Ambisonic Surround Sound FAQ, version 2.8: Section 7

7. What is UHJ?

The Ambisonic format recommended for recording and studio processing is called B-Format and is just the W, X, Y and Z direction signals. If only horizontal surround sound is required, then the Z signal can be omitted. However, this destroys the possibility of periphonic (full-sphere) reproduction.

Established transmission media (LP, FM radio, CD) are all two-channel and, unfortunately, it is impossible to obtain reasonable surround sound using only two of the B-Format signals. To overcome this, two-channel UHJ matrix encoding was developed. Not only can two-channel UHJ be decoded back into horizontal surround sound, but also this C-Format is mono and stereo compatible.

When two-channel UHJ is played in stereo, the front- and side-stage material is reproduced with sharply defined images. The rear-stage material is reproduced, but given a less focused, more "recessed" quality. This helps to provide an audible distinction between front and rear sounds when played in stereo.

When two-channel UHJ is played in mono, sounds from all directions, including due back, are reproduced in the single speaker at a level within 5 dB of one another.

Two-channel UHJ was extended into a hierarchy of C-Formats for 2, 2.5, 3 and 4 transmission channels, termed BHJ, SHJ, THJ and PHJ, respectively. The extra channels are used to augment the two base channels to give improved horizontal surround sound and, for four-channel UHJ (PHJ), periphonic (full-sphere) surround sound. In practice, only two-channel UHJ (BHJ) encoded material has ever been released. For this reason UHJ has become a synonym for BHJ, and UHJ is the symbol you will see on LPs and CDs.

The advantages of UHJ over B-Format are that it is mono and stereo compatible, and allows horizontal surround sound to be transmitted using two-channel media. The disadvantages of UHJ are that both encoding and decoding require the use of 90-degree phase shifters, and that encoding into only two channels requires compromise. ("No-compromise" horizontal surround sound requires three transmission channels.)

It is UHJ which has caused Ambisonics to be described as a "matrix" system, but Ambisonics is much more than UHJ.

Readers interested in seeing the set of encoding/decoding equations should consult the appendices of the Gerzon 1985 reference.

UHJ is more symbolism than initialism. The 'U' stands for Universal, and is taken from the UMX quadraphonic system which pioneered the technique of using supplementary channels to enhance directional resolution. The 'H' represents the BBC's Matrix H and their work on mono and stereo compatible matrices. The 'J' is taken from System 45 J, the name of a progenitor of the UHJ system. (The 'J' was simply a code letter used to describe a possible surround sound system.)