a sawtooth voltage oscillator synchronizable by synchronizing pulses , a capacitor being charged and subsequently discharged , between two threshold levels . the oscillator is switchable between two states under the influence of a synchronizing signal detector which establishes the presence of the syncrhonizing pulses , to wit a first state in the absence of the synchronizing pulses wherein the free - running frequency of the oscillator is substantially equal to the nominal repetition frequency of said pulses and a second state in the presence of the synchronizing pulses wherein the free - running frequency of the oscillator is lower than said nominal repetition frequency , the oscillator being thus in its second state synchronizable by the synchronizing pulses .