Introduction to Ambisonics

The development of Ambisonic surround sound systems started in the 1970’s. They were based on a mathematical model of directional psychoacoustics, which was developed by Michael Gerzon. The model which he developed, described directional psychoacoustics in a mathematical form, so that it could conveniently be used in calculations relating to surround sound system (Mallham 10) Ambisonics can be defined as a method of recording information about a soundfield and reproducing it over some form of loudspeaker array so as to produce the impression of hearing a true three dimensional sound image.

Simply put, Ambisionics is a system of recording and playback of sound fields. The model was developed by Gerzon. It used several previous theories of loaclization, and hence was more directional as compared to other systems, resulting in benefits such as reduced listening fatigue, good inter-loudspeaker imaging with reduced susceptibility to the detent effect – the tendency for apparent source locations to be pulled towards the closest loudspeaker. This later feature improved image stability (Cotterell ch-1 16)

Ambisonics is basically a two-part system of recording and playback, which is built upon strong mathematical foundations and theories of human hearing. The term soundfield, mentioned above, is a word used for describing sounds in the environment. It usually implies 3 dimensions. For recording in Ambisonics, an array of microphone capsules are built into one microphone called a Soundfield microphone. This Soundfield mike recording are then processed and encoded into a special format, called B-format, which may be written on and distributed using many different types of media.

In the endy, a decoder is used to process and recover the soundfield, This can be played back in many types of listening setup, giving a unique output for every speaker available (Adams 2) As is seen in above, Ambisonics employs a signal set, known as “B-format”. This is based on the principle of encoding direction, without reference to the loudspeaker layout used for reproduction. This results in Ambisonic systems being adaptable to multiple loudspeaker layouts. Using an Ambisonic decoder, appropriate loudspeaker feed signals can be derived from the transmitted B-format signals.

Generally the number of loudspeakers exceeds the number of B-format signals. This is done to obtain good performance from Ambisonic systems, since in the number loudspeakers usually gives better results (Adams 2) It is observed that the Brain can process Soundfield information in several ways. However, much of this processing is seen to rely on the differences between sounds reaching the ears. As an example, if a sound is played to on the right side, because the right ear is closer, it will first receive the sound than the other ear.

This is termed as Interaural Time Delay (ITD). Again, in this case the sound would be quieter in the left ear, not only because it is farther away, but because the body and head will absorb some of the sound. This is termed as Interaural Level Difference (ILD). The brain also pays attention to the timing between and relative loudness of sounds coming directly from the source and sounds reflected off of walls and other surfaces. This means that sources that are farther away tend to have more reflected energy than direct energy reaching the ears.

These ear-brain interactions are called psychoacoustics. When a Soundfield recording is done, more efforts are made to gather and record as much information about a very small volume of space, as possible. This information comprises of just – measurements of air pressure and air pressure changes. By just adding an omnidirectional microphone to the pair of figure eight units, it is seen that one can capture all this information, with simple low order microphones. The three hree capsules should be acoustically at exactly the same place in the soundfield i.e.

The capsules are arranged to be truly coincident. This becomes even more difficult when theer is an addition of an up-down oriented figure eight capsule in order to record height information.. This problem has been overcome in the Soundfield microphone which uses four small capsules situated on the surface of a notional sphere to sample the incoming sounds.) Building upon recording techniques for stereo developed by Alan Blumlein in the 1930s, Ambisonics extends this recording of information into the third dimension.

The idea is to record not only the pressure at a location, but also the pressure changes in different directions – front-back, left-right, and up-down. With proper decoding of this information using mathematics as well as psychoacoustics, a soundfield can be produced by speakers in the room, which reconstructs what was going on in that small volume at the listener’s location. In Ambisonics the horizontal figure eight units are mounted front-back and side-to-side rather than at 45 degrees.

Basic Ambisonics Technology The Ambisonic surround sound system can be considered as a two part technological solution to the problems of encoding sound directions, and reproducing them over loudspeaker systems in such a way that the listeners ears are under the impression that the sounds they are gearing are from speakers which are correctly located. Technically speaking, this can take place over a 360 degree horizontal only soundstage i. e. pantophonic systems or over the full sphere i. e. periphonic systems.

The system would be able to satisfactorily replay the recordings of mono and stereo material; while sound recorded using the technology would replay satisfactorily on conventional stereo systems or would be able to readily convert to match them. For example, FM stereo being broadcast as a mono-compatible Sum and Difference rather than directly as a Left and Right signal. (Chen 2) As mentioned above, B+ Format consists of B format ambient-recorded signal plus two channels of L/R direct recorded signal. It is recorded on 6 channels and when reproduced it is decoded into as many channels as the listeners wishes.

By using the two channels of L/R direct recorded signal , the directional sound clues from the direct signal as well as the sound of the instrument are present only in the front i. e. – instrument setting experience. There is no vertical height information in this pair, which is added to the frontal speaker array (Chen 3) By enabling the ambient and direct information to be stored separately, the listener can choose the balance of direct and ambient information at the time of reproduction. This will allow for many different sized reproduction spaces. The larger space will need less ambient information while the smaller spaces will need more.

In addition the listener can choose the amount of preferred ambience. Time, Location and Spatial Representation The direct sound gives the best directional and instrumental timbre information. Hence, the direct sound should arrive earlier than the ambient signal. Also, the direct sound doesn’t have any spacious or enveloping qualities. The ambient sound best gives spaciousness, spatial depth and envelopment to the reproduction of sound. B format presentation is known to be inaccurate in presenting location and therefore many scientists are trying to improve the reproduction with higher order of B format decoding.

Also, first order of B format is considered to be satisfactory for presenting the ambient sound because it is not well localized (Chen 4). Storage Requirements As mentioned in the section above, B+ Format recordings require 6 channels of storage. This can be obtained with either SACD or DVD-A using MLP compression. Chen says that the advantage of using B+ format disks is that one is not limited to the speaker arrangement that the format will dictate but rather a decoder will determine your listening set up or you can set the decoder for your listening set up (Chen 5) Recording Techniques for B+ Format

Direct Recording Stereo Mics – Recording of the direct channel done by conventional means. Can use stereo microphone techniques: X/Y, M/S or OCT. Stored as L/R or M/S. Direct signal should contain little reverberant information. Recorded using direction microphones. Spot Mics – Spot microphones or flanking microphones to balance recorded sound. Need to add ambience to the spot microphones and to flanking microphones. The ambience added should be in B format and added to the B format channel. Multi Track Techniques – Multi-track techniques used to generate the direct signal and to add ambience in B format.

B-Format Recording Room Recording – Room ambient recording done with the Soundfield microphone generating a B format output. The location of the Soundfield microphone is typically at the location in the room where the direct and reverberant sounds are equal. B-Format ambience by convolution –This is a digital technique where a room ambience is measured in B format and then is impressed upon another signal. B format ambient information can also be obtained with multiple reverberation devices. At least 3 engines are needed to generate B format reverberance, one for each axis (Chen 5)

Techniques for Monitoring B+ Format Ideal decoding with 16 channels Decoding of the B+ format is ideally presented as 16 channels. This is arranged as two rings of 8 speakers with 45 degrees of separation between them. From stereo, it is known that the maximum separation between speakers without a hole in the middle is 60 degrees Thus by using an upper and lower ring of eight speakers vertical/height information can be presented. The listener usually presents the L/R stereo on the front speakers without height information.

In this case the listener should be able to adjust the balance of dry versus ambient sound. For this Chen has developed decoders for 16, 12, 10 channels of 3D surround or without height as 8, 6, or 5 channels (Chen 6) Decoding in the present control room The listener can use the present control room to work in B+ format. In this case the decoding is done with 60-degree speaker spacing with two rings of 6 speakers. The front up and down speakers will be combined and the present control room speakers will be used for the sum of the front up and down signals.

After this listener can use his present console to obtain the L/R stereo mix and use a pair of auxiliary sends to sent to the B format ambience generator (Chen 6) Software for the B+ format Dr Thomas Chen has developed exciting and powerful software for the Creamware Pulsar system, providing multiple, configurable B-format and B+ format decoding, mixing and signal manipulation subsystems. He intends to make this software available commercially, either through Creamware or direct, in the near future.