Circuit Arrangement for Unobjectionable Switching Between TV-channels as in a TV Receiver

This invention relates to circuitry in a television (TV) receiver for switching between one channel and another without generating an objectionable display during switchover.

In current TV receivers, switching between channels or between the tuner as a source of signal and for example a video recorder as a source of signal, produces a disturbed image for a period of time. This occurs because the receiver which is synchronized to the original signal requires time to self synchronize to the new signal. The period for achieving resynchronization is nominally several hundred milliseconds which corresponds to approximately seven frame intervals. In professional equipment switching artifacts are precluded because the signals from the different sources are synchronized to each other before switching is attempted. However it is not practical to incorporate such synchronization circuitry in home receivers.

An object of the invention is to provide cost effective apparatus in a consumer receiver to permit channel or source switching without generating an objectionable display during the switching interval.

The present invention is switching circuitry for a video signal processing system which permits switching between various video signals without generating objectionable display artifacts. Video signal from selection circuitry such as a tuner or a signal source selection switch is coupled to a first input terminal of a controllable signal combining means and to the input port of a frame store. Output signal from the frame store is coupled to a second input terminal of the controllable signal combining means. Control circuitry responsive to signal selection controls and synchronization components of the selected signal conditions the controllable signal combining means to provide signal from the frame store during switching intervals and signal from said selection circuitry at other times.

The sole drawing is a block diagram of a portion of a TV receiver exemplifying the switching circuitry embodying the invention.

The video signal processing system in the FIGURE includes a first signal source, 1, which may be a tuner for the reception of several TV channels, and a second signal source, 2, which may be a video cassette recorder (VCR). Channel selection by the tuner, 1, is controlled by a microprocessor 4. Signals provided by the first and second sources are coupled to respective input terminals of a switch 3. An output terminal of the switch 3 provides video signal to the system processing circuits.

The operation of the circuit illustrated in the drawing will be described in terms of interchannel switching by the tuner. The system operates in similar fashion for intersource switching by the switch 3.

Assume that the tuner 1 is tune to a certain TV channel. Baseband video signal provided by the tuner is coupled to a controllable amplifier 5. The output signal from amplifier 5 is coupled to a signal adding circuit 6 the output of which is coupled to a display tube 8 via video signal processing circuitry 7. Video signal from the switch 3 is also coupled to circuitry 9, which, responsive to the synchronizing components of the video signal, extracts the horizontal and vertical synchronizing components and generates signals H and V having horizontal and vertical frequencies respectively. The signals H and V are coupled to deflection circuitry (not shown) for producing a raster scan on the display tube 8.

Video signal from the switch 3 is also coupled to the input terminal of a frame store circuit 18 which successively stores the current frame or field of video signal. Output signal from the frame store circuitry 18 is coupled to the input terminal of a second controllable amplifier 12. Output signal from amplifier 12 is coupled to a second input terminal of the signal adding circuit 6.

Amplifiers 5 and 12 are controlled by complementary signals provided by a control circuit 10, so that normally amplifier 5 applies the current video signal from switch 3 to the signal adding circuit 6 and amplifier 12 provides a zero value to signal adding circuit 6. During switching between channels or sources, amplifier 12 is conditioned to couple stored video signal from the frame store circuitry 18 to signal adding circuit 6 and amplifier 5 provides a zero value to signal adding circuit 6.

Amplifiers 12 and 5 may be gated amplifiers, that is, in response to control signals from control circuit 10, they are either turned on or off. Alternatively amplifiers 12 and 5 may be designed to multiply the respectively applied video signal by the factor K and its complement 1-K provided by circuit 10. In this instance the control signal K may gradually be incremented from values of K=0 to K=1 to effect a fading in of signal from amplifier 5 to signal from amplifier 12 and vice versa. The combination of signal adding circuit 6 and controllable amplifiers 5 and 12 form a controllable signal combiner.

To effect channel switching, the microprocessor 4 provides a channel selection signal to the tuner 1 over the bus 13. Simultaneously a switch-over signal is applied to a coincidence circuit 15 over connection 14. The vertical frequency signal provided by circuitry 9 is also coupled to coincidence circuit 15. Coincidence circuit 15 generates an enable signal on connection 16 when the vertical signal indicates the occurrence of vertical sync in the received video signal. The enable signal is initiated at horizontal line 17 of the vertical retrace, period to insure that the frame store circuitry 18 contains image data from a complete field or frame. The enable signal is coupled to the tuner 1, frame store 18 and control circuit 10 to simultaneously enable the tuner to change to the channel indicated by the signal on bus 13, switch the frame store from a write mode for storing video signal to a read mode for outputting stored video signal and to condition control circuit 10 to generate a control signal to disable amplifier 5 and enable amplifier 12. The duration of the control signal is selected to be long enough to insure that the system will synchronize to the new signal. At the termination of the control signal amplifier 5 is enabled to pass the video signal from the new channel at switch 3 and amplifier 12 is disabled. In this manner the switching arrangement provides continuous video signal to the video processing circuitry 7 which is synchronous with the system and precludes picture disturbance during channel switching.