dPMR, in common with other fully digital radio systems, does not transmit analogue voice signals. The voice signals transmitted have to be converted to digital and this process is accomplished by the vocoder. In order to ensure that all types and modes of dPMR will interoperate, it is clearly essential that use is made of the same vocoder. For this reason, the dPMR association has evaluated suitable vocoder technologies and has selected the standard vocoder to be used by all dPMR radios. Users can now be assured that radio products which carry the dPMR approved trademark will interoperate for all types of voice call. It is of course possible, using the dPMR protocol, to implement radio equipment with more than one vocoder type for specific applications. Provided such equipment also offers the standard vocoder, it may also be marked with the dPMR approved trademark.