An Introduction to Spat

Did you ever wonder what the Spat Revolution Immersive Audio Engine advanced source parameters were all about? How some panning techniques avoid sweet spots for live concert and installation? Wondering how multiple room soundscape using different set of loudspeakers and delivering different output stream can be achieved? What to learn on how live theatrical show are using Spat Revolution?
A blog read by Hugo Larin, Business Development for Spat Revolution
 





If you already had a chance to explore Spat Revolution, you’ve rapidly realised that control options for source go much deeper than what you may have expected or have used before. To many parameters? Maybe as a first flight into Spat you’ll find but understanding the power of each of them may have you realise what other options are brought to the table
Spat Revolution provides a variety of source parameters which affects each of the different mono, stereo or multi-channel sources in the Room soundscape. The common position parameters (derived from  the center ref. point) called radiation, includes azimuth (effectively the 360 degree pan ), the distance (the distance of the source) and the elevation (the + or – 90 degree elevation of the source ). These position parameters, known as polar coordinates, can also be managed using common cartesian  coordinates parameters; X, Y and Z axis
Then there are three parameters affecting the sonic properties of the source and the entire Room quality. The Aperture, is the “sound cone” projected by the virtual source in the acoustic space (going from a default 80 degrees to a tighter opening at 10 degrees, or a full spread up to 360 degrees). You can also consider it as a ratio between direct sound and reverbered ones. Increasing the Aperture will ultimately diffuse it wider,  excite more the room and then increase the reverb sensation. The orientation of the source is controlled by Yaw (the ability to turn the direction of the source 360 degrees, pointing it to another direction) and Pitch (pitching the source signal up or down + or – 90 degrees.
When dealing with multichannel sources or multiple simultaneously selected sources, the BARYCENTRIC (center of mass where the sources orbit) parameters allows the source group to rotate around the X, Y and Z center of mass. This give the soundscape designer some very rapid ways to work with multi-sources. You can also adjust the scale (distance from the center of mass) of the multi-channel or multi-selected source. The scale can be used to rapidly scale up many source distances in the soundscape provided.
In addition to this is  the SPREAD FACTOR controlling the way the source will be spread out to the different speakers depending on the selected Room panning type. For example, when set at 0%, the source is spread only to the closest speakers. When set at 100%, the source is spread to all the speakers. So the source’s sound image will be located at the center of the speakers.














Figure 1: Spat Revolution Source Radiation, Position, Spreading and Barycentric





Perceptual Controls
Some controls are proposed which allows to specify the effect the source has in a room using perceptual terms, rather than the technical terms. This is derived from psychoacoustic research with Flux:: partners the IRCAM.https://www.ircam.fr/recherche/equipes-recherche/eac/
The kind of source parameters we are talking about are; Presence (prominence of the direct sound with respect to the reverberated sound), Warmth (presence of the low frequency content part of the source) and Brilliance (presence of the high frequency content part of the source).
Further on we have the Room Presence  (Prominence of the reverberation with respect to the source,), Running Reverberance  (amount of perceived reverb when feeding a continuous music message, where the overall sound is a tight blend of the dry and wet signals and the reverb part cannot be mentally separated.), and the Envelopment (corresponds to the perceived notion of how much the listener is being surrounded by the ambient sound).
Now that we are touching on the reverb you may be interested to know that each room in SPAT have a reverb sound that can be enabled. For that matter of fact each sources in the room has an individual reverb on or off setting.  Spat is not trying to simulate any specific venue room but giving the ability to create the reverb effect (artificial reverberation of sounds). The reverb parameters include Room size, Reverb Start or Decay, some perceptual factors and parameters on the room response controlling early reflection and the cluster.





Figure 2: Spat Revolution Perceptual and Reverb Controls













Multiple Rooms, Panning Techniques, Dealing with Speaker Setup for Live
How to plan a speaker design and how Spat deals with panning over these your multi-channel immersive system remains a popular conversation. Some manufacturers will propose sort of a standard stage setup where 5, 7 or more speaker hangs are spread across the stage with rather equal separation in distance between each hangs. This can indeed achieve some good panning with a fix technique over the front stage but reinforces the need to work within the framework proposed.
Spat is not trying to give a fix proposition, rather it proposes a variety of options, some as creativity tools, others as flexibility in speaker design and approach. It can obviously and easily organize an arrangement of 5, 7 or 9 array hang reproducing the front of the stage with equal spacing and nicely pan sources across with nice and smooth VBAP panning technique thus providing great localisation and distribution in this case. The Azimuth parameter will provide the panning degree and Distance will move the source front to back.
An important factor to introduce is that Spat comes with a concept of many what is called room. A room is basically a soundscape using either a predefined or custom channel based speaker arrangements (Predefined includes stereo, all the common surround, various 2D, cubes, Auro 3D, Atmos and DTS, etc) The custom arrangement is a speaker configuration containing all the speakers of your design or actually a portion of the speakers that you may want to use in a specific soundscape. The power of multi-rooms means that a mix of strategies and arrangements could be used with Spat. While a Room could take care of the 3, 5 7 or more front of stage hang using a VBAP panning strategies, a second Room could deal with other speakers for effects.
Now, on with panning techniques. The Spat engine offers a variety of spatialisation and panning techniques which may suit different applications (Binaural, High order Ambisonic HOA or 2D/3D Channels based with VPAB, DBAP, KNN, SPCAP and more panning types).
A very important point to understand when working with immersive audio is how some panning techniques are relative to a central listening position. Various panning techniques commonly exist (non-proprietary) and a manufacturer would typically pick one of those methods.  VPAB and High Order Ambisonic room panning for example will typically require the listener to be positioned in a ‘’sweet spot’’ (although sweet spot size can be controlled in some case). This is clearly a challenge for concert, theatrical stage and installation where the audience can be all over the place.
Now we’ve talked about the fact that multiple techniques can actually be used simultaneously in Spat with these different Rooms allowing for different intentions, for different effects and requirements, in parallel (basically delivering multiple content at the same time). One room with some connected sources may be using a panning method with no assumptions on position of the listener, the central location (or speaker setup for that matter of fact). DBAP (Distance Based Amplitude panning) is actually one of those.  Contrary to VBAP where a source will typically use 2 (or 3) speakers only to reproduce the virtual source (which will work only for a portion of the audience) DBAP will distribute signal. It is also possible to use KNN (amplitude panning based on the distance between the virtual source and the K-nearest speakers.) allowing the user to define the number of speakers a source will use.
A more sweet spot centric strategy, such as VBAP, could actually still be used in a live situation. An example of this can be in a big top for an all-around setup where speakers are spread around the audience. Panning a source around will have a very nice smooth effect of movement and localisation which may be desired when spinning something in the soundscape and vocal intelligently (and distribution in the audience) is not of any concern.
Delivering simultaneous output streams on different speaker arrangements may not be something you need to do in an everyday live setup, but what about if you want to deliver a binaural feed to the video capture of the show in order to deliver the immersive mix while you’re simultaneously delivering the signal to your speaker outputs? With Spat you could be recording; a binaural, a standard stereo, a 5.1/ 7.1  or any channel based configuration simultaneously in the same project and setup.
In creation phase or studio style scenario, delivering simultaneous output streams is the power of bouncing all formats at the same time of the same immersive mix. You could think as well at the sound designer delivering himself for preview a binaural output for headphones while on a plane, a surround speaker arrangement for his studio work, while actually creating the content for the multi speaker arrangements  of the actual show itself, that may have speakers by the dozen!
Endless it is a very powerful creative feature. Allowing for e.g. to use a VBAP channel base room for very localized sound in addition to a first order Ambisonic ones for blurry sound etc…





  Figure 3: All Speaker with a KNN Pannings





Figure 4: 5.1 Soundscape deliver to multi zones





Figure 5: Various different Speaker Arrangements and Panning Types used simultaneously in one and the same project





Interested in this conversation?
Read our next blog that covers how Spat  deals with speaker alignment, the hardware resources to run Spat, the system latency and more specifically used in regards to Live Theatrical Shows.
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