Ambisonic UHJ format

Ambisonic UHJ format is a development of the Ambisonic surround sound system designed to be compatible with mono and stereo media. It is a hierarchy of systems in which the recorded soundfield will be reproduced with a degree of accuracy that varies according to the available channels. Although UHJ permits the use of up to four channels (carrying full-sphere with-height surround), only the 2-channel variant is in current use (as it is compatible with currently-available 2-channel media). In Ambisonics, UHJ is also known as "C-Format".

Ambisonic B-Format is the standard format for use in the studio. While it is possible to distribute B-Format recordings for decoding and listening by end-users, this is only starting to be more widespread with the advent of software-based players. Traditionally, Ambisonic recordings have been distributed in the form of 2-channel discs, CDs etc. using the 2-channel version of the UHJ encoding hierarchy, which, unlike B-Format, is designed to be stereo compatible.

UHJ was developed by the Ambisonic team, incorporating work done by the BBC (on their quadraphonic system, Matrix H) and Duane Cooper (on Nippon Columbia's UD-4/UMX quadraphonic system) and others, and building on the then-current version of Ambisonics, System 45J. The initials indicate some of sources incorporated into the system: U from Universal (UD-4); H from Matrix H; and J from System 45J.

UHJ is a unique hierarchical system of encoding and decoding directional sound information within Ambisonics technology. Depending on the number of channels available, the system can carry more or less information - but at all times, UHJ is fully stereo- and mono-compatible. Up to four channels (L, R, T, Q) may be used.

In its most basic form, 2-channel (L, R) UHJ, horizontal (or "planar") surround information can be carried by normal stereo signal channels – CD, FM or digital radio, etc. – which may be recovered by using a UHJ decoder at the listening end. Summing the two channels gives a highly compatible mono signal which in fact is a more accurate representation of the two-channel version than summing a conventional "panpotted mono" source. If a third channel (T) is available, this can be used to give improved localisation accuracy to the planar surround effect when decoded via a 3-channel UHJ decoder. The third channel does not have to have full audio bandwidth for this purpose, leading to the possibility of so-called "2½-channel" systems, where the third channel is bandwidth-limited to 5 kHz. The third channel can be broadcast via FM radio, for example, by means of phase-quadrature modulation. This configuration was tested by the Independent Broadcasting Authority (IBA) in the United Kingdom as a method of broadcasting surround recordings. 2½ or 3-channel UHJ delivers the same accuracy as 3-channel (WXY) B-Format. Adding a fourth channel (Q) to the UHJ system allows the encoding of full surround sound with height, known as Periphony, with a level of accuracy identical to 4-channel B-Format.

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