Introducing SPS format and its first practical implementation, called Mach1

 

  SPS stands for Spatial PCM Sampling, and is a modern alternative to Ambisonics for recording, synthesizing, manipulating, transmitting and rendering of spatial audio contents. This page provides the definition of the SPS concept, show similarities and differences with Ambisonics, and presents the hardware and software tools employed for working with SPS signals having various numbers of channels.

Finally, the first real world application of the SPS approach is presented, the Mach1 8-channels soundtrack currently supported by the Samsung VR portal and by the New York Times VR app

 

Ambisonics is a widely employed format for capturing, transmitting and decoding spatial audio, using 4, 9, 16, 25, 36 channels or even more (High Order Ambisonics, HOA). It was invented in the seventies by Michael Gerzon and colleagues, and has a long history of successes (few) and failures (many). In Ambisonics, the spatial information is obtained representing the sound field as a multichannel track, in which each channel corresponds with a virtual microphone having a very complex polar pattern, mathematically described by the corresponding Spherical Harmonics function.

SPS is a modern audio format providing all the same capabilities of recording, synthesizing, processing, transmitting and decoding as Ambisonics, adding some inherent advantages which will be explained here.

 


History of the SPS approach

SPS was invented by Alberto Amendola and Angelo Farina, during acoustical measurements performed inside the Grand Théâtre de Provence, Aix en Provence, on 19 June 2007. In that case, for mapping the sound reflections inside the theatre, Alberto Amendola suggested to use a Sennheiser shotgun microphone, mounted on an automatic two-axes turntable, for sampling the sound arriving by hundredths of directions while measuring room impulse responses.

So the first implementation of SPS was with room impulse responses, measured one after the other: each one was the response of a very directive microphone pointing in one of hundreths of directions, covering almost uniformly the whole sphere.

In 2010 we did buy our first Eigenmike, and this allowed to record SPS signals simultaneoulsy with 32 ultradirective virtual microphones, pointing in the same directions of the capsules of the Eigenmike:

Thanks to these realtime capabilities, SPS-32 was first used as a tool for creating colour maps of the spatial distribution of sound, employed both in concert halls and inside cars, as it was published in these papers:

Marco Binelli, Andrea Venturi, Alberto Amendola, Angelo Farina

Experimental analysis of spatial properties of the sound field inside a car employing a spherical microphone array

130th AES Conference, London, 13-16 May 2011

Angelo Farina, Alberto Amendola, Andrea Capra, Christian Varani

Spatial analysis of room impulse responses captured with a 32-capsules microphone array

130th AES Conference, London, 13-16 May 2011

Angelo Farina, Lamberto Tronchin

3D Sound Characterisation in Theatres Employing Microphone Arrays

Acta Acustica united with Acustica, vol. 99, n.1, pp. 118-125, ISSN 1610-1928, January/February 2013

Here a frame captured from the video rendering software written by Alberto Amendola for realtime mapping of room reflections inside the La Scala theater:

The usage of SPS as an alternative to Ambisonics for recording and playback was also explored successfully, as reported in the following papers:

A. Farina, M. Binelli, A. Capra, E. Armelloni, S. Campanini, A. Amendola

Recording, Simulation and Reproduction of Spatial Soundfields by Spatial PCM Sampling (SPS)

International Seminar on Virtual Acoustics, Valencia (Spain), 24-25 November 2011

Angelo Farina, Andrea Capra, Alberto Amendola and Simone Campanini

Recording and playback techniques employed for the Urban Sounds project

134th AES Convention, 2013 May 4–7 Rome, Italy

Angelo Farina, Alberto Amendola, Lorenzo Chiesi, Andrea Capra, Simone Campanini Spatial Pcm Sampling: A New Method For Sound Recording And Playback

AES 52nd International Conference, Guildford, UK, 2013 September 2–4

Angelo Farina, Simone Campanini, Lorenzo Chiesi, Alberto Amendola, Lorenzo Ebri Spatial Sound Recording With Dense Microphone Arrays

AES 55th International Conference, Helsinki, Finland, 2014 August 27–29

Angelo Farina From High Order Ambisonics to Spatial PCM Sampling – application to auralization of measurements and computer simulations

EPSRC funded workshop on numerical acoustic simulation and auralisation as built environment design consultation tools

Manchester (UK) 22-23 September 2015

 

Ambisonics signals

An Ambisonics multichannel sound track is not based on the concept of "position of the loudspeakers", as in a stereo (Left-Right) or "surround 5.1" (Left, Right, Center, Sub, Left Surround, Right Surround) formats.

Instead, the complete spatial information is represented by a number of "spherical harmonics" signals, at increasing orders. The number of signals for each order is increasing, and the corresponding polar patterns of the equivalent "virtual microphones" are shown here:

An Ambisonics stream is said to be "of order n" when it contains all the signal of orders 0 to n. For example, an Ambisonics signal of order 3 (the most widely used, nowadays) contains 16 channels: 1 of order 0, 3 of order 1, 5 of order 2 and 7 of order 3. Please note that, following the Ambix convention, numbering of Ambisonics channels starts from 0 instead of 1.

 


SPS signals

An SPS multichannel track consists in  bunch of signals recorded by a set of coincident directive microphones, pointing all around, covering (almost) uniformly the surface of a sphere. As the microphones are coincident, the SPS signals do not contain time differences between the channels, only amplitude is different, depending on the position of the sound source. In this, SPS is exactly as Ambisonics, coding the spatial information based only on amplitude and not on phase. However, in SPS there are never "reversed polarity" signals, as the virtual microphones being employed are cardioids of various order, whithout any rear lobe with reversed polarity.

The number of channels can be chosen among the known regular polyedrons (Platonic solids), up to 20 channels. Above 20 it is necessary to use not-exactly-uniform geometries (Archimedean solids), such as the truncated icosahedron (the Eigenmike). The most common number of channels, and the corresponding geometries, are listed here:

Figure N. channels Description
4 faces of tetrahedron
6 faces of a cube
8 faces of an octahedron
12 faces of a dodechaedron
20 faces of an icosahedron
32 faces of a truncated icosahedron
62 faces of a rhombicosidodecahedron
92 faces of a snub dodecahedron

 

There is not yet a standard absolute orientation and a standard channel ordering for all of these geometries, except for the tetrahedron, for the octahedron (Mach1, described later) and for the truncated icosahedron (SPS-32, which employs the same geometry as the capsules of the Eigenmike), as shown here:

Tetrahedron

 

 Mic #

Azimut [°]

Elevation[°]

 EM32 Mic #

1

45

35.26

6

2

315

-35.26

10

3

135

-35.26

26

4

225

35.26

22

Octahedron (Mach1)

 Mic #

Azimut [°]

Elevation[°]

 EM32 Mic #

1

45

35.26

6

2

315

35.26

12

3

135

35.26

28

4

225

35.26

22

5

45

-35.26

8

6

315

-35.26

10

7

135

-35.26

26

8

225

-35.26

24

SPS-32 (Eigenmike)

Truncated Icosahedron

 Mic #

azimut [°]

elevation [°]

Mach1 ch.

1

0

21

 

2

32

0

 

3

0

-21

 

4

328

0

 

5

0

58

 

6

45

35

 1: FLU

7

69

0

 

8

45

-35

5: FLD

9

0

-58

 

10

315

-35

6: FRD

11

291

0

 

12

315

35

2: FRU

13

91

69

 

14

90

32

 

15

90

-31

 

16

89

-69

 

17

180

21

 

18

212

0

 

19

180

-21

 

20

148

0

 

21

180

58

 

22

225

35

4: BRU

23

249

0

 

24

225

-35

8: BRD

25

180

-58

 

26

135

-35

7: BLD

27

111

0

 

28

135

35

3: BLU

29

269

69

 

30

270

32

 

31

270

-32

 

32

271

-69

 

 

Please note that, in the tables above, azimuth and elevation coordinates are conforming to current ISO2631 standard, as shown here:

Be aware that some software is not compliant with ISO standards, for example Ambix plugins and Facebook Spatial Workstation, which employ azimuth with reversed polarity. Eigenstudio instead has correct azimuth, but uses an elevation counting "down" from 0 at North pole, 90 at Equator and 180 at South pole.
 


Comparison between SPS and Ambisonics

The relationship between SPS and Ambisonics is substantially the same relationship occurring, for time domain waveforms, between a standard PCM waveform and its Fourier spectrum.

A time domain continuous (analog) waveform, can be represnted digitally by a sequence of pulses (PCM Pulse Code Modulation) as shown here:

Each pulse is also called a "Dirac's Delta Function".

In space, a "balloon" expressing the amplitude of sound arriving form every direction can also be approximated by a spherical distribution of "spatial pulses" or "spatial Dirac's Delta Functions", which can be seen as the spikes of a sea urchin:


 

Coming back to a waveform in time domain, we can also represent it as the summation of a number of sinusoids and cosinusoids with increasing frequency, each with proper amplitude (the amplitudes are given us by the Fourier transform):

Similarly, a spatial balloon can also be represented as the summation of a number of spherical harmonics functions of increasing spatial frequency ("order" in Ambisonics terminology), each with proper amplitudes (the amplitudes are given use by the Ambisonics encoding formulas, or by the A-format to B-format filtering):

In conclusion, it can be said that both SPS and Ambisonics are intermediate formats capable of decomposing a complete three-dimensional sound field in a number of discrete components. These are just Dirac's Delta pulses for Spatial PCM Sampling, and instead are complex oscillating functions ove the surface of a sphere for Ambisonics.
But conceptually both are just projections of the original data set over different kernels of base spatial functions, and the complete workflow going from recording to playback is exactly the same:

The red arrow means that the intermediate format (called B-format for Ambisonics, P-format for SPS) can be easily manipulated, although some sort of manipulations are easier in one of the two formats, and more tricky in the other.

For example, performing rigid rotatons of the soundfield is trivial in Ambisonics, and tricky with SPS signals. On the other side, doing "spatial equalization", which means boosting the gain in some directions and reducing the gain in other directions, is trivial with SPS signals and very tricky with Ambisonics.
As it is possible to go back and forth from SPS to Ambisonics and vice versa (exactly as we can go from time domain to frequency domain and back, using FFT and IFFT processors), the user can choose the intermediate format more suitable for his needs.
However, in most cases, SPS is advantageous (exactly as a PCM wave is advantageous over the usage of complex spectra for representing and processing a sound recording):

For playback of panoramic video with spatial audio soundtracks over a VR device such as Gear VR, Google Cardboard, HTC Vive, Oculus Rift, etc. the playback software becomes tricky with Ambisonics (as you need an advanced Ambisonics decoder, feeding each virtual loudspeaker with a complex mixture of all the Ambisonics signals), whilst it is really trivial with SPS: it is just necessary to place a spherical distribution of sound sources around the spherical video projection screen, each one fed with one channel of the SPS stream, as shown here for our Unreal Engine player for SPS-32 soundtracks:

 


Mach1 signals and other "spatial audio" formats

Mach1TM is a registered trademark of Mach1studios (http://www.mach1studios.com), a startup company based in New York (US) and founded by Dražen and Jacqueline Bošnjak; Dylan Marcus is Mach1's Technical Director. These guys were the first to create the tools necessary for employing SPS signals in the field of "VR videos", developing software both for generating Mach1 spatial audio soundtracks and for rendering them on Head-Mounted Displays, in particular on the Samsung Gear VR platform.

A Mach1 multichannel track corresponds to SPS-8, where the 8 virtual microphones are pointing at the vertexes of a cube, as shown earlier on this page.

Mach1 is becoming widely accepted as one of the best "spatial audio" formats accompanying 360-degrees videos on VR head-mounted displays. A "spatial audio" sound track ensures that users of these head-mounted displays equipped with headphones hear a binaural rendering of the spatial scene, which maintains the absolute position of the sound sources unchanged, independently on the direction where the user is looking.
Currently 4 "spatial audio" formats are available for different VR platforms and software:
 

Name N. channels Platform Software
Ambix (1st order Ambisonics, FOA) 4 Cardboard, Daydream, Gear VR Youtube, Facebook 360, Jump Inspector
TBE (almost 2nd order Ambisonics) 8+2 Gear VR, Oculus Rift, HTC Vive Facebook 360, Oculus Video, EccoVR
Ambix (3rd order Ambisonics, TOA) 16 Cardboard, Daydream Jump Inspector
Mach1 (SPS) 8 Gear VR, Cardboard, Daydream Samsung VR, New York Times VR

For most people, "spatial audio" means 1st order Ambisonics, but this is definitely the worst of the 4 available formats, providing a spatial resolution which is not enough for providing good realism. On the other hand, TOA (Third Order Ambisonics) produces significantly better results, but working with TOA signals requires knowledge and expertise, and most producers of 360 videos simply do not have knowledge, nor time to learn how to master TOA, nor even higher orders (Ambix software goes up to 7th order, which means 64 channels...).

Furthermore, 3rd order Ambisonics requires 16 channels of uncompressed PCM audio, something which is not graceful for videos to be streamed live. In practice, these 3rd-order recordings need to be downloaded first, then sideloaded on the HMD for being watched with Jump Inspector. Currently no VR portal allows for streaming videos with 3rd order Ambisonics soundtrack.

Facebook did a great effort releasing easy-to-use software tools for their proprietary TBE format, but in the hurry of releasing them, they incurred in a severe bug regarding encoding of elevation of the sound source, resulting in a completely distorted soundscape. This error has now been fixed (as of 20 August 2017), and so, finally, Facebook's TBE format becomes a good option for delivering spatial audio soundtracks with decent spatial resolution and good quality. This format has the additional advantage of making it possible to add two not-spatial channels, which remain "head locked" and can be useful for comment or not-diegetic audio information.

But now we have finally a competitive alternative to TBE: Mach1. Understanding SPS is much easier than understanding Ambisonics, and standard production techniques can be used for panning sounds in the SPS format: you just need to know "where" in space each virtual microphone is pointing to. The information presented above provides this knowledge, so synthetic Mach1 soundtracks can be created with a normal mixer. And the 8-channels stream can be easily compressed with AAC or other efficient codecs, making it possible to use it for realtime video streaming.
There are cases, indeed, where the "spatial audio" soundtrack was already captured or created in another format, possibly at higher spatial resolution, such as SPS-32, as raw 32 capsule signals form an Eigenmike, as a Third Order Ambisonics (TOA) with 16 channels, or as TBE 8-channels.
The following chapters explain how to transcode from these formats to Mach1, and vice versa.
 


Converting Ambisonics signals to Mach1

We start from the most basic case, a 1st order Ambisonics (4 channels) stream ready for Youtube. In this case, we generate 8 virtual cardioid microphones employing the Visual Virtual Mic plugin, as shown here:

It must be noted that the Ambi Converter plugin was required, as the recording was in Ambix format, whilst Visual Virtual Mic is an old plugin, still using the obsolete Furse-Malham (FuMa) format. Hence a conversion plugin is required for trasforming the modern Ambix signal in the old FuMa signal, before feeding the Visual Virtual Mic plugin.
This is making use of the maximum number of possible virtual microphones (8) which have been aimed at the vertexes of the cube, according to Mach1 requirements.
It must be clear that a 1st-order Ambisonics stream does not possess all the spatial resolution available with the Mach1 SPS format, hence the sound scene will still remain "smeared" and badly focused, exactly as the original 1st-order Ambix soundtrack.
However, it is possible to get more sharp virtual microphones, with better channel separation, if instead of the Visual Virtual Mic plugin we employ the Harpex-X plugin:

The only problem is that Harpex-X is not free, and quite expensive.

Even better results can be obtained if, instead of starting with just a mere 1st order Ambisonics recording, we have a full third-order Ambisonics recording, which means 16 channels. In this case we can make good use of the additional spatial resolution available in the Mach1 format. The processing in this case requires a third-order Ambisonics plugin for creating the 8 virtual microphone signals, as shown here:

The Ambix Vmic plugin allows for creating virtual microphones with "rectangular" cross-section, hence making it possible to split the acoustical scene in 8 "pyramidal" beams, which cover almost perfectly the sphere, without overlaps or uncovered regions. It is also possible to control the horizontal and vertical beamwidth of the virtual microphones, providing a tradeoff between sharpness and "speaker detent" effect.
The first experiments done transcoding form TOA to Mach1 show excellent results, with a resulting spatial resolution which is almost the same, despite halving the number of channels, and making it possible to store them in highly compressed AAC soundtracks.

 


Converting Mach1 signals to Ambisonics

There are cases when a Mach1 soundtrack needs to be converted back to Ambisonics. Of course, if we want to minimize the loss of spatial information, at least a 2nd order Ambisonics stream must be generated (9 channels), but at that point better to go directly to 3rd order, which is much more widely employed than 2nd order. It is obvious that the resulting 3rd order Ambisonics will never have the same spatial resolution as a "native" 3rd order, as it comes from a source which had just 8 channels.

As the 8 Mach1 signals represent sound sources coming from 8 cardinal directions (vertexes of a cube, as shown earlier on this page) it is quite straightforward to employ an Ambisonics-encoding plugin for converting them back to TOA. For example you can use Ambix Encoder or 03A Panner, as shown here below:


Of course, if one just needs 1st or 2nd order Ambisonics, the same plugin can be employed, recording just the first 4 or 9 channels coming out from it.

And, as usual, if you do not like to install and use these advanced VST plugins, you can always use X-volver with 8 inputs, 16 outputs, with the proper filter matrix called Mach1-to-Ambix-TOA.wav as it will be explained later.

 


Converting TBE signals to Mach1

TBE stands for Two Big Ears, a small Scottish company which was acquired by Facebook for providing a free spatial audio platform for Facebook's 360-degrees "VR" videos.
The TBE format is an 8-channels stream containing a subset of 2nd order Ambisonics signals, by dropping one of the 9 channels (R) and reorganizing and rescaling the others, as explained in details here.

Albeit Facebook's 360-degrees videos with TBE sptial audio soundtrack can be enjoyed only on a small installed base of Samsung Gear VR devices, and hopefully shortly on Oculus Rift devices, the format quickly gained popularity as the sptial resolution provided by its almost-2nd-order Ambisonics, with two additional available channels for "head locked" stereo signals, provides much better results than the currently-dominant Youtube platform with its limited-resolution First Order Ambisonics (4 channels only).

This is due also to the fact that, while on Youtube watching a 360-degrees videos with spatial audio using a browser on a compuer results in a plain stereo mixdown of the signals, instead on Facebook good binaural rendering is provided also for people watching the video on their PC.

Furthermore, for people using HMDs, the Gear VR platform ensures excellent video rendering and high quality headphone listening, at a quality level far superior to Youtube "Google Cardboard" devices. This means that, for a content producer which does not want to release his video and audio streams with reduced quality, the Facebook platform is currently the preferred choice.

But now we have an alternative, thanks to the Samsung VR app (available for Gear VR devices, as an alternative to Oculus Video player): Samsung VR allows to even better video resolution (thanks to the native hardware support for the H265 codec, which allows to break the limit of 4096x2048 of the H264 codec, barely supported by Oculus Video and only on European S7 smartphones), and supports Mach1 audio, which, albeit using 8 channels as TBE, provides more definite spatial resolution, due to the different rendering approach (SPS instead of Ambisonics).

So it could be useful being able to convert from TBE to Mach1 and vice versa.

For converting TBE to Mach1 one has to create a set of 8 virtual microphones with proper directivity (ideally a 2nd order cardioid) pointed at the 8 vertexes of a cube.

Unfortunately the Facebook Spatial Workstation is NOT equipped with a "virtual microphone" plugin, and the "converter" module does not address directly the TBE format, just 1st order Ambix, which means a significant loss in spatial resolution.

The Ambisonics theory says that a virtual microphone V can always be obtained as a linear combination of the available Ambisonics components Ai, with proper gains Gi:

V = A1*G1 + A2*G2 + ... + A8*G8

The gains Gi can be found by a least-squares approach, attempting to minimize the deviations between the polar patterns of the "target" microphone and of the "virtual" one obtained by the above-described linear combination of available components.

The solution has been found using the Solver add-on in Excel, and is shown here:

  TBE(1)* TBE(2)* TBE(3)* TBE(4)* TBE(5)* TBE(6)* TBE(7)* TBE(8)*
Mach1(1) = 0.682217 -0.590557 0.590557 0.591337 0.00 -0.152414 -0.152615 0.152615
Mach1(2) = 0.682217 0.590557 0.590557 0.591337 0.00 0.152414 0.152615 0.152615
Mach1(3) = 0.682217 -0.590557 -0.590557 0.591337 0.00 0.152414 -0.152615 -0.152615
Mach1(4) = 0.682217 0.590557 -0.590557 0.591337 0.00 -0.152414 0.152615 -0.152615
Mach1(5) = 0.682217 -0.590557 0.590557 -0.591337 0.00 -0.152414 0.152615 -0.152615
Mach1(6) = 0.682217 0.590557 0.590557 -0.591337 0.00 0.152414 -0.152615 -0.152615
Mach1(7) = 0.682217 -0.590557 -0.590557 -0.591337 0.00 0.152414 0.152615 0.152615
Mach1(8) = 0.682217 0.590557 -0.590557 -0.591337 0.00 -0.152414 -0.152615 0.152615

The Excel file used for computing these gains can be downloaded here.

So, matrixing the 8 TBE signals with the set of coefficients provided here converts them to Mach1 format.

In Plogue Bidule an 8x8 gain matrix is available as one of the standard processing effects, as shown here:

The problem is to setup this module with all the gains contained in the 8x8 matrix shown above.

The user interface allows for entering the gains in dB. And in dB there are no negative-polarity gains...

So this is not the correct way; for entering the proper gains inside the Matrix Mixer 8 module, it was necessary to save a preset and to edit it manually, as shown here:

The preset was saved in a file named com.plogue.MatrixMixer_8.bprs (it is a Bidule's preset), which in practice is an XML file, which can be opened and edited with a good text editor.

The 64 gain values of the matrix above has been entered as the values of parameters 0 to 63, one column after the other.

After saving the edited preset and loading it inside the Plogue Bidule module Matrix Mixer Mono 8, the module performs the required conversion from TBE to Mach1 with minimal CPU load (it is just a matrix of gains).

Of course, doing the same in different DAWs can be almost impossible, as only Plogue Bidule comes with this handy matrixing module. Hence, for users of DAWs such as Reaper, the preferred way to operate the conversion from TBE to Mach1 is using the X-volver VST plugin, as explained later, and using the proper FIR filter matrix, named TBE-to-MACH1.wav, which is shown here:


Converting Mach1 signals to TBE

Of course also the inverse transformation is possible.

The conversion had been trivial if the "Spatialiser" plugin included in the Facebook's Spatial Workstation had 8 discrete inputs. Instead, for unknown resons, it was equipped with just 7 inputs, so a single instance of the plugin cannot be used for performing the Mach1 => TBE conversion. Currently one has to use TWO instances of the Spatialer plugin, each dealing with 4 channels, as shown here:

Of course in Spatailser_2 (not shown here) the elevation will be set to -35.3 for all the 4 inputs, the azimuth is the same as in Spatialser_1.

It must also be noted that in Spatialser the azimuth angle has reversed polarity, as the Facebook Spatial Workstation does not comply with ISO standards, hence uses a "wrong" azimuth polarity (positive towards right, whilst the ISO standard is that azimuth is positive towards left).

Finally, it must be noted that the "spread" slider has been set to minimum, for ensuring maximum channel separation, and the distance has been set to 4m.

An alternative way to transform SPS (Mach1) to TBE is to first convert Mach1 to Third Order Amsbionics, as shown in previous chapter, and the convert from Ambisonics to TBE using the guide provided here. However, usually a direct conversion is better than two subsequent ones, even if purely linear as in this case.

 


Are these transformations lossless?

All the transcodifications presented here are purely linear operations, so one could think that they are lossless also from a spatial point of view.

Unfortuntaley, this is not true. If we start with an SPS signal, this has the theoretical possibility of infinite channel separation. Which means that we could create a sound field where the sound comes just from one of the SPS directions, and there is no sound coming from elsewhere.

In Ambisonics this is not possible, the sound distribution is always a continuous balloon. This ballon can be stretched significantly towards one direction only by using High Order Ambisonics (order 5 or more). But in practice we are limited to the usage of Ambisonics order 2 or 3, maximum.
This inherently spread the sound all around, reducing spatial "sharpness" in comparison with the SPS format.

Here we present an example of an SPS-8 (Mach1) signal containing white noise being hardpanned in just one of the 8 channels, namely on channel 2 (Front-Right-Up).

This signal is first converted to TBE, and then converted back to Mach1, using the conversion methods described in the previous two chapters.
the following figure shows the Plogue Bidule setup and the VU meters showing the input and output Mach1 signals:

It can be seen that originally there was signal just on Mach1 channel #2, and after going to TBE format and coming back, now there is also some signal in adjacent Mach1 channels, and only the channel 7 (Back-Left-Down), being exactly opposite to the channel which was initally carrying signal, has remained silent.
In practice here you see the gains of second order virtual microphone cardioids, which are not able anymore to separate perfectly the 8 spatial components of the original SPS signal.

The lesson here is simple: if you want maximum spatial resolution, you should generate your signals natively in the SPS format, and do all your processing with P-format signals. If, for any reason, you need to switch to Ambisonics and then come back to SPS, you should use very high order Ambisonics (7th order, possible with Ambix plugins) for preserving as much as possible the spatial sharpness of the SPS signal.

If instead you start with low-order Ambisonics signals, for example with the very handy and free Facebook Spatial Workstation, do not think that converting to Mach1 (SPS-8) at the end of your work will increase the spatial resolution: it will remain exactly the same that was available with the TBE format. The only way of improving the spatial resolution is employing not-linear (parametric) processing, such as using Harpex.

 


Using X-volver for transcoding to Mach1 (and back)

If you do not want to deal with the pieces of software shown above, and you want to use an unique plugin capable of any kind of transcoding, then X-volver is for you.
You just need to employ the proper matrix of FIR filters for operating the transcodification between one format and the other. For example it is shown here how to use X-volver for converting a 16-channels TOA signal into an 8-channels Mach1 signal:

 

The same approach can be employed also for converting from 1st order Ambisonics, from TBE or from raw 32-channels Eigenmike recordings, or for coming bach from Mach1 to other formats.

The FIR filter matrices for all these possible conversions can be downloaded here:
http://www.angelofarina.it/Public/Xvolver/Filter-Matrices/Mach1-Encoder/

 


Native recordings in Mach1 format

Although currently no native Mach1 microphone is available on the market yet, some companies such as Brahma and Core Sound are developing prototypes of these new 8-capsules microphone arrays, as shown here:
 

It must be clarified that the raw signals coming out of these 8 capsules are not the Mach1 signals! Please note that:

  1. SPS signals are COINCIDENT, exactly as Ambisonics signals. The 8 capsules of these new microphone array are SPACED by a little amount from the acoustic center of the microphone array, hence if their signals are used as SPS signals, a number of artifacts will occur due to the small delays, such as comb filtering, chorus effect, etc.

  2. The directivity patterns of the 8 capsules are at best cardioids (1st order), hence the spatial separation among channels is poor. SPS-8 (Mach1) requires that the 8 virtual microphones exhibit at least 2nd order cardioid directivity, ensuring proper channel separation

Hence for recording a Mach1 stream the 8 capsule signals (A-format) must be converted to SPS (P-format) by means of a proper 8x8 filter matrix, or by means of a dedicated A2P plugin which will be supplied together with the microphone. This also means that, if you are not able to create such an A2P filter matrix, you are discouraged to build yourself an 8-capsule microphone array for performing native Mach1 recordings.

As of today, it is possible to record native Mach1 signals employing existing high order microphone arrays, such as the Eigenmike or our own AIDA-RAI cylindrical microphone array:

 

                                 

Eigenmike (Left) and RAI-AIDA Cylindrical Microphone Array (Right)

 

From a 32-channels raw recording (A-format) coming form one of these high-end microphone arrays it is quite easy to derive directly a set of directive virtual microphones with proper directivity and aiming (P-format), employing the software tools provided with them (Eigenstudio and 3DVMS respectively). For example, here it is shown how to create the 8 Mach1 signals from an Eigenmike recording:

In this case it is possible to adjust the beamwidth of the virtual microphones, choosing among cardioids of order 2, 3 and 4.
The results are significantly better than first creating TOA signals (B-format) and then transcoding them as shown in previous chapter, as a single filtering stage going directly from A-format to P-format is always better than two, and making a direct beamforming of the capsule signals avoids some of the known problems of High Order Ambisonics occurring at very low and very high frequencies.
So currenty using an Eigenmike with direct beamforming of the 8 signals is the preferred method for obtaining high quality spatial audio recordings in Mach1 format. This can also be operated in realtime, with reasonably small latency, so the Mach1 signals can be used even for live streaming.

Here it is shown instead how to use the 3DVMS software developed for the RAI-AIDA Cylindrical array (this software, indeed, can be used also for other microphone arrays, such as the Eigenmike, planar and dome arrays with various number of capsules):

In this case the directivity pattern of the virtual microphones can be changed smoothly from order 0.0 (omni) up to order 8.0, but of course for generating an SPS-8 (Mach1) stream the 8 virtual microphones should be chosen in the range of orders 2.0 to 4.0.

 


Guide for preparing video files with Mach1 spatial audio for Samsung VR

1. Sideloading

Here we provide instructions and examples for creating 360 videos including a Mach1 8-channels soundtrack, which can be side-loaded on a Samsung Gear VR device, for being viewed with the Samsung VR app.

The container must be a MP4 file, and the video must be an equirectangular panoramic video, either monoscopic (2:1 format) or sterescopic top-bottom (1:1 format).

Recommended resolutions are 3840x1920 or better for monoscopic, 3840x3840 for stereoscopic.

The Samsung VR app can decode smoothly both H264 and H265 videos. Of course the latter is recommended, providing faster downloads and better quality.

Of course, the equirectangular 4k H264 video coming from a Samsung Gear 360 camera is just perfect, as it could be expected.

Coming to the soundtrack, Samsung VR supports natively the Mach1 8-channels spatial audio format. But the soundtrack cannot be stored as a single stream containing the 8 channels, as the Samsung VR app cannot understand this format. It is necessary to split the 8 channels in 4 stereo soundtracks (2 channels each), and pack these 4 streams together with the video stream inside the MP4 container as 4 seperate AAC streams.

This can be done with the command line tool called FFMPEG. Suppose that we have an H264 video file called test.mp4 and an 8-ch WAV file (48 kHz, 16 bits) called test.wav. The following commands first split the 8-channels WAV file in 4 stereo WAV files (1.wav, 2.wav, 3.wav, 4.wav), and then create a new MP4 file with the proper contents (and proper file name) for being side-loaded onto a Gear VR device running the Samsung VR app:

ffmpeg -i test.wav -map_channel 0.0.0 -map_channel 0.0.1 1.wav -map_channel 0.0.2 -map_channel 0.0.3 2.wav -map_channel 0.0.4 -map_channel 0.0.5 3.wav -map_channel 0.0.6 -map_channel 0.0.7 4.wav

ffmpeg -i test.mp4 -i 1.wav -i 2.wav -i 3.wav -i 4.wav -map 0:v -map 1:a -map 2:a -map 3:a -map 4:a -c:v copy -c:a aac -shortest test_m1spatial.mp4

Please note that the filename of the resulting file, to be side-loaded in the Gear VR device, ends with _m1spatial : this is required for the Samsung VR player app for Gear VR to understand that this is a file containing a Mach1 spatial audio soundtrack.

 

2. Uploading to the Samsung VR portal

First of all, be aware that the MP4 file you just created (and possibly succefully tested) for being side-loaded on your Gear VR device IS NOT GOOD for being uploaded onto the Samsung VR portal.

You need to use a different container, a .MOV file, as the spatial audio soundtrack is better preserved if uploaded as uncompressed PCM. And the MP4 container does not allow for uncompressed PCM audio, so you need to use the .MOV container.

Furthermore, the 8 audio channels must be inside a single stream, the Portal does not recognize the structure made of one video stream and 4 stereo AAC streams.
However, the video stream to be embedded in the .MOV file is exactly the same which was good for side-loading...

So, in a case where the original audio and video files were named test.mp4 (H264 equirectangular video) and test.wav (48 kHz, 16 bits, 8 channels), as in the previous example, the FFMPEG command for packing them inside a .MOV container is the following:

ffmpeg -i test.mp4 -i test.wav -map 0:v -map 1:a -c:v copy -c:a copy -shortest -channel_layout octagonal test_m1spatial.mov

Before being allowed to upload your video to the portal, you need obviously to create an account on it. Once you are logged in, you can select the Creator Portal tab, where you can click on Upload in the menu on the left. This brings you to this page:

Here you need to specify the name associated with your video, a description of it, and to select the proper formats for the video and the audio types. Of course we specify Mach1 Spatial audio type...

Clicking on Next brings you to the true Upload page, where you can drag-and-drop your video file, as shown here:

You can also add tags. The upload is usually quite fast, but after it, you have to wait for a much longer time, during which the uploaded contents are reformatted with different codecs, suitable for the different platforms which can access the Samsung VR contents.

When finally the processing is finished, you can find your video online, using the search button while wearing your Samsung Gear VR HMD....

 

3. Sample files

All the sample audio and video files employed for the FFMPEG commands in the two previous subchapters can be found here:

http://www.angelofarina.it/Public/Xvolver/Filter-Matrices/Mach1-Encoder/Samples/

 


All the contents are Copyright by Angelo Farina, 2017