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into details one can say that the line time base is allowed to run
continuously, producing pulses of its own at every flyback. These
are compared with the incoming sync, signals in what is called a
phase detector. If both occur at the same frequency, nothing
happens. But directly the line generator gets out of step, the phase
generator products a voltage, the polarity of which depends on
whether the generator is running fast or slow. This voltage is applied
to the generator in such a way as to correct its speed. The time
constant of the system is chosen so that correction is gradual;
occasional mutilation of the line sync, pulses by interferenc ...