Constant-bitrate rate control is very useful for real-time video transmission. However, it is difficult to realize a good CBR rate control in an MPEG4-AVC video codec. According to the invention, an adapted ρ-domain rate control processing for real-time rate control is applied, whereby ρ is the percentage of zero amplitude transform coefficients following quantization. A couple of tables are used for mapping between the quantization parameters and ρ. A detailed analysis at transform level is carried out in order to get the slope of the relationship between the bitrate and ρ. The slope of the linear model is constrained by a look-ahead procedure with respect to the neighbor frames or blocks.