--- In Scott Fraser
<> wrote:
> If you create the mono by simply removing the Side channel, this is
> more or less true,
It is *precisely true*, not "...more or less true". There is nothing
vague, ill-defined or "more or less" about it.
> although all the ambience in the original Mid
> channel still exists
It damn well better "still exists" or there is something very, very
seriously wrong!!!
> If mono is created by summing the decoded MS
> information the result will be identical to summed XY & will
contain
> all the side information collapsed to the center.
No, no, no, no, no, no, no... and NO!
Summing a stereo signal (that has been properly decoded from MS) to
mono is the same as "...simply removing the Side channel", except that
summing to mono also causes the M signal level to increase by 6dB. In
both cases, there should be *none* of the S mic's signal in the summed
mono signal. If there is, something is broken. The S signal in the
right channel is a polarity-inverted version of the same S signal in
the left channel, at precisely the same amplitude, and therefore
*must* cancel out when summed to mono.
There is only ever *one* S signal provided in and from the MS source.
When decoded to stereo it is panned hard left to add with the M signal
and create the L signal. A 'mirror image' of it is panned hard right
to subtract itself from the M signal and create the R signal when
reproduced in stereo, and to cancel itself out altogether when summed
to mono.
It's all here in the outrageously simple yet highly misunderstood
maths, which I'll repeat from an earlier reply:
L =3D M + S
R =3D M - S
Therefore:
Mono =3D L + R =3D (M + S) + (M - S) =3D 2M
Let's say M was an apple and S was an orange, and we're going to 'sum
to mono' by putting the whole lot into one fruit basket via the laws
of MS. Then:
Contents of fruit basket =3D (one apple plus one orange) + (one apple
minus one orange)
We have put two apples into our fruit basket. At the same time, we put
one orange in and then took one orange out.
Contents of fruit basket =3D 2 apples and (wait for it) NO ORANGES!
In contrast, if we sum an XY or coincident pair to mono we get:
L =3D L
R =3D R
Mono =3D L + R =3D LR
If L was an apple and R was an orange, our fruit basket would contain
one apple and one orange. NOT 2 apples!
So the result of MS stereo summed to mono won't "...be identical to
summed XY" and won't "...contain all the side information collapsed to
the center". That is the main difference between MS and XY. It is also
one of the reasons why some film/television audio people have moved
away from MS and towards XY (to bring us back to the catalyst behind
this messy fruit salad).
You can create just about any XY equivalent with MS by choice of M
capsule polar response and M:S ratio, but it will behave differently
to the XY equivalent when summed to mono because the S will always
cancel out completely. When it comes to summing to mono, MS and XY are
as different as apples and oranges.
Sigh. I need to find a new wall to bang my head against...
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