Television receiver synchronizing arrangement

Claim 1

A television receiver suitable for the reception of a television signal incorporating line and field synchronizing pulses, a plurality of fields constituting an image, the receiver including a synchronizing arrangement which comprises: a line oscillator for generating in the synchronized condition thereof a signal having a frequency equal to the line frequency or to an integral multiple thereof, a line oscillator control circuit, means for applying received line synchronizing pulses and a reference signal derived from the line oscillator signal to said line oscillator control circuit which is adapted to produce a control signal depending on the phase relationship between said line synchronizing pulses and said reference signal for controlling the frequency and/or phase of said line oscillator, a frequency divider circuit connected to said line oscillator for producing pulses of field frequency from said line oscillator signal, a field comparison stage, means for applying received field synchronizing pulses and pulses produced by said frequency divider circuit to said field comparison stage for comparing the phase of said received pulses with that of the divider pulses, means for determining the time during which a given phase relationship does not exist in said comparison stage between the pulses compared therein, means for applying received field synchronizing pulses to said frequency divider circuit for the resetting thereof, a field deflection output stage, a field synchronization control stage connected to said field deflection output stage, said field control stage being switchable between an external synchronization operating mode in which it is adapted to apply in operation the received field synchronization pulses or a signal derived therefrom to the field output stage and an internal synchronzation operating mode in which it is adapted to apply in operation the divider pulses to the field output stage, said field control stage being adapted to be switched from said external to said internal synchronization mode when the given phase relationship in said field comparison stage has existed for a predetermined period while said field control stage is adapted to be switched from said internal to said external synchronizaton mode when said given phase relationship has not existed for said predetermined period, a received field synchronizing pulse being then allowed to be applied to the frequency divider circuit for the resetting thereof, wherein said field control stage is also adapted to be switched from the internal to the external synchronization mode under the control of the line oscillator control circuit when the phase between the line synchronizing pulses and or a further reference signal also derived from the line oscillator signal departs from a given phase relationship.