turn Double MS into a super-directional virtual mic?

[realsnd]

Dear all,

I’ve recorded some tracks in double MS
and wandered if anyone has ever
attempted to use the rear and fig-of-8 channels
to increase (in post) the lateral and rear rejection
of the front mic. The end result would be
a super-directional mono (virtual) mic.

This would be useful to adjust in post tracks
recorded in uncontrolled environments,
e.g. documentary setting, direct cinema style,
no time to change mics or lav people.

Is the idea making sense at all? If it is, any
suggestion to make it work?

My set up was a CMIT (front), CCM41 (rear),
and CCM8 (Fig-of- inside a Cinela PIA-3,
recorded on a Nomad with the three inputs
with identical fader and trim settings, no
compressor/limiter involved.

The typical track: a quiet river surrounded by open
fields, a man bathing 3m in front, an engine 100m
in the back, kids babbling on the sides from
various distances. The three channels are quite
contrasted but the front mic still got too much
engine and kids. The goal is to get a mono track
with the kids, and more importantly the engine,
attenuated.

I played around with Schoeps' DMS plugin.
http://www.schoeps.de/en/products/dms_plugin/overview
The displayed polar patterns suggests that I could
achieve complete rejection of rear and attenuation
of side with some settings and using only the center
channel (from DMS decoded as 5.1). But the actual
result is unimpressive and there is some distorsion
if I push it too far.

So, what’s your take on this?

 

[studiomprd]

welcome snd: " So, what’s your take on this? "

trial and error,  have fun.

[VM]

Of  course you can. You should have better results with "double MStools BF", the other Schoeps plug in that is more complete.

[realsnd]

Thanks for the replys. The plugin I was refeering to was actually the

Double MS tool BF from Schoeps (wrong link in my previous mail, correct link is

http://www.schoeps.de/en/products/dms_plugin_bf).

Cancellation of the CMIT's rear lobe was not very effective, some phase-related

distortions appeared with the most extreme settings of the "Focus' parameter,

which was the most effective. So, I'd appreciate alternative ideas, other plugins,

insights on the theoretical problem, etc.

[pkautzsch]

The Sanken CS-3e works with multiple capsules, the Super-CMIT uses an "intelligent" DSP. The new Sanken CSR-2 is supposed to have nearly as good cancellation as the Super-CMIT, but in the analog domain. So although the principle of multiple capsules and phase cancellation works, the details apparently need lots of engineering.

 

I think the basic problem is this.

A shotgun's or hyper's rear lobe usually is polarity inverted. The more "shotgun-y", the more angle-dependent and frequency-dependent these rear lobes are. So if you just put a backward-facing cardioid on top, you might improve the pattern in some frequency ranges, but make it worse in other ranges. You'd have to have a "cancelling" pattern that's very close to the backward lobes to be effective.

[Wandering Ear]

The capsules in a sanken shotgun are so close together they can remain phase coherent up to fairly high frequencies. The capsules on your dms rig are too far separated to be truly phase coherent. This will always be a limiting factor in this type of rig, as your tests have proven.

[realsnd]

The capsules in a sanken shotgun are so close together they can remain phase coherent up to fairly high frequencies. The capsules on your dms rig are too far separated to be truly phase coherent. This will always be a limiting factor in this type of rig, as your tests have proven.

 

As matter of fact, my tests suggest you're right, but there is something that bother me.

If capsules' coincidence is good enough for 5.1 decoding---which involves some cancellation---why isn't it good enough for rear cancellation? In this set up, the CMIT and the CCM41 capsules are on the same plane,

approximately orthogonal to the mic-rear sound axis. The wave length at 1000Hz is about 34cm,

the distance between the capsules on this axis is about 100 times smaller. So it seems negligible,

considering we are talking about an diesel engine sound quite muffled, 100m away.

[studiomprd]

rs: " If capsules' coincidence is good enough for 5.1 decoding---which involves some cancellation---why isn't it good enough for rear cancellation? "

the laws of physics, perhaps..?

[pkautzsch]

A Schoeps 41 capsule is a "hyper" too, so it has that back lobe. Pointing it backwards on top of a shotgun will result in the hyper's back lobe pointing forward.

[realsnd]

A Schoeps 41 capsule is a "hyper" too, so it has that back lobe. Pointing it backwards on top of a shotgun will result in the hyper's back lobe pointing forward.

That must be a limiting factor, indeed. According to Shoepd polar diagram

the lobe is -10dB compare to front below 2khz, I don't know if that's the reason

for cancellation failure. (see http://www.schoeps.de/en/products/ccm41/graphics)

 

Regading coincidence, I have assumed the mic placement displayed on the Cinela

site was correct (see attached image), but I am not 100% sure of where exactly

the capsule is inside the CMIT. Is there someone expert enough

to confirm this is the optimal placement? (I can satisfactorily decode MS on

both rear and front, so it is at least OK, but perhaps not optimal).

 

Since I am on the topic of mic placement, Does any one see any

specific reason why is the CCM8 on top and the CCM41 below?

It seems odd because the CMIT suspension gets in the way of the

CCM41 in the set up and would be better place on top, no?

[realsnd]

After an inquiry to Schoeps about capsules positions

within the mics:

 

----

The distances from front grill to diaphragm are:
 
CMIT: 92,5mm
CCM8: 14,5mm
CCM41: 4mm
----

 

This helped me and will possibly help others

to precisely position the mics (the perspective

in the photo above gives slightly distorted

picture of the optimal position in my view).