My first plugin release was a free first order encoder called the o1Panner. Today it gets an update to version 1.0.1 and a new name. Here is a list of the updates:
name change from o1Panner to a1Panner to match the naming convention of the rest of the aX Ambisonic Plugins.
Today the aXMonitor plugins get their first major update to version 1.2.2. There are two major updates and one minor updates. Let’s start with the major updates:
The HRTFs used for binaural 3D sound have been regenerated using Google’s own Resonance Audio toolkit for VR audio. These are the same HRTFs used by Google in YouTube 360. The code released by Google is only up to 5th order, but was actually quite simple to extend to 7th order.
A gain control has been added to boost or cut the overall level for convenience.
The minor update is a fix to make sure the plugin reports the correct latency to the host when using the Binaural or UHJ Super Stereo (FIR) methods.
Google have just open sourced their Resonance Audio SDK, including all sorts of tools for spatial audio rendering. This update to ensures that you can aXMonitor ensures that you can mix your content on HRTFs that will be widely used across the industry.
The aXMonitor is available in 3 versions, providing up to first, third and seventh order Ambisonics-to-binaural decoding.
So if you’d like to start mixing your VR/AR/MR audio content just head over to my store. With your support, I can continue to update the aX Ambisonics Plugins to bring you the tools you want and need.
The aXRotate plugin receives an update today to version 1.2.0 and it’s a big one! What’s more, it now comes as a (Universal Binary) AudioUnit format for Mac!
If you have already bought it, you can download the update from the download section of your Account page. If you haven’t, you can pick it up at my online shop!
Version 1.0.0 was a plain vanilla Ambisonics rotation with yaw, pitch and roll control. Version 1.2.0 adds two new features that massively increase its usefulness:
Get head tracking by connect an EDTracker module.
Increase the spaciousness of your static binaural mixes by adding micro oscillations to the sound scene.
The aXCompressor is a compressor VST plugin (Windows and Mac) made specifically for Ambisonics signals. It comes in three variations: first order (a1), third order (a3) and seventh order (a7), allowing you to process . They accept any Ambisonics format that has the W channel as the first channel. This means it works for the more modern AmbiX and legacy FuMa format.
There are plenty of Ambisonics encoders and decoders but not so many things to process between these two points on the signal chain. I wanted to help bring some of the tools we take for granted when working in stereo to VR/AR and immersive audio, hence the aX Plugins. If you’re interested in trying out any of the plugins, including the aXCompressor, you can download the demo versions. You can support future development by making a purchase at from my web shop. Continue reading Product Spotlight: aXCompressor
Today I am very happy to be releasing my latest work: the aX Ambisonics plugins. They are the result of a lot of work and it is great to be able to finally release them into the world.
The aX Plugins are a set of VST plugins intended to make your work with spatial and immersive audio that little bit easier. They come in three variations each with equivalent plugins – a1, a3 and a7.
Which one you choose will depend on the level of spatial resolution you need for your project (how accurately the spatial properties are reproduced to the final listener). The different levels are known in the Ambisonics world as the order and can theoretically go to infinity. In practice we can (thankfully!) stop somewhere quite a bit before infinity! The aX Plugins give you a choice between basic, advanced and future-proof version.
What are the plugins and what can they do?
There are currently seven plugins in each suite with a different purpose. Here is a quick summary:
aXPanner – a stereo to Ambisonics encoder to bring your sounds into the spatial domain.
aXRotate – this plugin will let you rotate a single track or a full sound scene to make sure you have everything exactly where you want it.
aXMonitor – Ambisonics needs a decoder to be listened to. This plugin decodes to binaural 3D audio (over headphones) or to standard stereo. This means you can always share your creativity via traditional channels.
aXCompressor – Ambisonics requires careful handling of the audio to avoid changing the spatial balance. aXCompressor lets you compress the signal without alteration.
aXGate – simiarly, this plugin acts as a noise gate and downwards expander while preserving the spatial fidelity.
aXEqualizer – safely sculpt the tone of your signals.
aXDelay – get creative with five independent delay modules that can be rotated independently of the original signal.
I will be doing a series of posts going into more detail about each plugin. You can also get more information on the product pages. In the meantime, if you are curious, you can download demo versions of these plugins (for evaluation purposes only) here and if you like them you can support future development by buying them from the shop. Thanks!
Audio for VR/AR is getting a lot of attention these days, now that people are realising how essential good spatial audio is for an immersive experience. But we still don’t have as many tools as are available for stereo. Not even close!
This is because Ambisonics has to handled carefully when processing in order to keep the correct spatial effect – even a small phase change between channels significantly alter the spatial effect – so there are very few plugins that can be used after the sound has been encoded.
To avoid this problem we can apply effects and processing before spatial encoding, but then we are restricted in what we can do and how we can place it. It is also not an option if you are using an Ambisonics microphone (such as the SoundField, Tetra Mic or AMBEO VR), because it is already encoded! We need to be able to process Ambisonics channels directly without destroying the spatial effect.
So, what is missing from your 3D sound toolbox? Is there a plugin that you would reach for in stereo that doesn’t exist for spatial audio? Maybe you want to take advantage of the additional spatial dimensions but don’t have a tool to help you do that. Whatever you need, I am interested in hearing about it. I have a number of plugins that will be available soon that will fulfil some technical and creative requirements, but there can always be more! In fact, I’ve already released the first one for free. I am particularly interested in creative tools that would be applied after encoding but before decoding.
With that in mind, I am asking what you would like to see that doesn’t exist. If you are the first person to suggest an idea (either via the form or in the comments) and I am able to make it into a plugin then you’ll get a free copy! There is plenty of work to do to get spatial audio tools to the level of stereo but, with your help, I want to make a start.
I am working on some spatial audio plugins to provide some more tools for VR/AR audio and I am kicking things off with a freebie: the o1Panner. It is free to download from the Shop.
What is it?
The o1Panner a simple first-order Ambisonics encoder with a width control.
How to use it
There are two display types: top-down and rectangular. The azimuth, elevation and width are controlled in different ways in each of these views. The views are selected by right clicking on the display.
For the top-down view, azimuth is controlled by clicking and dragging on the main display, the elevation is controlled by holding shift and dragging up/down and width is controlled by holding ctrl and dragging up/down.
For the rectangular view, azimuth and elevation correspond to the x- and y-coordinates respectively and width is controlled by holding ctrl and dragging up/down.
What does it output?
The output is AmbiX (SN3D/ACN) Ambisonics. This is the format used by Google for YouTube 360 and is quickly being adopted as the standard for Ambisonics and HOA.
What’s coming up?
I am working on several Ambisonics and HOA plugins that will be available in 2018. Some of them will do things that other plugins do, but most of them should do something new. Some of them will do something more creative and experimental. If you want to see a certain effect for spatial audio, just get in touch and let me know what you want. If you’re the first person to suggest a plugin that gets developed then you will get a free copy to say thanks!
In my last post I detailed two methods of converting Ambisonics to stereo. Equations and graphs are all very good, but there’s nothing better than being able to listen and compare for yourself when it comes to spatial audio.
With that in mind, I’ve made a video comparing different first-order Ambisonics to stereo decoding methods. I used some (work-in-progress) VST plugins I’m working on for the encoding and decoding. I recommend watching the video with the highest quality setting to best hear the difference between the decoders.
There are 4 different decoders:
Cardioid decoder (mid-side decoding)
UHJ (IIR) – UHJ stereo decoding implemented with an infinite impulse response filter.
UHJ (FIR) – UHJ stereo decoding using a finite impulse response filter.
Binaural – Using the Google HRTF.
The cardioid decoder more quickly moves to, and sticks in, the left and right channels as the source moves, while this is more gradual with the UHJ decoder. To me, the UHJ decoding is much smoother than the cardioid, making it perhaps a bit easier to get a nice left-right distribution that uses all of the space, while cardioid leads to some bunching at the extremes.
The binaural has more externalisation but pretty significant colouration changes compared to UHJ and cardioid decoding, but also potentially allows some perception of height, which the others don’t.
The VSTs in the video are part of a set I’ve been working on that should be available some time in 2018. If you’re interested in getting updates about when they’re release, sign up here:
Ambisonics, especially Higher Order Ambisonics, is great for 3D sound applications. But what if you have spent a long time mixing for a 3D audio format but want to share it with listeners who are only listening on stereo?
The first thing depends if they’re going to be using headphones or loudspeakers. If they’re using headphones then you can create a binaural mix in the usual way. If they are using loudspeakers then binaural is no longer an option (unless you want to go down the fragile transaural route). In this post we will focus on how you can decode from first order Ambisonics to stereo using one of two common options.
Mid-Side Decoding
The first option is probably the simplest – treat the Ambisonics signal as a mid-side recorded scene by taking the W and Y channels, with W being the mid and Y being the side. Then you can make your left and right (L and R) stereo playback channels using \begin{eqnarray} L = 0.5(W+Y),\\ R = 0.5(W-Y) \end{eqnarray}
This is effectively the same as recording a sound field with two cardioid microphones pointing directly left and right. Sounds panned to 90 degrees will play only through the left loudspeaker and those at -90 degrees through the right.
The advantage of this sort of decoding is that it is very conceptually simple and, as long as your DAW can handle the routing, it is even possible to do without any dedicated plugins. It also results in pure amplitude panning, meaning that it has all of the advantages and disadvantages of standard intensity-stereo. However, we’ve got another option to choose from when we want to play back over a stereo system that has some advantages.
UHJ Stereo
A more complex and interesting technique is UHJ. We’re only going to go over how UHJ for stereo listening, but it is worth noting that UHJ is mono compatible and that a 4-channel version exists from which full first-order Ambisonics information that can be retrieved via correct decoding. 3-channel UHJ can get you a 2D (horizontal) decoder by retrieving the W, X and Y channels. A nice property of the 3- and 4-channel versions is that they contain the stereo L and R channels as a subset. This means, importantly, 2-channel UHJ does not require a decoder when played back over two loudspeakers. All you need to do is take the first two channels of the audio stream.
The stereo L and R channels can be calculated using the following equations:\begin{eqnarray} \Sigma &=& 0.9397W + 0.1856X \\ \Delta &=& j(-0.3430W + 0.5099X) + 0.6555Y\\ L &=& 0.5(\Sigma + \Delta)\\R &=& 0.5(\Sigma – \Delta)\end{eqnarray} where \(j\) is a 90 degree phase shift.
You can see from these equations, converting to UHJ from first-order Ambisonics results in signals with phase differences between the L and R channels. This creates quite a different impression to the kind of mid-side decoding mentioned above. There will obviously be some room for personal taste as to whether or not UHJ is actually preferred to mid-side decoding. Sound sources placed to the rear of the listener are more diffuse when reproduced of a stereo arrangement than those at the front, while for mid-side decoding there is no sonic distinction between a sound panned to 30 degrees or to 150 degrees.
Beyond front-back distinction, UHJ can actually result in some sounds appearing to originate from outside the loudspeaker pair by a small amount. This is why it is sometimes referred to as Super Stereo. In my experience, this effect is very dependent on the sound being played, both its frequency content and how transient it is.
Figure 1: The (high frequency/broadband) localisation curves for UHJ and Mid-side decoding of an Ambisonic sound source over two loudspeakers at +30 and -30 degrees.
Because UHJ stereo relies on phase differences between the two channels, any post-processing or mastering applied should preserve the phase relationship between L and R, otherwise there is a very real risk that the final presentation will be phase-y and spatially blurred.
Figure 1 shows the localisation curves for a sound played back over a stereo system where the signal in the Ambisonics domain is panned fully round the listener. Obviously the sound stays to the front, but the actual trajectories between UHJ and mid-side decoding are quite different. (These localisation curves were calculated using the energy vector model of localisation, so they are most appropriate for mid/high frequencies and broadband sounds).
Which of the two stereo loudspeaker decoding strategies you’ll want to use will depend on the needs of your project. Mid-side decoding is simpler and results in pure amplitude panning. UHJ can result in images outside of the loudspeaker base, but relies on the phase information being preserved. If you want to retrieve any spatial information then UHJ is absolutely the way to go.
Tools for Stereo Decoding
I have an old Ambisonics to UHJ transcoder VST that you can download here, but they are old and I am not sure how compatible they are with newer version of Windows and Mac OSX. To remedy that, I’ve been working on an updated version that will provide simple first-order to stereo decoding. Just select which method you want to use and pump some Ambisonics through it. Keep an eye out in the near future for when it is made available!
I’m curious to hear from anyone who has used both techniques what you prefer. Leave a comment below!
This post is part of a What Is… series that explains spatial audio techniques and terminology.
The last post was a brief introduction to Ambisonics covering some of the main concepts of first-order Ambisonics. Here I’ll give an overview of what is meant by Higher Order Ambisonics (HOA). We’ll be sticking to the more practical details here and leaving the maths and sound field analysis for later.
Higher Order Ambisonics (HOA) is a technique for storing and reproducing a sound field at a particular point to an arbitrary degree of spatial accuracy. The degree of accuracy to which the sound field can be reproduced depends on several elements: the number of loudspeakers available at the reproduction stage, how much storage space you have, computer power, download/transmission limits etc. As with most things, the more accuracy you want the more data you need to handle.
Encoding
Spherical harmonics used for third-order HOA (image by Dr Franz Zotter https://commons.wikimedia.org/wiki/File:Spherical_Harmonics_deg3.png)
In its most basic form, HOA is used to reconstruct a plane wave by decomposing the sound field into spherical harmonics. This process is known as encoding. Encoding creates a set of signals that depend on the position of the sound source, with the channels weighted depending on the source direction. The functions become more and more complex as the HOA order increases. The spherical harmonics are shown in the image up to third-order. These third-order signals include, as a subset, the omnidirectional zeroth-order and the first-order figure-of-eights. Depending on the source direction and the channel, the signal can also have its polarity inverted (the darker lobes).
An infinite number of spherical harmonics are needed to perfectly recreate the sound field but in practice the series is limited to a finite order \(M\). An ambisonic reconstruction of order \(M > 1\) is referred to as Higher Order Ambisonics (HOA).
An HOA encoded sound field requires \((M+1)^{2}\) channels to represent the scene, e.g 4 for first-order, 9 for second, 16 for third, etc. We can see that very quickly we require a very large number of audio channels even for relatively low orders. However, as with first-order Ambisonics, it is possible to do rotations of the full sound field relatively easily, allowing for integration with head tracker information for VR/AR purposes. The number of channels remains the same no matter how many sources we include. This is a great advantage for Ambisonics.
Decoding
The sound field generated by order 1, 3, 5 and 7 Ambisonics for a 500 Hz sine wave. The black circle in the middle is approximately the size of a listener’s head.
The encoded channels contain the spatial information of the sound sources but are not intended to be listened to directly. A decoder is required that converts the encoded signals to loudspeaker signals. The decoder has to be designed for your particular listening arrangement and takes into account the positions of the loudspeakers. As with first-order Ambisonics, regular layouts on a circle or sphere provide the best results.
The number of loudspeakers required is at least the number of HOA encoded channels coming in.
A so-called Basic decoder provides a physical reconstruction of the sound field at the centre of the array. The size of this physically accurately reconstructed area increases with increasing order but decreases with frequency. Low frequency ranges can be reproduced physically (holophony) but eventually the well-reproduced region becomes smaller than the size of a human head and decoding is generally switched to a max rE decoder, which is designed to optimise psychoacoustic cues.
The (slightly trippy) animation shows orders 1, 3, 5 and 7 of a 500 Hz sine wave to demonstrate the increasing size of the well-reconstructed region at the centre of the array. All of the loudspeakers interact to recreate the exact sound field at the centre but there is some unwanted interference out of the sweet spot.
Why HOA?
Since the number of loudspeakers has to at least match the number of HOA channels the cost and practicality are often the main limiting factor. How many people can afford 64 loudspeakers needed for a 7th order rendering? So why bother encoding things to a high order if we are limited to lower order playback? Two reasons: future-proofing and binaural.
First, future-proofing. One of the nice properties of HOA is that you can select a subset of channels to use for a lower order rendering. The first four channels in a fifth-order mix are exactly the same as the four channels of a first-order mix (see the spherical harmonic images above). We can easily ignore the higher order channels without having to do any approximative down-mixing. By encoding at a higher order than might be feasible at the minute you can remain ready for a future when loudspeakers cost the same as a cup of coffee (we can dream, right?)!
Second, binaural. If the limiting factors to HOA are cost and loudspeaker placement issues then what if we use headphones instead? A binaural rendering uses headphones to place a set of virtual loudspeakers around the listener. Now our rendering is only limited by the number of channels our PC/laptop/smartphone can handle at any one time (and the quality of the HRTF). The aXMonitor is an example of an Ambisonics-to-binaural decoder that can be loaded into any DAW that accepts VST format plugins, plug Pro Tools | Ultimate in AAX format.
The Future
As first-order Ambisonics makes its way into the workflow of not just VR/AR but also music production environments, we’re already seeing companies preparing to introduce HOA. Facebook already includes a version of second-order Ambisonics in its Facebook 360 Spatial Workstation [edit: They are now up to third order!]. Google have stated that they are working to expand beyond first-order for YouTube. I have worked with VideoLabs to include third-order Ambisonics in VLC Media player. This is in the newest version of VLC.
Microphones for recording higher than first-order aren’t at the stage of being accessible to everyone yet, but there are tools, like the a7 Ambisonics Suite that will let you encoded mono signals up to seventh-order, as well as to process the Ambisonics signal. There are also up-mixers like Harpex if you want to get higher orders from existing first-order recordings.
All of this means that if you can encoded your work in higher orders now, you should. You do not want to have to go back to your projects to rework them in six months or a year when you can do it now.
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