UHJ Ambisonic Decoder

Posted on Feb 9, 2013

The decoder is intended to be a reference quality prototype with maximum flexibility for experimental use with different speaker layouts. A new improved version has been added at the end. There are two main sections to the instrument. The UHJ decoder board, amb02 accepts 'stereo' UHJ sources and converts them to B format. The B format decoder board, amb01 generates the audio signals for each speaker from the B format signal. This B format input signal can come either from the UHJ decoder or from a separate B format source. The stereo UHJ signal enters on two domestic phono sockets. The decoded B format output is on a 5 pin male XLR which conforms to the pin assignment used on the historic Audio and Design Ambisonic units. The B format input to the B format decoder is a female 5-pin XLR. Finally, the outputs to the amplifiers are once more on domestic phono sockets.

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The decoder deals only with horizontal surround. The Z signal is ignored. The Texas TL074 quad op-amp is used extensively, as it is a good audio performer while being cheap. Shelf filters are not included. Shelf filtering alters the spectral content of the W signal relative to the X and Y signals to compensate for the blocking effect that the head has on the right ear when a sound comes from the left direction. The degree of filtering is different for different speaker layouts, making it difficult to apply in a generalised decoder like this one. Phase distance compensation is not included in this decoder either. Phase distance compensation attempts to make the wavefronts more spherical in some way by applying a phase equalisation at a very low frequency. Once more I elected to omit this feature as it seemed to be a very small finessing of the system, and again is dependant on speaker layout. Amplitude distance compensation is available for each speaker in steps of 2dB. This sheet shows the voltage regulators which are standard. Approximately ±14V is chosen for the voltage rails as this allows the use of a 12V transformer. There is about 4V drop available to the regulators which lets them stay well out of drop-out without dissipating too much power. The B format input enters the board from the rear panel XLR onto a six pin header. The W signal is buffered and amplified by root 2 at this point so that it is at the correct...

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