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This paper studies the function that links end-to-end distortion with channel signal-to-noise ratio, the D-SNR curve, for systems using Joint Source-Channel Coding (JSCC) bit rate allocation. The D-SNR curve is shown to be well approximated for certain multimedia systems using practical source and channel codes, by a linear function in log-log scales. The D-SNR curve is characterized by following an approach that considers a set of carefully selected points, which are those where the relative contribution of channel errors to end-to-end distortion is small. The D-SNR characterization is applied to study the effects on performance of channel codes of different strength or source codes with different compression efficiency. The approach followed to characterize the D-SNR curve has the potential advantage that it could be applied to represent performance of many practical systems using JSCC bit rate allocation, as it is illustrated for a scheme carrying voice traffic.