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The application of code conversion techniques to digital audio systems

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The recent development of low cost digital storage has made digital audio systems attractive. However its popularity depends on the development of accurate low cost digital to analogue (D/A) converters. Existing converters require the fabrication of precision analogue components which prohibit their mass production. An alternative strategy is investigated which involves code converting the digital P.C.M. into a different digital code which can be simply decoded.
This type of D/A converter removes the need for precision analogue hardware at the expense of increased digital complexity and is therefore more suited to mass production.
Code conversion to delta sigma modulation is investigated and found to be unsuitable for high quality audio at practical clock frequencies. Methods of improving the performance of the basic d.s.m. by shifting the encoding noise out of the message band by changing the P.D.F. of the input signal are investigated and shown to be unacceptable for a linear system.
An error feed-forward converter is described which uses a second d.s.m. to encode the residual error produced by the main d.s.m., thus achieving a useful reduction in quantization noise. The order of the feedback filter is increased in the proposed second order system.
This is shown to give 14-bit P.C.M. accuracy at a 4 MHz clock frequency.
An additional advantage of using these code converters is illustrated when they are used in conjunction with a class D power amplifier. This arrangement eliminates the need for any analogue processing.


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