In a differential pulse code modulator for image-representative signals, a coder codes difference signals to produce coded signals which have a highly variable rate, which depends upon motion in the image being represented. A rate buffer receives the coded signals and generates a control signal representative, at least in part, of the rate of fill of the rate buffer. Various filters, decimators and/or coarse quantizers associated with the modulator have characteristics controlled by the control signal in order to tend to control the rate of fill of the rate buffer. This aids in preventing loss of information at the receiver.