Most source coded bit streams exhibit unequal sensitivity to bit errors. Certain source bits can be much more sensitive to transmission errors than others. For example, errors in control bits may cause severe error propagation. When entropy coding, e.g. Huffman coding, is used, error propagation also occurs among the audio bits in an audio coder bitstream. Unequal error protection (UEP) offers a mechanism for matching error protection capability to sensitivity to transmission errors. A UEP system typically has the same average transmission rate as a corresponding equal error protection (EEP) system, but offers an improved received signal quality at equal channel signal to noise ratio. In this work we introduce methods of unequal error protection to the perceptual audio coder (PAC). An error sensitivity classifier divides the bits in classes of differing sensitivity to transmission errors. This can be done in a fixed scheme on a frame by frame basis or in a more flexible, adaptive scheme over several consecutive audio frames. The second approach requires an additional control channel. Different channel codes are applied to the different classes of audio bits with different levels of sensitivity. Control information is often treated separately. We show how both Reed Solomon codes and punctured convolutional codes can be used for UEP of the PAC bitstream. Experimental results are also presented. The systems with UEP exhibit graceful degradation and extended range for digital audio broadcasting compared to their EEP counterparts.
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