The math really is simple.
M =3D L + R (Mid or Sum channel)
S =3D L - R (Side, or Difference channel)
(L + R ) + (L - R) =3D 2L
(L + R) - (L - R) =3D 2R
that's where the 6 db is coming from.
umashankar
----- Original Message ----
From: maxfrick78 <>
To:
Sent: Wednesday, March 14, 2007 5:34:19 AM
Subject: [Nature Recordists] Re: X-Y -> M-S & Perfect Reversals
Rob, Lou, Walter & others,
Good point about the latency, Lou.
Sorry for causing some confusion about the +6dB gain. Let me try to make it=
more clear:
What I meant was that actually an MS DECODER is exactly the same as an MS E=
NCODER
(except for the 6dB). That's because of the math behind the MS-technique (a=
ny
programmers out there who can explain?)
That means that you don't even have to download Tom Erbe's free +matrix (wh=
ich is great
for sure, I LOVE SOUNDHACK!!) if you own Logic, for example. In Logic there=
's the
"direction mixer", a little plug that has an MS-decoder built in (but there=
's no MS ENcoder,
and that's no problem as you will see..).
Lets say you have an MS-ambience. You put that on a stereo track in Logic, =
insert the
DirectionMixer, set its input to MS and voila, you have your normal LR-ster=
eo-ambience.
But the funny thing is: You can go back to your initial MS-encoded signal b=
y inserting
another DirectionMixer right after the first one, setting the input to MS, =
too (although that
feels wrong) and boosting the output by 6dB! (Because using an MS-decoder "=
in reverse"
will result in a signal that is 6 dB too low; so MS-DEcoder + 6dB =3D MS-EN=
coder).
If you compare the initial MS recording to the decoded+decoded+6dB signal, =
they will be
exactly the same. In Logic the difference between them was somewhere around=
60dB
below peak level (due to rounding precesses in computing I guess), just lik=
e Rob described
in his test with +matrix).
If you have an LR-stereo recording that is MS-compatible (e.g. recorded in =
MS..) then you
can use the same setup to go from LR to MS to LR, so you could adjust the s=
tereo-width in
the MS-phase of this setup.
But also the DirectionMixer does exactly that if you set its input to LR, s=
o if all you want is
adjusting the stereo-width of a LR-stereo-file use the "Base" slider in the=
plug, all it does
should be adjusting the relative volumes of Mid and Side..
Hope it's clear now...
greets,
Max
--- In Lou Judson <> wrote:
>
>
> On Mar 13, 2007, at 9:02 AM, Walter Knapp wrote:
>
> > You should be adjusting the gain on your recorder to optimum for each
> > channel when you record, which would adjust for the extra output of the
> > MKH80. Don't gang the gain of the two channels in M/S. Set each
> > independently and correct according to your headroom preferences. This
> > will give the optimum recordings to work with.
> >
> > Note that this usually results in a side that if mixed without gain
> > adjust is too strong and tries to push the stereo wider than it wants
> > to
> > go. So, during decode you also adjust gain separately.
>
> This is debatable. Until I got the SD 744 I always set the two channels
> for natural sound in L/R, with the side set maybe 6dB down. Then it
> approximates the final sound I wanted, with the sides remaining
> adjustable on post or playback. I did my first (concert, not nature) MS
> recording recently with the gains set equal (ganged), and it is
> unlistenable without adjusting the gains on playback. But the sources
> are well recorded... with 24 bit recording there is no need to boost
> the side on the recording, there is plenty of both headroom and SNR or
> bit use.
>
> Caveat is that I do a lot more music recording than nature, but still
> my approach works for both.
>
> On another hand, adjusting the levels for appropriate soundstage width,
> then reversing it to MS, will result in a corrected level that can be
> played back in MS with decoding without the need to adjust the side
> levels!
>
> Side note - the SD recorders do not allow the ajustment of the relative
> levels in the headphone matrix, so the only monitoring you can do is
> the mic recording levels. In the future I wll lower the side mic by 6dB
> for a natural monitoring sound. On the SD 7xx series, you can adjust
> the relative level and still gang (link) the two channels, a big plus!
>
> I don't think there is a hard rule on this, YMMV as you wish it to. LR
> will reverse to perfect MS in any case, it just won't match the raw
> tracks.
>
> One more thing missing so far in this discussion - when working in the
> computer with MS decoding, to keep perfect phase relationships between
> the two channels, I find it neccessary to put the exact same plugs on
> both channels, even of one is set to bypass, so any latency does not
> throw the perspective off. I learned this from experience (as well as
> theory) when taking live recordings done in LR or XY and splitting them
> into MS so I can manipulate the center separately from the stereo image
> (for example, if a music recording has the centered vocal too loud and
> I want to tone it down or compress it without changing the stereo
> field, I have to put the same compressor and/or EQ on both tracks even
> if the side one is bypassed. Without the same latency on both, the
> stereo perspective changed unpleasantly!). If your chosen software
> automatically compensates for latency, then this may not be necessary.
>
> <L>
>
> Lou Judson =95 Intuitive Audio
> 415-883-2689
>
"Microphones are not ears,
Loudspeakers are not birds,
A listening room is not nature."
Klas Strandberg
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