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, only the 2-channel variant is in current use. In Ambisonics, UHJ is also known as "C-Format".

UHJ hierarchy

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 two-channel discs, CDs etc. using the two-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 and Duane H. Cooper 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 ; H from Matrix H; and J from System 45J.
Image:Uhj-hierarchy.gif|frame|right|The theoretical path from B-format to the various stereo/mono-compatible UHJ variants. In fact many mixing applications went straight from multitrack to 2-channel UHJ before the advent of digital audio workstations and current Ambisonic mixing developments.
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 may be used.
In its most basic form, two-channel UHJ, horizontal 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 is a more accurate representation of the two-channel version than summing a conventional "panpotted mono" source. If a third channel is available, it can be used to give improved localisation accuracy to the planar surround effect when decoded via a three-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 in the United Kingdom as a method of broadcasting surround recordings. 2½ or three-channel UHJ delivers the same accuracy as three-channel B-Format. Adding a fourth channel to the UHJ system allows the encoding of full surround sound with height, known as Periphony, with a level of accuracy identical to four-channel B-Format.
Although there are some compromises with accuracy of localisation in the two-channel UHJ system, two-channel UHJ is the format commonly used for distribution of Ambisonic recordings. Two-channel UHJ recordings can be transmitted via all normal stereo channels and any of the normal two-channel media can be used with no alteration.
It is stereo compatible in that, without decoding, the listener perceives a stereo image, but one that is significantly wider than conventional stereo. The left and right channels can also be summed for a very high degree of mono-compatibility. Replayed via a UHJ decoder, the surround capability is revealed.

Number of channels	Decoder?	Capabilities	Typical applications	Channels	B-Format equivalent	Original designation*
4	Yes	Full-sphere surround	DVD, HD disc	LRTQ	WXYZ	PHJ
3	Yes	Full horizontal surround	DVD, HD disc	LRT	WXY	THJ
"2½"	Yes	Full horizontal surround	FM radio	LR band-limited T	WXY	SHJ
2	Yes	Horizontal surround	CD, Stereo Radio General 2-channel media	LR	-	BHJ
2	No	Stereo	CD, Stereo Radio General 2-channel media	LR	-	-
1	No	Mono	Radio	LR	-	-

*These designations are seldom used today

Decoding UHJ

A UHJ decoder, primarily for the 2-channel format, was the original component required for consumer experience of Ambisonics. When the system was first being promoted the push was two-pronged: on the one hand encouraging record companies to endorse the system and release discs, while on the other hand encouraging hardware manufacturers to sign up as Ambisonic licensees and include decoders in their equipment.
Inside a UHJ decoder, the signals are processed in a phase amplitude matrix to recover a set of signals that resembles B-Format. In most cases the signal will actually be B-Format, but in the case of 2-channel UHJ there is insufficient information available to be able to reconstruct a true B-Format signal, only something that behaves in a similar way.
The information is then passed to an amplitude matrix that develops the speaker feeds, via a set of shelf filters, which improve the accuracy and performance of the decoder in smaller listening environments. Ambisonics was designed to suit actual living rooms and practical speaker positions: most living rooms are rectangular and as a result the basic system was designed to decode to four loudspeakers in a rectangle, with sides between 1:2 and 2:1 in length, thus suiting the majority of living rooms. A layout control is generally provided to allow the decoder to be configured for the loudspeaker positions – an important aspect of Ambisonic replay where it differs from other surround systems: the decoder is specifically configured for the size and layout of the speaker array. The layout control may take the form of a rotary knob, a 2-way or a 3-way switch.
Four speakers is the minimum required for horizontal surround decoding and while this is suitable for a typical consumer environment, larger spaces require more speakers to give full surround localisation.

Super stereo

A disadvantage of the use of 2-channel UHJ is that to obtain a surround effect the listener requires an Ambisonic UHJ decoder. These have traditionally not been widely available, but have become so as plugins on modern PC equipment. However, listeners to a 2-channel UHJ signal without a decoder notice a "super stereo" effect. Because of the inter-channel phase relationships inherent in the encoding scheme, the listener experiences stereo that is often significantly wider than the loudspeakers.
At the same time, a feature of domestic UHJ decoders has been the inclusion of a super stereo feature. This allows conventional stereo signals to be "wrapped around" the listener, using some of the capabilities of the decoder. A control is provided that allows the width to be varied between mono-like and full surround. This provides a useful capability for a listener to get more from their existing stereo collection.
Both features were used as selling points in the early days of Ambisonics, and especially Ambisonic mixing. They helped to overcome a "chicken and egg" situation where record companies were reluctant to release Ambisonic recordings because there were few decoders in the marketplace, while hi-fi manufacturers were unwilling to license and incorporate Ambisonic UHJ decoders in their equipment because there was not very much mainstream released content. On the one hand, it was worth having a decoder because you could get more out of your existing record collection; while on the other it was worth making UHJ recordings because even people without a decoder
could gain appreciable benefits.

Undecoded UHJ

In a world in which the majority of listeners do not have Ambisonic decoders, most listeners to 2-channel UHJ-encoded material will be listening undecoded.
Some engineers have preferred the perceived additional spaciousness, width and stability of undecoded 2-channel UHJ to conventional panpotted stereo mix techniques. The late John Timperley, for example, used the UHJ Transcoder extensively and produced several series of UHJ recordings even though he was fully aware that few people would ever hear them decoded, and mixed material deliberately for the best undecoded result. Unfortunately these recordings have not yet been fully documented.
Similarly, on the Tina Turner album, Break Every Rule, the backing vocals are in fact UHJ encoded – and in this case there is no point in the listener decoding the tracks, because the rest of the mix is in regular panpotted stereo and contains no surround information – the vocals were encoded purely for the spacious effect without decoding.
When listening to undecoded 2-channel UHJ material, the rear information is folded down over the front stage at 3 dB down, so that the soundstage retains some degree of 'distance' between front and rear. For example, a sound panned around the listener in surround will, when auditioned undecoded, appear more distant as it passes across the 'rear'; in addition, the interchannel phase relationships associated with rear localisation make the sound attempt to appear behind the listener as a result of a form of 'aural decoding'. However, while this is generally effective in the case of Ambisonically mixed content, in the case of material recorded with a soundfield microphone system the ambience, for example in a concert-hall, may be sufficiently high in level when undecoded to make the recording appear over-reverberant, where this is not the case when the UHJ is decoded into surround. This has been cited in the past as a criticism of some SFM recordings by some record reviewers: the problem can be ameliorated by careful mic array positioning.