Embodiments of the present invention provide a compact representation of a cumulative bit curve formed from piecewise straight line approximations between upper and lower bounds about an actual cumulative bit curve (CBC). In one embodiment the lower bounds are found by applying a constraint such that if a delivery rate was to be calculated using the count at the lower bound it would result in a delivery rate which was greater than the delivery rate that would be calculated using the actual CBC data by a particular amount, for example 10 %. The actual CBC data is then used as an upper bound. As a result, the approximated CBC will lie for each GoP between the actual CBC value and the lower bound, with the result that one can be certain that any data rate that is calculated using the approximation will be at least as high as a data rate that is calculated using the actual CBC data. In terms of line- fitting algorithms that are used, several different algorithms may be used of differing complexities, with the intention of trying to minimise the number of end-points required of within the piecewise approximation.