The disclosure relates to symbols of data being encoded into visible light emitted by a light source, at a symbol rate fsym having a symbol period Tsym = l/fsym. Each of the symbols is encoded as one of a set of at least two different symbol waveforms formed in a level of the emitted light as a function of time, each of the symbol waveforms representing a respective one of a corresponding set of different data values. The difference between the symbol waveforms, which represents the different data values, is formed only within a predetermined time window at a given phase within the symbol period, the predetermined time window having a duration less than 0.75.Tsym, the light level inside this time window being substantially different for the different symbol waveforms, and the light level outside the time window being substantially the same for the different symbol waveforms.
