ambisonics filtering 2 of 2



My goal is to find the best and effective method to isolate, in an Ambisonics recording (alone, done with only one first-order mike and no ancillary mikes of any type), the different instruments (or groups of instruments) being part of an ensemble or orchestra. This goal is, in my opinion, the main reason for recordists to use the Ambisonics recording method, so they can adjust the single levels of the various instruments in post and obtain a well balanced mix. I think this goal is primary for professional use, and this overcome in importance the spatiality 3D rendering, that is also important but only on a secondary plane. Obviously, when our goal is simply a 360 VR leisure filming, the 3D sound rendering jumps ahead at the first position, but here I want to talk and experiment in a more professional oriented field.

Here I have at my disposal two different Ambisonics VST plugins filters that can handle the directionality aspect of the soundfield: the “Rode SoundField” and the “Ambix Directional Loudness”, both freeware. I made some comparisons with these, but if you know about other plugins or methods to reach the main goal stated above about filtering single sources by directionality, let me know!

In this experiment I try to isolate as much as possible a trumpet in a small jazz ensemble. The original Ambisonics recording is the Rode sample that comes with their SoundField VST plugin, recorded in NT-SF1 format with the mike placed in the center of the players circle, and converted to b-format Ambix standard by using the same SoundField VST plugin (because the ambix directional loudness VST can only be fed with an Ambix format input).

The parameters used in the SoundField plugin are clearly visible on the video, while in the Ambix Directional Loudness plugin I used all the 8 filters the plugin has, all in a passive -99 gain configuration. I placed four of them overlapping along the entire horizontal and vertical soundfield, roughly spanning horizonally from -90 to -135 degrees, passing from 0, and from -90 to +90 degrees vertically. This leaves a small vertical window from -135 to -90 horizontal degrees where the trumpet resides. Then I added two more filters above and two more filters below 0 degrees in this window, in order to isolate perfectly the trumpet instrument. The filters areas are marked by blue rectangles and a white circled number for identification in their geometrical center.

By taking into account Angelo Farina’s advice to use a cardioid in order to nullify better the back of the jazz ensemble, in the second video I first compare the cardioid VS shotgun filtering operated by the Rode plugin: to my ear, the cardioid wins in isolating better the trumpet and lowering the other instruments. Then I compared the cardioid of the Rode plugin VS the Ambix Directional Loudness plugin and, to my ear, the ADL plugin wins again.

So, to summarize, the best isolation methods are, in order of effectiveness: ADL, Rode cardioid, Rode shotgun. Obviously I posted here my findings in order to be discussed, because perhaps I have a not an enough trained ear, or perhaps there are more effective ways to reach the main goal: anyway, a good confrontation is always beneficial for all of us !

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