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

M/S recording creates two mono channels not three. Might be good to read this:

http://www.uaudio.com/blog/mid-side-mic-recording/

There are ways to decode the M/S recording back to stereo. You can use three channels of a mixer as per the diagram in the above article. There are many plugins that decode it as well.

Acoustic guitar is a good candidate for M/S recording especially around the 12th fret. You will end up with a wider stereo recording after decoding and you can control the width of the stereo image as well after the event. Mono compatibility is also excellent. 

And if you don't like it you can always just use the M channel for the direct in front sound. 

What is good about M/S recording is the M channel is pointing directly at the source. With XY co incident and near coincident techniques neither microphone is actually pointing directly at the source.

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" -

You record 2 channels.  The mid and one of the side channels.  You pan the side channel hard in one direction (left for example) and then make a copy of it, pan the copy hard right and invert its phase.

You now have 3 mono channels, 2 recorded and 1 copy.

Adjust the level of these 3 channels to taste.  The 2 side channels will cancel in mono leaving you only the center channel.

A typical M/S set up has the mid recorded with a sdc (cardioid) pointed directly at the source and a side mic with a figure 8 p/u pattern turned 90 degrees to the source.
 
EDIT - in other words you have 3 mono "tracks".

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.  

The statement that there are two side channels is factually correct and so there is enough info on the table to both interpret the intent of Dave's statement and agree with the facts represented.


You use the one side channel that you actually record to produce the second side channel.

BTW the two side channels do not have to be sourced by a figure 8 mic but can also be sourced by two separate mics pointed to their respective sides.







To answer the OPs question:

The result of running the source files through the MS matrix decoder is to end up with a single "stereo" file that is also completely mono compatible.



Cakewalk's Channel Tools works OK but Voxengo's free MSED does it with a simpler GUI.

Dave has suggested a way to do it without any plugins.  

It's so simple... and any way you do it works real good.



best regards,
mike

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?

This all makes good sense. My experimentation is going well. Once the channels are in place and spread to taste - then comes tweaking the EQ of each to have one side highlight the guitar's highs, and the other lows. Otherwise it all ends up a little too flat - I might do as suggested with the mid-recording and leave the mids and pull down each end.

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'm just chiming in to say thanks! I have an RME Fireface which has a Mid Side setting on input mics. My guess is that this enables me to bring a figure 8 patterned mic input into a mono channel and process it on the fly, flipping the phase of one of the sides and providing a stereo panner so you can adjust the width and volume. This has been a great lesson. Thanks!! I can't wait to get home and try it!