VBAP and VBIP are both panning algorithms, based on level differences between the loudspeakers. VBAP is based on amplitude differences whereas VBIP works with intensity differences.
Understanding VBAP & VBIP
Those algorithms perform a pairwise (2D) or triplet-wise panning (3D). They select the two or three speakers surrounding the source, and feeds them the source signal with gain differences.
If a source is perfectly aligned with one loudspeaker, then only this speaker is fed with the source signal. In 3D, when a source is aligned with the arc between two speakers, only those two will be used.
With VBAP/VBIP, the speakers surrounding the source are considered as a vector base, and the source direction is used to calculate the gain values. These algorithms only consider the azimuth and elevation of the sources and the speakers.
The main difference between VBAP and VBIP resides in the way the gains evolve with the source position. VBAP was designed to offer an accurate perception of the source position for frequencies below 700 Hz, whereas VBIP optimizes this criterion for frequencies over 700 Hz.
The figure below shows the way the gain associated with two speakers (respectively at azimuth -30° and +30°) evolves in VBAP or VBIP when moving a source between those two speakers.
Use the spread to change the apparent width of sources spatialized by the bus. This parameter spreads replicas of each source in multiple directions around the position of the source. It can be used for artistic purposes or to smooth the width and coloration of a moving source.
The divergence applies a spread to the sources progressively, when they start entering the divergence circle (2D) or the sphere (3D) defined by the Radius parameter.
- When the source's AED Distance is higher than the radius, no spread is applied.
- When the source's AED Distance lower than the radius, a spreading is progressively applied, reaching 100% when the source is at the center of the coordinates system.
Available in 3D only, the phantom speakers allows you to create a virtual speaker placed above (Top) or under (Bottom) the speaker setup. When speakers are not evenly distributed, this can solve sound homogeneity issues if a sound source is sent at the top or at the bottom of the setup.
VBAP and VBIP were designed to work on 2D and 3D surround speaker systems. Try to use speakers that are as regularly spaced as possible across the entire setup, for a maximum homogeneity.
However, VBAP and VBIP are efficient on quite asymmetric layouts, as long as there are enough speakers on a trajectory to offer smooth variations. Check for possible optimizations in "Amplitude Panning Algorithms Optimization", page 77.
Speaker Placement Requirements
- This algorithm requires the loudspeaker layout to include frontal and surround speakers.
- The center of the coordinates system needs to be inside the speaker layout.
HOLOPHONIX will display an error message in the 'LOG' section if an invalid speaker layout is used. Otherwise, speaker positioning is free.
If the setup you have is not valid for the algorithm, you can still use 'virtual' loudspeakers that will not be connected to any real loudspeaker, but will allow the algorithm to see a valid setup.
First, create a new bus with the right amount of loudspeakers (real speakers and virtual speakers), and assign the first bus outputs to the real loudspeakers. Open the bus 'SPEAKER PARAMETERS', deactivate the 'LINKED TO SPEAKER POSITION' mode and manually adjust the position of the additional loudspeakers to match the algorithm requirements.
For example, if you want to use VBAP or VBIP algorithms on a frontal speaker layout (with no surround speakers), you will need to use 'virtual' speakers placed at the rear of the venue (i.e. behind of the origin of the coordinates system).
Elevation Layout (3D)
On the elevation plane, VBAP and VBIP offers better results for setups where the different layers are organized in staggered rows (i.e. the speakers are not aligned with one another), thus forming triangles in the elevation plane. If speakers are all aligned in the elevation plane (forming squares), use LBAP, as it was designed for such layouts.
When using VBAP or VBIP on 3D setups that are open at the top, the speaker triplets computed by the algorithm can end up being not optimal for the setup. For the algorithm to ensure a good accuracy in source localization, the setup would have required a speaker at the top or the bottom of the setup.
Without such speaker when a source is moved upwards (or downwards), its signal will be sent to three speakers, where the user would have selected two loudspeakers.
Adding an imaginary (phantom) loudspeaker causes the algorithm to compute new speaker triplets in the problematic area. The signal sent to the phantom speaker being ignored, the algorithm ends up using only two speakers instead of three.
However, if a source is positioned in the same direction as the phantom speaker, its audio will be lost, as the phantom speaker signal is simply dismissed.