Here we briefly discuss the main speech coding techniques which are used today, and those which may be used in the future. In order to simplify the description of speech codecs they are often broadly divided into three classes - waveform codecs, source codecs and hybrid codecs. Typically waveform codecs are used at high bit rates, and give very good quality speech. Source codecs operate at very low bit rates, but tend to produce speech which sounds synthetic. Hybrid codecs use techniques from both source and waveform coding, and give good quality speech a intermediate bit rates. This is shown in Figure 5, which shows how the speech quality of the three main classes of speech codecs vary with the bit rate of the codec.