Abstract: The performance of an adaptive digital doppler tracker having a sample data phase locked loop is improved by a circuit that detects and demodulates interfering signals. The tracker has a coherence sensor that provides signals indicating coherence and non-coherence conditions. An amplitude sensor generates a strong signal when interfering (time coincident) signals are received. The tracker includes both a coherent data detector and an SSB processor. An interference detector receives the outputs of the coherence sensor and the amplitude sensor and generates an SSB enable signal in response to the coincidence of a strong signal and a non-coherent condition signal. The SSB enable signal disables the coherent data detector and enables the SSB processor which detects both interfering signals.

Abstract: Multilevel code mates A and B expanded in accordance with the expression a)I (b.sup.K) and B=(a.sup.K)I (b) are compressed to a lobeless basic mate pair by repeatedly compressing corresponding code mate pairs in successive stages to provide diminishing codes of half the number of code bits by amplifying the code mates by a predetermined amplification factor and then adding and subtracting the code mates in accordance with expressions A.sub.i-1 =A.sub.i +B.sub.i.sup.K.sbsp.i and B.sub.i-1 =A.sub.i.sup.K.sbsp.i -B.sub.i where i is the i.sub.th compression stage and K.sub.i is the amplification factor applied to the codes A.sub.i and B.sub.i.

Abstract: A line equalizer including a .sqroot.f equalizer for compensating a .sqroot.f-characteristic of a transmission line, a BT equalizer connected in series with the .sqroot.f equalizer for removing an echo component caused by a bridged tap (namely, BT) on the transmission line, and a circuit for controlling the .sqroot.f equalizer is disclosed. A signal applied to the .sqroot.f equalizer is subjected to over-equalization to make the time domain length of impulse response at the output of the .sqroot.f equalizer small. The equalization state of the output of the .sqroot.f equalizer is judged by signals formed in the BT equalizer, and the gain of the .sqroot.f equalizer is controlled on the basis of the result of judgement. In this manner, the .sqroot.f equalizer is controlled without suffering any interference between a control loop of the .sqroot.f equalizer and a control loop of the BT equalizer.