A method of controlling an input signal supplied from an encoder to a modulator, wherein the input signal causes modulation of a current flowing through a light source in order to embed data in light emitted by the light source. The method comprises: for each of a plurality of possible logical transitions allowed by the encoding scheme from an adjacently preceding symbol to a present symbol, determining a respective observed value for the current level of the present symbol; for each of the plurality of logical transitions, determining an associated error between the respective observed value and a corresponding one of a set of nominal values for the current levels; and based on the respective error for each of the plurality of logical transitions, applying a respective compensation to the corresponding input in order to bring the resulting current level closer to the corresponding nominal value.