8. What are BHJ, SHJ, THJ and PHJ?
Description
This article is from the Ambisonic Surround Sound FAQ, by Martin Leese with numerous contributions by others.
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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