2005/12/02, 21:18:34, UTC

 File Format for UHJ

Many transmission media (LP, FM radio, CD) are 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 it is also mono- and stereo-compatible.

An official file format for downloadable two-channel UHJ files has been defined to allow easy exchange of audio files. The file format is in the public domain for anybody to use. Here are the details:

Many people already offer plug-ins and other Ambisonic software. Such people are encouraged to support the above format (in addition to other formats).

Mono- and Stereo-Compatibility

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.

Encoding from B-Format

Two-channel UHJ is produced from first-order horizontal B-Format using the following equations (where j is a +90 degree phase shift):

S = 0.9396926*W + 0.1855740*X
D = j(-0.3420201*W + 0.5098604*X) + 0.6554516*Y
Left = (S + D)/2.0
Right = (S - D)/2.0

UHJ with more than two channels

UHJ is a hierarchy of stereo compatible standards for 2, 2½, 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, full-sphere surround sound.

Unfortunately, WAVE-EX files containing more than two channels do not play on stereo systems. Further, on multi-channel systems, the audio device renders the extra channels to output ports not in use. This can happen even when the extra channels are masked off. Therefore, such WAVE-EX files can not be made stereo-compatible. As the only advantage of UHJ over B-Format is stereo compatibility, it is suggested that three- and four-channel UHJ material be converted to B-Format in the ".amb" file format.

2½-channel UHJ (where the "½" is a third channel band-limited to 5 kHz) cannot be converted to B-Format. However, as no material is known to exist in this format that does not also exist in B-Format, this is not a problem.

Martin Leese