| 摘要 |
A television receiver including a synchronizing arrangement for providing line and field drive pulses for line and field output stages, which synchronizing arrangement comprises a line oscillator control circuit for controlling the frequency and/or phase of a line oscillator the output signal of which is divided by a divider circuit to produce pulses of field frequency. The divider pulses are applied to a field coincidence stage together with received field synchronizing pulses where their phases are compared and the number of times there is coincidence is counted by a counter and to a controllable switch which also receives field synchronizing pulses from a field oscillator which is driven by received field synchronizing pulses. A mode selection circuit controls the operation of switch which switch is driven into an internal synchronization mode when coincidence is present in the field coincidence stage for a given period, in which mode divider pulses drive the field output stage, and into an external synchronization mode when the above coincidence is not present, in which mode pulses from the field oscillator drive the field output stage. In order to prevent a black bar appearing for a finite period on the display tube of the receiving when changing television channel or the camera is changed in the studio the line oscillator control circuit produces an output which output drives the switch from the internal to the external synchronization mode. |
