When a Virtual Room is Channel Based, the speaker configuration layout is modelled as fixed speakers which appear in the Room graphically according to their programmed positions ( see section 4.5 Custom Speaker Configuration ) - you will not be able to move those speakers around with the mouse. These fixed speaker positions are virtualising a sound system configuration which is often the same one that will be used for diffusion. These virtual speakers interact with the virtual acoustics of each Room as designed by the Artificial Reverberation editor (see section 8).
★ Each room can have a DIFFERENT Virtual Acoustic design and different sources.
The idea is that by simulating speaker positions and acoustic interactions, the process of mixing for a particular speaker layout becomes more predictable. For example, by monitoring binaurally a scene from a channel based room, it is possible to get an impression as to how the mix might sound diffused by a particular speaker arrangement (including the space between speakers and gain characteristics belonging to selected panning types). As we have mentioned earlier in section 4.6 on Ambisonics, there are also options for mixing and monitoring a particular Channel Based Room simulation transcoded to a different setup than that being simulated, which may also be useful in certain contexts.
This is really all about mixing. Mixing in stereo is already a significant challenge, some have devoted their entire careers to it. Mixing for multiple speakers, is even more complex.
As mentioned in section 4.6 and other places in this guide, it is advisable to think in parallel as far as room mixing gœs. You could think of SPAT Rooms as different kinds of spatial busses with integrated acoustic emulation units. Use them in parallel, send some or all of the sources to multiple Rooms at the same time and sum the outputs together into one output format. One room might have a huge reverb, and one might be a much smaller space. The fact of the matter is that different
Channel Based Rooms will have different sounds, but they can all be summed into the same channel based format.
It is only in this kind of Channel Based room workflow that the Nebula Spatial Spectrogram will appear, as it works by computing a diffusion map from the combined outputs of statically positioned sound sources - i.e. speakers.
In practice, the best mixes are attained by mixing on the same sound system in the studio or venue as you have represented in the Channel Based Rooms. But there is also the possibility of monitoring Channel Based mixing on headphones using the binaural monitor or a dedicated, parallel, binaural room as mentioned in section 5.1. This is workable enough to allow you to keep on arranging a mix and spatial composition ‘off-location’ without too many surprises when it comes to the final result.
★ Channel Based simulations render localisable virtual sources by controlling the virtual speaker array in a similar way that panning-laws do on real world physical systems. They do not model the positions of virtual sound sources, but instead control the gains of the virtual speakers in the model to render perceived positions.
All inputs to a Room must be in some kind of Channel Based format, even when the Room is an Ambisonic or Binaural Room. Even a Mono source can be considered as a channel based signal of one channel (see section 6.41). For any source that has more than one channel, it will be structured as a Channel Based configuration - and that configuration will be virtualised inside the Room as one single source in an inter-connected cluster formation - this is designed to maintain interchannel spatial image relationships as much possible, keeping the multichannel source in a discrete self-contained system. The sound, size and positioning of that configuration can now be manipulated as a unified group.
The better alternative to Channel Based simulations is to use a Binaural or High Order Ambisonic room to mix. In these Room simulations, speakers and panning are not simulated. Instead all the inputs into the room become Virtual Sources they become sound emitters in the virtual space. To get a grasp on how mindbending this is, we need to go through it a little.
In a fully virtualised Ambisonic room, the Nebula Spatial Spectrogram will no longer be available, as it is too complicated to compute the spatial-spectra of moving sources. No visualisations of speakers will appear either, as they are not being modelled.
So far, we have established how the audio technology at the heart of SPAT is capable of simulating precisely the emissions of virtual speakers at fixed positions, and the way they interact with virtual acoustic space.
But the real big achievement of SPAT and High Order Ambisonic is the modelling of virtual sound sources that are changing orientation, size and position. SPAT can model these and simulate the changing response of the acoustic space to these complex, dynamic and imaginary sound diffusors.
This is where we touch on what ‘virtualisation’ of sound sources really means.
The technology of Virtual Sound Systems and Virtual Spaces invites us to stretch our imaginations beyond what is possible on a physical installation and into a place where sound emitters can continuously alter their structural dimensions, orientation and acoustic characteristics - the idea of speakers as fixed objects no longer applies in the virtual acoustic space. Instead, sound emitters grow wings and become complex musical elements to be organised and composed in time and space, contributing even more to the cultural experience of music.
One workflow that is often required, is that of re-mixing a pre-rendered channel
based mix of a particular format, to get that mix expressed into a different channel
based format. For example, the need to down mix a 5.1 surround into stereo is
quite common, but also the other way - perhaps a 5.1 surround mix needs to be
up-mixed to a 7.1.
One way to do this, is to use an Ambisonic Virtual Room as a way to simulate the source configuration as if it were being diffused in a space, in its correct speaker format and with simulated, full sphere acoustics and then transcode the output to the desired format.
Here is an example of that in the Room view. Notice how it is possible to re-balance and re-mix the original using the perceptual factors or positional parameters. A room simulation reverb is also playing a significant role in the cross-format mixing process, as the reverb simulation adds new spatial information to the mix, which might help get a more immersive result in the output format.
This is an example of a multiple format changing signal graph, which changes format in two ways, to provide an alternative result. Firstly, by transcoding the output of an HOA Room into various other Channel Based formats, but also a room that simulates how the target speaker configuration might render the source speaker configuration. Both methods will give different results and the choice is up to the designer.
Options are available when transcoding (aka decoding ) from a High Order Ambisonic room into a Channel Based stream. These decoding options will affect the sound in quite different ways - luckily it is possible to listen to the results of different Ambisonic decoding options in realtime. More about this in section 6.7.8 which details more on the HOA Room concept.
When a Virtual Room is set to be Channel Based / Stereo it will model a stereo speaker system in the virtual room acoustics and virtualise stereo microphones in real-world stereophonic recording configurations. This is a hybrid approach, where you ‘record’ the sources positioned in the Room through virtual matched pair microphones at the listener position, as if you were miking up a concert for stereo broadcast or playback. The microphone modes were described in section 5.68 and become available under the Panning Type menu of the Room when it is set to Channel Based Stereo.
You will notice how the Spat Reverberation handles the stereo image naturally, because it is modelling the entire mix scene in the acoustic context of a Virtual Room, rather than the conventional ‘voltage controlled’ left-right pan pot that all audio mixers offer for stereo mixing.
This method of working in a stereo Room invites a different approach to stereo panorama source mixing in the studio or on twin stack PA systems. It can also be a way to down mix from a surround format to stereo by simulating the speaker configuration and ‘recording’ it in stereo.
One final thing to note when working in an Ambisonic room, rather than a channel based room, is that the feeling of Artificial Reverberation (see section 8) that gets encoded into the High Order Ambisonic will have a much reduced strength than what is heard during a channel based room mix. This needs some trial and error to get a feel for