MS Audio Mikes in Final Cut Pro

[ This article was first published in the Sept/Oct, 2007, issue of
Larry’s Final Cut Pro Newsletter. Click here to subscribe. ]

Phil Ashby, cofounder of the West-of-England Final Cut Pro User Group, sent in the following on a microphone style I had never heard of — the M/S microphone — which I first learned of last month.

[Last month you had a question about handling M/S mikes in Final Cut Pro.]

 

MS (pronounced M S or sometimes M ‘n S) is very popular for pro crews shooting in the UK – but because it’s specialist sound, expensive(ish) mics and generally done via an SQN or similar mixer, it’s not practiced unless there’s a pro sound recordist in the crew. Generally too it’s for high-end documentaries (many of which don’t need ‘genuine’ stereo from location apart from wild tracks) – dramas will need more channels.

 

Another way of thinking about it is that it’s matrixed sound: so M (mid or mono) = (L + R) and S (stereo or side) = (L – R) . Think of (L-R) as a difference signal. M-S was originally (I’m talking mid 80’s I think) done with two identical mics and the SQN mixer used to phase invert one of them, and supply the two signals for the (Nagra) tape deck, but very quickly the true M/S mic was developed, which is as described in the original post from Dave Coulter. The BIG advantage of recording M/S was apparent in the 16mm sepmag (sprocketed tape) edit – even slight misalignment of L/R cuts gave phase ‘shushes’ over an edit, not so problematic with M/S systems. And (sub-frame) phase-locking of sepmag tape drives was never that stable on playback either, in the dubbing theatre.

 

The sound has to be de-matrixed during the sound dub – and again as Dave describes, life is made much easier for the dubbing mixer, with the ability to adjust the stereo ‘presence’ via the level on the S channel. But to de-matrix, a phase invert filter (or in the physical world, channel!) is required, i.e. doing the ‘math’ : L = (M+S)/2, R=(M-S)/2 – it’s the minus S that’s the problem.

 

That’s why you can’t do it (yet) in FCP – no easy phase invert filter for audio in the mini-mixer.

Larry continues: Thanks for writing, Phil. Bill Southworth then provided some comments:

I just read the question about using MS microphones recorded into FCP. I use a Sanken CMS-10 extensively and I’ve spent countless hours searching for a plug-in that would work with FCP or Soundtrack Pro. It is theoretically possible to do the conversion in Logic but I haven’t had any success doing this.

 

For a variety of additional reasons, I now use a separate four-channel mixer (Sound Devices 442) and it decodes the mic, add it to the mix, and then pass on the mixed stereo to the camera for recording.

 

If the MS is simply converted to stereo and recorded as stereo on the camera, it CAN BE REVERSED later by passing it back through the mixer to turn it back into an MS signal. MS recording is a reversible process. A mid-side lobe signal when demuxed is converted to stereo. The stereo when demuxed goes back to MS. When you adjust the levels on the MS signal you control the width of the stereo image. When you adjust the stereo version you change the left-right balance. So an MS mic can be used to focus your zoom in on your subject or discriminate unwanted side noise.

 

A plug-in for FCP to mux-demux the audio would be an extremely useful thing for documentary guys. I’d buy it in a nanosecond.

Larry adds: Just when I thought we agreed that there was no solution to MS Mikes, Andrew Commiskey sends in the following which solves the whole decoding issue.

The decoding process is more mechanical than digital. The mic works by having 2 signals: a middle or mono signal and a figure 8 or left-right side signal.

 

To decode it bring the mid-side (mono) signal into the 1st channel and pan it center, then take the side(figure 8 ) make a copy of it and bring it into channels 2 and 3. Pan channels 2 and 3 left and right then select the clip on channel 3 and invert it (making it 180 degrees out of phase). This gives you a stereo sound field than doesn’t cause phase problems when played back mono.

 

Once again:

• Mid-channel – mono center
• Side-channel – split to left-right with one of them inverted (or 180 degrees out of phase)

Larry replies: One quick note. Phil is correct that there is no phase inverter for Final Cut. But there IS one in Soundtrack Pro. So, if you are mixing in STP, working with M/S mics should be possible.

Thanks to Phil, Bill, and Andrew for their comments — I learned a lot just reading these.

 

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