Solution for unstable stereo

[FV]

I was intrigued by the latest mix review (Jan 2016) in which an unstable stereo image was discussed on Hilary Hahn's "Coming To". This is a problem I encountered once when trying to close mic an animate source with an MS array. The slightest sound movement becomes greatly exaggerated!

To avoid this, I am thinking a mono close mic in addition to a backed-off stereo array could work. This would stabilise the image by making the mono mic dominate both in loudness and TOA—a setup not dissimilar to a Decca tree.

I would be interested to know what solutions others would suggest. Perhaps I am missing something.

[Exalted Wombat]

I'd suggest that "stereo" and "ckose-miked" are mutually exclusive concepts.

[Hugh Robjohns]

I was intrigued by the latest mix review (Jan 2016) in which an unstable stereo image was discussed on Hilary Hahn's "Coming To". This is a problem I encountered once when trying to close mic an animate source with an MS array. The slightest sound movement becomes greatly exaggerated!

Unstable stereo imaging is almost always caused, fundamentally, by placing a stereo array too close to the source, such that small physical movements relative to the array result in large angular changes in the reproduced spatial image. Arrays with small stereo acceptance angles exacerbate the problem, of course, as do spaced-mic arrays (rather than coincident arrays) because of their additional sensitivity to time-of arrival differences. The Hahn recording suffered from the double-whammy of a too-close spaced mic array -- a rather strange choice, in my view, which very obviously didn't work at all well in that situation.

To avoid this, I am thinking a mono close mic in addition to a backed-off stereo array could work. This would stabilise the image by making the mono mic dominate both in loudness and TOA—a setup not dissimilar to a Decca tree.

It is true that the centre-forward mic on a Decca Array is there to help stabilise the stereo image and remove the 'hole-in-the-middle' effect you typically get with wide spaced omnis. It's also true that a close spot or accent mic can be used to provide a stable static image for a specific source within a larger stereo image.

However, the approach you describe introduces the potential problem of conflicting imaging information provided by a moving source in front of central and static spot mic versus the distant stereo array, in addition to possible comb-filtering effects from two separate mic systems. This approach might work acceptably for a large scale orchestra and soloist, but would probably be less effective or satisfactory for a more intimate soloist recording.

I would be interested to know what solutions others would suggest. Perhaps I am missing something.

I would advocate a slightly more distant coincident (XY or MS) stereo array configured with a broader stereo acceptance angle to capture the performer's physical movements with a realistic and acceptable scale as a more appropriate and simpler solution. MS (with a moderate Side level) or crossed-cardioids, would be my starting point, for example.

If the room acoustic does not allow a more distant stereo array, then some compromise is required. Either don't record in stereo (stick with a closer spot mic to eradicate the problematic movement), or find a different recording venue with a more appropriate acoustic that warrants the stereo approach, or, configure the stereo array into the vertical plane instead of horizontal. This radical alternative technique will convey a sense of scale to the instrument (presumably the reason for wanting to record in stereo in the first place), but will hopefully remove much of the side-to-side movement effects since the array is only sensitive to up-down movement in this orientation.

...You just have to hope the source doesn't lean forwards and backwards as well as swinging from side to side!

H

[FV]

Thank you Hugh, your post is very enlightening and I think I grasp all of your points but, since I know you love to talk about these things, I will be bold and ask something more.

...the approach you describe introduces the potential problem of conflicting imaging information provided by a moving source in front of central and static spot mic versus the distant stereo array, in addition to possible comb-filtering effects from two separate mic systems.

How does a Decca tree avoid the problems you describe here?

[Hugh Robjohns]

Comb filtering can occasionally be a problem with Decca Tree arrays (when auditioned in mono), especially those with larger dimensions, of course. But since they are typically used with large scale orchestras the complexity of sound arriving from multiple directions usually does a good job of minimising the problem.

The potential imaging conflict between spot mic and Tree doesn't occur in normal use because orchestral soloists don't tend to move about much, and are normally at a relatively large distance from the Tree array, so the angular variation is negligible. Any spot/accent mics are panned into the appropriate position to match the image obtained from the Tree, and once done the mix image is very stable.

The problem in your situation was the close proximity of moving source to the stereo array imposed large angular variations in the array image, which would conflict with the fixed static spot mic image position. That's not a situation which occurs in normal Decca Tree applications.

H