for example the multiplication and mixing could be moved to the right of the codec 13 alternately either of the volume controls may be made digital and the other made analog 2 the amount of sidetone is independent of the line impedance z l in fig3 transmitted reflections have been largely eliminated from the received signal so that little or no microphone sidetone is contained within the received signal on bus 5 the amount of reflection induced sidetone depends on the amount of reflection which is reduced by filtering 3 the 2 to 4 hybrid in fig4 acts as an interface between two wire service and four wire service codec 2 acts as an interface between four wire service and a digital bus or the digital processing system represented by dashed block 35 codec 13 acts as a similar type of interface 4 the invention provides an increase in transhybrid loss thl over that which is normally found in prior art hybrids for example common thl values in prior art hybrids range from 0 db to 10 db and values in the range of 8 db to 10 db are common the invention by use of the adaptive filter shown in fig4 is expected to increase thl by at least an additional 6 db and more likely by an additional 12 db to 15 db or more 5 the invention can change the frequency response of the received signal in order to assist persons.