Abstract
Multichannel sound systems are being studied as part of the Eureka 95 and Radio-communication Bureau TG10-1 investigations into High Definition Television. One emerging sound system has five channels; three at the front and two at the back. This raises some compatibility issues. The listener might have only, say, two loudspeakers or the material to be broadcast may have fewer than five channels. The problem is how best to produce a set of signals to be broadcast, which is suitable for all listeners, from those that are available.
To investigate this area, a device has been designed and built which has six input channels and six output channels. Each output signal is a linear combination of the input signals. The inputs and outputs are in AES/EBU digital audio format using Βι¶ΉΤΌΕΔ- designed AESIC chips. The matrix operation, to produce the six outputs from the six inputs, is performed by a Motorola DSP56001. The user interface and 'housekeeping' is managed by a T222 transputer.
The operator of the matrix uses a VDU to enter sets of coefficients and a rotary switch to select which set to use. A set of analogue controls is also available and is used to control operations other than the simple compatibility matrixing.
The matrix has been very useful for simple tasks: mixing a stereo signal into mono; creating a stereo signal from a mono signal; applying a fixed gain or attenuation to a signal; exchanging the A and B channels of an AES/EBU bitstream, and so on. These are readily achieved using simple sets of coefficients.
Additions to the user interface software have led to several more sophisticated applications which still consist of a matrix operation. Different multichannel panning laws have been evaluated. The analogue controls adjust the panning, the audio signals are processed digitally using a matrix operation. A digital SoundField microphone decoder has also been implemented. The signal processing is a matrix operation, the analogue controls being used to adjust the characteristics of the decoded microphone.
DSP software for specific tasks not requiring operator control has also been used. Adaptive filtering and signal restoration are two examples. The transputer in this case can be left to perform the housekeeping.
The design of the HDTV digital audio matrix is such that it can be applied to a wide variety of signal processing tasks. The combination of a dedicated DSP chip programmed in assembly language for speed of operation and a general purpose processor for user interface tasks programmed in a high level language has been found to be extremely useful.
White Paper copyright
Β© Βι¶ΉΤΌΕΔ. All rights reserved. Except as provided below, no part of a White Paper may be reproduced in any material form (including photocopying or storing it in any medium by electronic means) without the prior written permission of Βι¶ΉΤΌΕΔ Research except in accordance with the provisions of the (UK) Copyright, Designs and Patents Act 1988.
The Βι¶ΉΤΌΕΔ grants permission to individuals and organisations to make copies of any White Paper as a complete document (including the copyright notice) for their own internal use. No copies may be published, distributed or made available to third parties whether by paper, electronic or other means without the Βι¶ΉΤΌΕΔ's prior written permission.
