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Version: 2.8
Last-modified: 21 January 1998

Ambisonic Surround Sound FAQ

Section 8

Copyright: © 1994, 1995, 1996, 1998 by Martin J Leese
Distribution is unlimited


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8. What are BHJ, SHJ, THJ and PHJ?

BHJ is the engineering specification for encoding the W, X and Y direction signals into two channels. The two channels, called Left and Right, can then be transmitted using conventional stereo media before being decoded back into W, X and Y. The BHJ format has been designed to be mono and stereo compatible. In practice, BHJ is the only UHJ encoding that has been used for commercial record releases. For this reason UHJ has become a synonym for BHJ and UHJ is the symbol you will see on BHJ encoded LPs and CDs.

SHJ specifies how W, X and Y can be encoded into 2.5 channels, called Left, Right and T, where the T channel is of reduced bandwidth (5 kHz). The original intention was to provide the reduced bandwidth channel in broadcasting by additional modulation of the 38 kHz sub-carrier. Presumably RDS, Minicall, etc, kills this possibility.

THJ specifies how W, X and Y can be encoded into three channels called Left, Right and T. This is the "no-compromise" horizontal C-Format.

PHJ specifies how W, X, Y and Z can be encoded into four channels, called Left, Right, T and Q, and is the "no-compromise" periphonic C-Format. Periphonic (full-sphere) reproduction requires speakers to be placed above and below the height of the listeners' ears.

BHJ, SHJ, THJ and PHJ are all inter-compatible. That is to say, to go from one member of the set to the next you add or delete additional signals without changing those that remain. A beauty of this is that each member of the UHJ set is mono and stereo compatible. In addition, a BHJ decoder, for example, can decode SHJ, THJ and PHJ material simply by ignoring the extra T and Q channels.

Version 1.0 of this FAQ suggested encoding a third channel into the subcodes of a CD to give THJ or SHJ. It transpires that the bandwidth available in the subcodes is less than 2 kHz using 16 bits, so this ain't gonna fly.

It is possible, however, to use "buried data" to encode a third channel of reduced bandwidth onto a CD such that an existing CD player is unaware the channel exists. This would allow SHJ encoded CDs to be produced that are completely compatible with conventional stereo CD players. That is to say, a stereo system would produce stereo, a BHJ decoder would produce surround sound, and an SHJ decoder (fed from a CD player with special digital electronics) would produce even better surround sound. All this from the same CD! The technique is too complicated to describe here, and interested readers should consult the Gerzon and Craven 1995 reference.

Peter Knight has pointed out that the CD format specification includes a four-channel quad format that would be suitable for PHJ encoded material. The problem, of course, is that existing CD players are not quad CD "aware" and would produce a mishmash if asked to play a quad CD. He has also pointed out that quad CDs have to be spun twice as fast as stereo CDs and have only half the playtime.


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