D.c. restoration in amplifiers

Claim 1

AN ELECTRICAL SYSTEM COMPRISING A D.C. AMPLIFIER FOR AMPLIFYING A SIGNAL INCLUDING REGULARLY RECURRENT INTERVALS, DURING ALL OF WHICH INTERVALS IT IS REQUIRED THAT THE SIGNAL LEVEL SHOULD ASSUME A CONSTANT D.C. LEVEL, SAID AMPLIFIER HAVING AN INPUT AND AN OUTPUT STAGE, A SAMPLER, MEANS FOR RENDERING THE SAMPLER ACTIVE ONLY DURING THE SAID INTERVALS, THE SAMPLE BEING COUPLED TO SAID OUTPUT STAGE TO SAMPLE THE SIGNAL IN THE SAID INTERVALS AND PRODUCE THEREBY A POTENTIAL, A COMPARATOR FOR COMPARING SAID POTENTIAL PRODUCED BY THE SAMPLER WITH A FIRST FIXED REFERENCE POTENTIAL SELECTED EQUAL TO THE DESIRED CONSTANT D.C. LEVEL AT THE AMPLIFIER OUTPUT SO AS TO PRODUCE AN ERROR SIGNAL REPRESENTATIVE OF THE DIFFERENCE BETWEEN THE SAID PRODUCED AND FIRST REFERENCE POTENTIALS, A SOURCE OF A SECOND VARIABLE REFERENCE POTENTIAL AND A DRIVEN CLAMP COUPLED BETWEEN SAID SECOND SOURCE AND SAID AMPLIFIER, MEANS FOR RENDERING THE CLAMP ACTIVE ONLY DURING THE SAID INTERVALS TO CLAMP THE LEVEL OF THE INPUT SIGNAL TO THE AMPLIFIER TO THE SECOND REFERENCE POTENTIAL, THE SAID SECOND VARIABLE SOURCE BEING RESPONSIVE TO SAID ERROR SIGNAL TO VARY SAID SECOND REFERENCE POTENTIAL SO AS TO REDUCE ANY DIFFERENCE BETWEEN SAID POTENTIAL PRODUCED BY SAID SAMPLER AND SAID FIRST REFERENCE POTENTIAL.