An MPEG 1 layer II encoder can be programmed to add a data payload to a frame. It uses a conventional Musicam pyshoacoustic model to apply a sub-band resolution parameter that is constant across a window of a given number of samples. The encoder is further programmed to apply a sub-band resolution algorithm that generates a more accurate set of resolution parameters that vary across at least part of a given window, the difference between the constant parameter and the variable resolution parameters for the same window being indicative of bits which can be overwritten with the data payload.