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Taking the evolution of spectral parameters into consideration in speech coding has been shown to enhance the perceptual performance. In this study we examine and compare two methods that are designed for explicit control of spectral dynamics. One method operates on the encoder part of the coding system by incorporating a constraint in the distortion measure and the other method smoothes the trajectory of output vectors at the decoder side. The decoder method requires however an additional coding delay of one frame. By means of listening experiments it is demonstrated for three different vector quantizer structures that especially the decoder method gives significant improvements. For noisy channels, the preference for this method is even more emphasized.