* coincident stereo
> techniques *including* MS but with the possible exception of the
> Soundfield).
> JL} Not quite right, the mono compatibility of MS is totally down
to the
> ability to match the S & invS signal so they cancel completely in
mono. With
> XY the mono will be the sum of 2 off axis mics (in general the
results are
> ok in practice IMO)
>
> But if your definition of, or requirement for, 'mono compatible'
> means the mono version of the signal retains all the information
> contained in the stereo signal, MS fails by its very design while XY
> succeeds very well.
> JL} No, they are both ways of conveying essentially the same
information
> L+R =3D M
> L-R =3D S which if added give
> 2L
> And if added with a polarity Inv on S give
> 2R
>
> But to re-emphasise if using MS mics
> The L's & R's from the 2 mics must have no appreciable phase shift
from one
> another (only 0 &180deg polarity shift of the S re M). The degree
to which
> they do deviate from perfect coincidence will create out of phase
info if
> heard in stereo. So long as the mics are tolerably coincident this
is
> generally just fine. Move the mics some distance apart as in your
example
> and you will get an interesting stereo like effect with no
localisation at
> all (a bit like extreme AB spaced)
>
> You make a good point about the centre image. An XY pair means
> sounds in the centre are captured off-axis by two microphones,
> rather than directly on-axis to one microphone. Unless the two mics
> in XY have very good off-axis response then the centre image won't
> be as solid as it will with MS. So if you need a strong centre
> image, use MS. If the side information is more important, use XY.
> JL} Yes XY is often favoured for Orchestral recordings I suspect
going along
> with the prejudice that the strings placed L&R and therefore on
axis to the
> 2 mics have greater priority/ importance than the horns percussion
& stuff
> in the centre.
>
> For the record, I am a huge fan of MS with a cardioid M capsule; it
> is my primary microphone rig, I use it all the time and enjoy it
> very much.
>
> I have a Schoeps MS pair, which is a wonderful implementation of the
> MS technique and produces a fantastic stereo image when decoded
> properly. But from time to time the problem described above hits me.
> Most recently, I used my MS pair to record a direct-to-stereo album
> for an acoustic folk ensemble. For one of the more complex pieces
> with lots of performers and instruments, I had to place the
> musicians in a reasonably wide arc around the microphone, resulting
> in a stereo image that extended from hard left to hard right. I knew
> the balance and overall sound would change when summed to mono, but
> I thought "who's going to play this in mono these days?"
>
> Well... a short time after the album was released, one of the
> musicians called me complaining that it sounded very wrong when
> played on Radio National (an Australian broadcaster who apparently
> transmits in mono). After telephoning the station to enquire why the
> album sounded so wrong, he was told that the recording had poor mono
> compatibility because, when collapsed to mono, the instruments on
> the far sides of the recording dropped considerably in level (6dB or
> so relative to sounds in the centre) as did the reverberation,
> changing the entire balance and sound.
>
> Fortunately for me, the same station awarded it 'Album of the Week',
> which probably saved my butt from the financially-compensating wrath
> of the musicians!!!
>
> JL} Me thinks you may have overdone the amount of S in your de-
code, it can
> be very tempting. In essence you have got some out of phase stuff
in your
> mix, which can sound fine but means that it goes against the MS
idea that
> all sounds are in the M with the S steering those sounds.
>
> As you crank up the S the effective polar pattern of your simulated
mics not
> only change angle but also polar pattern do it a lot and you get
back to
> back Fig 8's. If you had a backwards facing cariod IE double MS you
can
> using Ambisonic like techniques have independent control of polar
pattern &
> mic angle see
> http://www.schoeps.de/dmsplugin.html
> for a great free VST plugin
> John L
>
>
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