Mid-Side recording question

When I do a mid-side recording of an acoustic guitar...should the resulting 3 channels be mono or stereo?

Ok read that article and maybe missing something still --- here is a quote from that article: "Now you've got three channels of recorded audio– the Mid center channel and two Side channels" -

I have quoted from the article, note the bold text. The three channels refers to using 3 channels of a mixer (internal or external) in order to decode. I think you will find in Sonar there is a plugin which does the decoding and you wont have to do this.  
  
The signal from each microphone is then recorded to its own track. However, to hear a proper stereo image when listening to the recording, the tracks need to be matrixed and decoded. 

Although you have recorded only two channels of audio (the Mid and Side), the next step is to split the Side signal into two separate channels. This can be done either in your DAW software or hardware mixer by bringing the Side signal up on two channels and reversing the phase of one of them. Pan one side hard left, the other hard right. The resulting two channels represent exactly what both sides of your figure-8 Side mic were hearing.
Now you've got three channels of recorded audio– the Mid center channel and two Side channels – which must be balanced to recreate a stereo image. (Here's where it gets a little confusing, so hang on tight.) MS decoding works by what's called a "sum and difference matrix," adding one of the Side signals—the plus (+) side—to the Mid signal for the sum, and then subtracting the other Side signal—the minus (-) side—from the Mid signal for the difference.

The two recording mics only generate two mono signals, M and S.  

what Mike said!

Put another way... when you make a mid-side recording you end up with one channel that is the sum (M+S) or mono signal, and a second channel that contains the difference (M-S) that can be used to create Left and Right channels.


This happen no matter the microphone configuration. My favorites are a cardiod facing front and a bi-directional facing sideways, but if the room is really good I'll use bi-directional microphones for both positions. You can also use three discrete microphones, but that's a bit trickier<G>!

The result is a stereo image that can be tweaked for apparent width.

Does that make any sense?

It is such a versatile technique and it can work not only for a single performer but a group of performers as well. ie single acoustic guitarist especially in a slightly ambient space. 

It is also great for a small group such as a string quartet. You can spread them in an arc eg 180 degree setup and put the M/S setup at the centre facing the two inner musicians. What is good about it is the S mic is picking up the two extreme players directly on both sides of the figure 8 mic so it covers all 4 players so well.

Putting the M Mic into omni can give a nice result too if you want a little more of the room and that will ensure the M pattern is very even too.

You can process the M and S signals differently and you will effect either the sound that is more centre or on the extremes of the stereo image depending on what you do to each signal in the decoding process. eg you could brighten the M Mic in an acoustic gtr recording and apply LPF to the S signal. What you will hear is the guitar sound will be the brightest in the centre after decoding and duller on the extremes of the stereo image after decoding. (or visa versa)

I have recorded a lot of foley and atmos effects using MS techniques. The ability to control how much you hear directly in front and the extremes of the stereo allows a lot of control over the effects later in post. 

You can always alter the M S decoding levels slightly to alter stereo width.

In mastering it can be handy too as a final stereo mix can be converted into M and S signals and then you can process the two differently and convert back to stereo again. This allows you to do things like EQ a slightly loud vocal that is panned centre leaving the stuff on the extremes of the stereo image alone. Or compressing the extremes of the stereo image a little more leaving the centre stuff alone. I have used it in mastering to reduce the amount of room reverb of a solo piano recording that was done using XY setup and the client thought there was no way to change it. Sure we lost a little stereo imaging but we ended up with a much drier and direct result.

I updated to the latest firmware on my Tascam DR-680 last week and the dsp mixer on it now has a M/S matrix built in just like your RME.

I thought that was a cool update.


:-)

So, can you use mid side recording of a guitar while also having instruments playing to the left and right of the performer? I'm thinking yes, but also thinking the m/s mic will pickup those other performers.