The invention relates to a method for converting a signal data rate and to a transmitter in a digital data transmission system, the transmitter comprising a number of data sources that generate a signal having a first data rate, and a coder for forming the signal into frames of a given length, which length corresponds to a second data rate, and a coder for grouping the signal into a first block the length of which corresponds to the first data rate. In order to convert the data rate flexibly, the transmitter comprises a coder, which calculates the difference between the frame and the number of symbols in the first block by subtracting the frame length from the number of symbols in the first block, which equalizes the difference by removing or repeating every ith symbol in the first block, i being determined as the nearest bigger or an equal integer when the number of symbols in the first block is divided by the absolute value of the difference that was calculated, which updates the number of the symbols that do not fit in the frame by subtracting therefrom the number of the symbols that have been repeated, and which forms the first block to consist of the symbols that have not been repeated or removed.