Introduction to Ambisonics

The increase of Ambisonic contact impenetrable magnetic discing dodges started in the 1970s. They were found on a mathematical model of take upional psychoacoustics, which was positive by Michael Gerzon. The model which he developed, described draw a bead onional psychoacoustics in a mathematical form, so that it could conveniently be utilize in calculations relating to surround bottom ashes (M exclusivelyham 10) Ambisonics discount be defined as a method of written text cultivation about a gruellingfield and reproducing it over any(prenominal) form of talker roam so as to produce the depression of hearing a true up three dimensional hearty image.Simply put, Ambisionics is a dodge of save and playback of sound fields. The model was developed by Gerzon. It personad several previous theories of loaclization, and accordingly was much than directional as comp atomic number 18d to other system of ruless, resulting in benefits such as reduced earreach fatigu e, uncorrupted inter-loud speaker system imaging with reduced susceptibility to the andiron effect the tendency for app arnt source locations to be pulled towards the closest loudspeaker. This later feature improved image stability (Cotterell ch-1 16)Ambisonics is basic wholey a deuce-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 host of mike capsules are built into one microphone called a Soundfield microphone. This Soundfield mike recording are then processed and encoded into a modified coiffure, called B- initialize, which may be written on and cared victimization galore(postnominal) a(prenominal) disparate types of media.In the endy, a decipherer is used to process and recover the soundfield, This brush tally be played back in many types of listening rectifyup, giving a unique output for every speaker available (Adams 2) As is seen in above, Ambisonics employs a portend set, known as B- set up. This is based on the linguistic rule of encoding direction, without reference to the loudspeaker layout used for counterpart. This results in Ambisonic systems being flexible to multiple loudspeaker layouts. Using an Ambisonic decoder, appropriate loudspeaker feed signals arsehole be derived from the transmitted B- do signals.Generally the number of loudspeakers exceeds the number of B-format signals. This is make to obtain good carrying into action from Ambisonic systems, since in the number loudspeakers usually hurls better results (Adams 2) It is observed that the chief nates process Soundfield education in several carriages. However, much of this impact is seen to rely on the differences in the midst of sounds reaching the ears. As an example, if a sound is played to on the right side, because the r ight ear is closer, it will firstborn 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 except if because it is farther away, however because the body and head will absorb some of the sound. This is termed as Interaural Level Difference (ILD). The brain alike pays attention to the measure between and relative loudness of sounds coming directly from the source and sounds reflected off of walls and other surfaces. This agent that sources that are farther away tend to become more reflected energy than direct energy reaching the ears.These ear-brain interactions are called psychoacoustics (Adams 2). When a Soundfield recording is done, more efforts are made to gather and record as much entropy about a very small people of space, as possible. This information comprises of just measurements of air insistency and air pressure changes. By just adding an omnidirectional microphon e to the pair of figure octet units, it is seen that one arse capture all this information, with simple low ramble microphones. The three hree capsules should be acoustically at exactly the said(prenominal) part in the soundfield i.e. , The capsules are coherent to be truly coincident. This becomes even more difficult when theer is an addition of an up-down oriented figure eight capsule in align to record height information.. This problem has been overcome in the Soundfield microphone which uses four small capsules situated on the surface of a high-risk sphere to sample the incoming sounds (Mallham 12) Building upon recording techniques for stereophonyscopic photographphonic 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 likewise the pressure changes in different directions bearing-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 populate, which reconstructs what was going on in that small bulk at the attendants location (Adams 2). In Ambisonics the horizontal figure eight units are mounted present-back and side-to-side rather than at 45 degrees (Adams 2).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 meeters 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 (Mallham 14)Ambisonic offers a hierarchy of encoding schemes from a stereo compatible UHJ format (Sinclair 27) There is no withdraw to consi der the actual details of the reproduction system when doing the original recording or synthesis Encoding Equations The position of a sound in spite of appearance a three dimensional soundfield is encoded in the four signals which make up the B format X = cosA. cosB (front-back) Y = sinA. cosB (left-right) Z = sinB (up-down) W = 0. 707 (pressure signal) Where, A is the anti-clockwise angle from centre front and B is elevation (Mallham 14)B- Format A number of different signal sets may be used at some stage of an Ambisonic system A-format signals The output signals of the microphone capsules making up a soundfield microphone. This signal set is not available to the outside orb it is utilised only within the soundfield microphone itself. C-format signal The signal set which is conveyed to the listener via a recording or transmission medium, when it differs from B-format, is termed C- format. The C is sometimes said to stand for consumer.The C-format signal sets were proposed as p art of the initial development of Ambisonics are together known as the UHJ hierarchy (Cotterell Ap-2 16) With Ambisonics, a sound field is decomposed into spherical harmonic components, termed W, X, Y and Z. These are collectively called B-Format (Leese 7). B-format is based on the principle of encoding direction, without reference to the loudspeaker layout used for reproduction. As a result of this, ambisonic systems are adaptable to multiple loudspeaker layouts an ambisonic decoder derives appropriate loudspeaker feed signals from the transmitted B-format signals.To obtain good performance from ambisonic systems, the number of loudspeakers should exceed the number of B-format signals used a further increase in the number of loudspeakers will usually give improved results (Cotterell ch-1 18) B- format signals can also be expressed in terms of derivatives of sound pressure. The nth-order B-format signal set consists of the signals obtained from coincident microphones having polar pa tterns corresponding to the (n +1) linearly unaffiliated spherical harmonics of all orders up to and including n.A signal set of any order can be extended to a higher order that by augmenting it with additional signals it is not necessary to change any of the actual signals. Thus, an nth order soundfield microphone is distinguished by its ability to forget outputs which are the nth-order B-format signal (Cotterell ch-4 1) B-format signal is the primary signal format for Ambisonic use. Ideally, the B-format signals would be communicated directly to the listener.Unfortunately, this has not always been possible in particular, the need to distribute recordings via two-channel media, retaining compatibility with existing stereo and mono equipment, led to the need for alternate(a) signal formats to be employed (Cotterell Ap-2 16) Enhanced B-Format BE-, BF- & BEF-Format Enhanced B-format signal sets puzzle been proposed in connection with B-format decoders, which are optimized speci fically for use with HDTV, or more generally for use in support of visual media (Cotterell Ap-2 19)The primary need is to produce a frontal sound stage which is more invariable with respect to movement by the listener, and specifically to lock centre-front acoustic images in place with respect to a screen. Thus, the motivation is substantially the same as for the use of the centre channel in cinema-oriented surround sound formats. Hence, two extra signals are defined, denoted as E and F, which deem directional resolution patterns. (Cotterell Ap-2 16) Ambisonics and Stereo The B format signals are not at all stereo compatible.However, it is possible to combine the three (X,W,Y) components required for horizontal march in such a way that not only is a good stereo compatible two channel system produced but with a suitable decoder much of the original surround sound image can be recovered. This resulting Soundfield is also not perfect, but carefully designing the encoding equation s can make it possible to place the defects in areas where the ear is less susceptible (Mallham 10) This encoding method, called UHJ coding, is used to produce stereo compatible Ambisonic records, tapes and broadcasts.The X,Y and W signals are combined in the form of a two-channel compatible stereo signal (Brice 225). This is done using the pursual equations Left = (0. 0928 + 0. 255j)X + (0. 4699 0. 171j)W + (0. 3277)Y Right= (0. 0928 0. 255j)X + (0. 4699 + 0. 171j)W (0. 3277)Y The above decoding equations are such that a decoder for any of the levels will always extract the correct information from high level inputs in other words the system is upward compatible (Mallham 10) B+ format Another extension of the B-format, mentioned above, was developed by Dr. Thomas subgenus Chen.He calls it as B+format. This gives an optional enhancement to the Ambisonics listening experience. Definition B+ format is the standard 4 bring of B format square known as W, X, Y, and Z plus 2 brin g of L/R dry stereo recording. This method of recording separates the dry/direct sound from the ambient/room sound in recording and reproduction. Each serves a distinct purpose and is ideally worthy to it. (Chen 1) The + which is seen in B+ Format denotes the presence of a close-miked standard stereo recording simultaneous with a soundfield recording.This is decoded to the front-half of the rig to give the impression that in that location is a set of stereo speakers on its surface in front of the listener. The angle between them is around 60 degrees (Adams 2) Chen is the inventor of B+ format. This format extends Ambisonic B format with two additional bring. Going by his definition of the format, one can see that, Chen suggested B+ format by combining the 1st order ambisonic B-format with two additional bring L/R dry channel recording (Trond 2) One way to look at this idea is that the format separates the dry signal and atm.This means that the dry and ambient signal can be dyna mically jumbleed depending on the space used for playback as well as the preferences of the listener (Trond 3) Why use the B+ format Chen considers that the sound system he ahs created, would provide the following feature. Ambient The system would be ambient. This means that the whole acoustic event could be captured and re begind, such that it manages to give convincing impression to the listeners ears, which a conventional stereo sound systems doe not. A soundfield must be throw on reproduction. Accurate The system would be accurate. This means that the direction of the sounds must be exact, as per the specifications or requirements i. e. front-stage, to the sides, above or below, behind. Sounds would be free from sonic distortions of tone, timbre or position. The system must equally suit all types of singers, without any particular voice sounding good or bad. Scaleable The system would support the group listening no matter the size of the group, and the same system coul d be used by an individual without compromising on the sound quality.The scaling factor would be adjustable Approachable The system would be easy use without any complications, physical or otherwise, on the behalf of the listener. For e. g. No head clamps or enforced body positions or nix head directions or microscopic sweet spots or dogmatic narrow listening seat. Practical The system would be easily orient to suit the ordinary domestic listening environments, at least as good as what conventional a stereo does. Compatible The system would be able to satisfactorily replay the recordings of mono and stereo material while sound preserve using the technology would replay satisfactorily on conventional stereo systems or would be able to quickly 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. (Chen 3)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 vestibular sense 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 (Chen 3) Time, Location and Spatial original The direct sound gives the scoop out directional and in strumental timbre information. Hence, the direct sound should arrive earlier than the ambient signal. Also, the direct sound doesnt have any spacious or enveloping qualities (Chen 4) The ambient sound best gives spaciousness, spatial depth and envelopment to the reproduction of sound. B format exhibit 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 copesettic 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 any SACD or DVD-A using MLP compression. Chen says that the good 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) save Techniques for B+ FormatDirect save 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. put down 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 impress 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 bloc (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 dishonor 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, intermixture and signal manipulation subsystems. He intends to make this software available commercially, either through Creamware or direct, in the near future.