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Real-time rate-control requires fast, accurate characterization of the relationship between wavelet subband step-size and rate. A rate-control algorithm is presented based on local properties of the function parameterizing this relationship, the rate-quantization or R-Q curve, for each subband. The behavior of this function is characterized by using the coder once, with step-sizes close to those that achieve the desired rate, which are determined entirely by a model-based technique. This solution requires a coder, a performance metric (which implies a subband-quantization strategy), and continuously differentiable (C1) R-Q curves. This approach differs from some previous approaches in that it is based on local structure of R-Q curves instead of a global property, and does not require any training. The proposed scheme produces step-sizes that achieve rates typically less than half a percent away from the target rate. This method is fast, accurate, coder independent, requires no training, and can be combined with any subband-based quantization scheme.