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. Wit=
h
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 alon=
g
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 no=
t
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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