M-S vs. X-Y for field use

[Gary Nattrass]

I also had an ABW (AB wide) on the AMS Neve logic and DFC consoles I used for many years and you could change the width of any XY or M/S signal to suit when mixing in Pro Logic but as others will know all too well you have to be careful or the compliance police will reject the content for being out of phase and I have had many an argument with some graduate who looks at a chromatec phase meter rather than doing a proper mono check!

One stereo trainer from ravensbourne (Neil Papworth?) once summed up phase meters wonderfully by saying that they wave at me and I wave back at them! ;0)

I would never record M/S though as there are too many problems and people who do not know what to do with it and if you lose the left M channel you are pretty much stuffed, I also knew an editor who did a whole conform with just the S channel and wondered why it all sounded a bit distant but I have used those sony prosumer M/S mic's the 907 and 957 for many years to record stereo sound effects for film and TV.

[Ty Ford]

Originally Posted by Steven Reid

Audio guru Ty Ford, who chimes in here from time to time, once wrote an exemplary review of this mic; it nudged me toward a purchase. I can't seem to find a link at the moment.

Hope this helps.

Hello Steven and thanks for the hat tip.

The BP4025 is the upgrade. I may have moved the audio file to a new location on my server.

It's now here: https://www.dropbox.com/s/7f0qg6se5b...4025ambi08.wav


BTW, as it was explained to me, the beauty of the Mid/Side method is that, during production, you can never know when you'll use the wide or closeup shot. Their thought was that being able to vary the stereoscape in post production would be of value. I haven't done any experiments with that to really get a feel of how it might affect the shot. Maybe it's very valid, maybe it's a "because we can" situation.

I was told some years ago that in France, shooting dialog in Mid/Side was popular for similar reasons. I don't have any data on that either, but shooting dialog in Mid/Side sounds like a lot of precarious work if you're doing it with the thought of using the wider settings during a mix.

Regards,

Ty Ford

[Ty Ford]

And, after digging around a bit, I found the original review.

Ty Ford Audio and Video: Audio-Technica BP4025 Stereo Mic - In A Field Of Its Own

Regards,

Ty Ford

[John Willett]

Originally Posted by Brian P. Reynolds

Seth.... have you ever broadcast MS mics LIVE to air?

I did a golf tournament in the early days of stereo TV using MS mics, to match some of the wide pretty shots we set the stereo mics to wide (nothing tricky JUST the 'wide' setting on the mics).

It went like this.... program open music xfade to stereo FX (wide MS mics) for 10 seconds to established the location, cue commentary..... BUT what occurred at ALL the transmission station(s) across the country was the auto phase correction detected an out of phase signal so it corrected it and phase inverted R channel to bring it into 'what it assumed as correct phase', and when the commentary started the mono commentary was completely canceled out.......... YEP thats right NO commentary across the country..... and ALL caused by some MS mics set to the wide setting on the mic.

So don't tell me there is NO problems with MS mics !!!!!!!!!!!!!!!!!!!!!!!

Since that incident many years ago I now use XY set VERY wide almost to a point of having a dip in the centre of the audio image, the mono commentary and spot FX sits in there nicely giving a VERY wide spatial sound image that suits wide screen TV fantastically.
This XY stereo technique is what I have used for the past 25+ years in live to air sporting Outside Broadcasts.

And thats why I have ALWAYS said that EVERYbody that has some ability to change the sound of a mix MUST be aware of how MS works and how it can be used and more to the point how it can destroy a Broadcast....
If you ever do a movie or music record using wide MS how do you know what will occur to the material you have created many years down the track...Could you be creating future problems for others?

This does not sound like an MS problem per say, but it sounds like that the raw MS was transmitted instead of being matrixed to left/right stereo.

This would be a finger problem at source by someone not understanding how MS should be handled.

[Brian P. Reynolds]

The mic used was NOT in an unmatrixed form, the Sony ECM 969 does NOT have that option...
I have demonstrated the effect on many MS mics, Sony MS (various models) Shure VP88, Audio Technica, AKG etc to other sound professionals from film / TV broadcasting. They were unaware of the potential problems of wide MS.....They are now!!!

While I have to agree with others it's not the direct fault of a MS setup.... YES it's an operator fault BUT if the settings appear on the MS mics people will use that setting.

MS technique gets discussed frequently on forums by people that have little knowledge on the subject and these are the exact people will obtain a MS mic set it to wide thinking it will be OK...... well under some (I admit very rare and very specific) situations the MS technique can cause problems, this needs to be kept in mind because some people on various forums promote the MS technique as 'the perfect STEREO' setup.

In an unmatrixed form the potential of problems has vastly increased if it's used by people with minimal knowledge on the subject of MS, this is why I have always said that ALL people in the production process that use MS should be FULLY aware of how its done / used / and its pitfalls.

[Peter Rummel]

I have a HDX-900 with an AT835ST as an on-board mic. I keep it in M-S mode, so when I switch a channel to use a wireless mic, I still have the M channel as the camera mic. If I had the 835 in X-Y then a single channel would be listening off to one side. I agree it's very important to make sure the editor knows that I've recorded M-S, and knows how to deal with it.

[Jim Andrada]

By the way, there are any number of plugins that will translate stereo to M-S and allow spread adjustment and translate back to stereo.

[Rick Reineke]

Giving a video editor M-S tracks to 'mix' is a disaster waiting to happen. An experienced audio post engineer is another story.

[Gary Nattrass]

Originally Posted by Rick Reineke

Giving a video editor M-S tracks to 'mix' is a disaster waiting to happen. An experienced audio post engineer is another story.

Amen to that and I think I said before I had one editor do a whole show with just the S track and wondered why it sounded very distant, it is always why I never ever record in M/S!

[Steve House]

It should be mentioned that many better field mixers allow one to use M/S micing with the matrixing to L/R stereo done in the mixer for output.

[Ty Ford]

Brian's comment about the golf match is interesting.

Brian, am I understanding you correctly, they went to air with raw M/S?

Regards,

Ty

[Gary Nattrass]

Ty I think Brian meant that an M/S source mic was matrixed very wide so the S content was greater than the M so it would look out of phase as a stereo signal.

I have had this happen a lot when mixing pro logic and using the AMS Neve AB-wide control to increase the S content but it then makes a phase meter on a chromatec go negative.

I have never heard of automatic phase flippers though and all I ever got was a phone call from an inexperience QA person who didn't understand phase meters or how pro logic mixes can cause this and once I tell them to check the mono mix all has been OK.

[Ray Turcotte]

Great discussion guys!

For Seth & Gary: I have a Sony ecm-680s Shotgun stereo mic in which the capsules are in m+s mode. However there is a chip in the mic that switches the signal to L+R stereo encoding for input to the camera. What can be done in post to get the M+S signal back?


Thanks in advance

[Seth Bloombaum]

Originally Posted by Ray Turcotte

... However there is a chip in the mic that switches the signal to L+R stereo encoding for input to the camera. What can be done in post to get the M+S signal back?

Originally Posted by Jim Andrada

By the way, there are any number of plugins that will translate stereo to M-S and allow spread adjustment and translate back to stereo.

I'd be interested in Jim's recc for a plugin, but here's how I did it.

Matrixing M-S to Stereo looks like this:
L = M+S
R = M-S

Think about how you'd set that up on a timeline: A single M track panned center, an S track panned hard left, and an inverted S track panned hard right. Group the two S tracks, and adjust the gain between this group and the M track to find the spread and image you want.

So, remembering algebra, if we use the math to de-matrix from Stereo to M-S, it looks like this:
M = (L+R)/2
S = (L-R)/2

Have I got that math right? If so, to set up on a timeline, you'd split your L+R file into two tracks, the method differs between NLEs/DAWs. Pan each track center and render; that's your M clip. Now, invert the phase of the right track and render, that's the S clip.

(We've ignored the "divide by 2" step in deriving our M & S, but do make sure you're not overmodulating the renders, you might need to reduce master gain before rendering.)

You've dematrixed, now start a new project and set it up as above, L=M+S, R=M-S and dial your spread!

With some judicious grouping, you can do it all on one timeline, the dematrix and new matrix back, but that gets needlessly complex, too easy to make a mistake, I recommend rendering as above.

Why do it the easy way with a plugin? This is so much more fun, and better learning, too! But after you've done it once and said "isn't this cool" you could go right to using a plugin... you'll know what it's doing.

(Won't my ears be red if I've flubbed the math, it's been a few years since I dematrixed, and a few decades since Algebra!)

[Brian P. Reynolds]

Originally Posted by Ty Ford

Brian's comment about the golf match is interesting.

Brian, am I understanding you correctly, they went to air with raw M/S?

Regards,

Ty

No Ty, it was a L / R FULLY matrixed output but the width setting was on the widest it would go (the mic used Sony ECM969 did NOT have an un-matrixed [raw] MS output).

The width setting controls change the ratio between M and S components, in the wide setting there is more S component than M component making the audio image very wide .... But if there is a sound (bird call, frog croak, car horn etc or any other sound) at 90deg to the mic on either the left or right it WILL result in an out of phase signal on the other side..... and if that sound is there long enough it will be regarded as being 180deg out between L & R and in a broadcast chain will possibly cause errors when going to air depending on the transmitter processing.

And its NOT just Sony mics that do this, I have been able to replicate the problem on ALL MS mics I have had acess to Shure VP88, Audio Technica [various models] , AKG and Sony [various models], I haven't tried the Sennheiser 418 but I am assuming it would be similar to other mics.

edit... Just checking the details on the Senni 418 it appears to be an UN-matrixed mic and ONLY has a discreet M and S outputs and NO Left / Right outputs.