• Rory Conder


15th November

The Ambisonic system offers a complete hierarchical approach to directional sound pickup, storage or transmission and reproduction, which is applicable to mono, stereo, horizontal, surround-sound, or full periphonic reproduction, including height information.

Depending on the number of channels employed it is possible to represent a number of dimensions in the reproduced sound.

A number of formats exist for signals in ambisonics, and these are as follows:

  1. A-format for microphone pickup,

  2. B-format, for studio equipment and processing,

  3. C-format for transmission

  4. D-format for decoding and reproduction

  5. UHJ (Universal HJ(earlier sound system) maintains multi-channel surround sound while sending stereo and mono channels to a non surround audience.

The source of an ambisonic signal may be an ambisonic microphone such as the Calred-Soundfield, or an artificially panned mono signal split into the correct B-format components and placed in position around the listener by adjusting the ratios between the signals.

In a microphone W,X,Y, and Z are corrected electronically for the difference in level, so as to compensate for the difference between pressure and velocity components. For example, W is boosted at very low frequencies since it is derived from velocity capsules which do now have the traditionally extended bass response of omnis.

B-format signals may also be created directly by arranging capsules or individual microphones in the B-format mode (two or three figure of 8s 90degrees plus an omni. the Z-component is not necessary for horizontal information, If B-format signals are recorded instead of speaker feeds (D-format), subsequent manipulation of the soundfield is possible, and the signal will be somewhat more robust to interchannel errors.

The C-format consist of four signals, C, R, T and Q, which conform to the UHJ hierarchy, and are the signals used for mono or stereo-compatible transmission or recording.The C-format is in effe, a useful consumer matrix format. L is a two-channel-compatible left channel, R is the corresponding right channel, T is a third channels which allows a more accurate horizontal decoding and Q is a fourth channel containing height information. The proportions of the B-format signals which are combined to make up a C-format signal have been carefully optimised for the best compatibility with the conventional stereo and mono reproduction.

Two, three and four channels of the C-format signal may be used depending on the degrees of directional resolution required, with a 2.5 channel option as well, where the 3rd channel, T has limited bandwidth. For stereo compatibility only L and R are used.

D-format signals are those distributed to loudspeakers for reproduction, and are adjusted depending on the selected loudspeaker layout. They may be derived from either B or C format signals using an appropriate decoder, and the number of speakers is not limited in theory, nor is the layout constrained to a square. Four speakers give adequate surround sound, whilst six provde better immunity against the drawing of transient and sibilant signals towards a particular speaker, whilst eight may be used for full periphony with height.

The decoding of B- and C- format components into loudspeaker signals can be explored further:

decoding involved the passing of two or more UHJ signals via a phase-amplitude martix, resulting in B-format signals that ar esubjected to shelf filters (in order to correct the levels for head-related transfer functions such as shadowing and diffraction). These are passed through an amplitude matrix which feeds the loudspeakers. A layout control is used to vary the level sent to each speaker depnending on thephysical arrangement of the speakers, e.g. ControlGris plugin.

Higher order ambisonics or "Spherical harmonics" can be enhanced by the addition of two further components on the horizontal plane, U and V. These can not so easily be recorded but can be synthesised for sound modelling and rendering applications.

B-format-to-5.1 decoding, using "Vienna decoders" after the AES conference that described them first. The sound image is in this case "front-biased" with better localisation characteristics in the frontal region than the rear, owing to the loudspeaker layout, see direct/ambient and multiple sound sources layouts of surround sound systems in other blog entries.

"The trick is to stop thinking horizontally and to start thinking and mixing vertically. Normal stereo is left to right. Stereo mics are made to provide left/right pickup. Pan controls on most portable mixing panels sweep left to right.

But when you are in the audience, staring at a relatively narrow screen that occupies only a fraction of your vision width – just how believable is a hard left or right sound? Probably more of a mental distraction than an enhancement of the viewing experience – considering that screen left and screen right are still pretty much centered in front of you.

Where is the viewer in terms of depth? We are looking through a spacial window. Objects are in front of us, but it is possible for them to travel to or from that window (rear to front or vice versa).

If we get used to a balanced left/right sound in front of us, and then it suddenly whooshes over and to the rear of us (or from the rear to forward) – it catches us unexpected. We think 3D or immersive.

Achieving this effect on a shoestring budget is remarkably simple. As you do your final mixdown at home, treat “left” as front, and “right” as rear. It might help to reposition your speakers in your edit bay.

When it comes time for the presentation, make sure that you have a minimum of four speakers to plug into your portable PA amp. Most of the field PA systems have enough jacks to feed at least four outboard speakers, if not more.

But do not place your speakers in the typical left/right arrangement on either side of the screen. Instead, place your LEFT speaker and AUX LEFT speaker flanking on both sides, near the front. Put your RIGHT and AUX RIGHT speakers at the back of the room, also on both sides.

Your audience will not hear left/right stereo, but they won’t miss it either, because of the relative small screen size and narrow seating arrangements. They will be impressed and surprised by front/back panning!

And if you are using stereo mics during production, try orienting them front/back rather than left/right. You will end up with sounds that will amaze."

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