set back to the pseudo c quam mode in step 36 and a return is made to the comparison in step 33 consequently the receiver operates in the pseudo c quam mode except when the stereo difference level is at a high level which can be more accurately received by using the c quam mode fig4 shows an alternative embodiment wherein the receiver is preferentially set to the true c quam mode thus the receiver is initially set to the c quam mode in step 40 and the i and q signals are continuously generated in step 41 in step 42 the i signal is compared with zero to identify the presence of over modulation step 42 repeats as long as the value of i has not fallen below zero when the i signal drops below zero then the receiver is set to the pseudo c quam mode in step 43 while in pseudo c quam mode the instantaneous value of the i signal is compared to zero in step 44 a series of comparisons is conducted for a predetermined time t 1 when the value of the i signal has been greater than zero for time period t 1 the receiver is set to c quam mode in step 45 otherwise the i signal continues.