Circuitry for automatically switching the control speed of a phase-control circuit

1. Circuitry for automatically switching the control speed of a phase-control circuit. 2.1. The switching is, especially in conjunction with different signal sources employed with a television receiver, either difficult or possible only by reserving prescribed program locations. The advantage of the invention is that it becomes possible to switch over without great expenditure to the higher control speed only while necessary and back to the low speed when it no longer needs to be high. 2.2. The invention is characterized in that a control circuit 12 increases the control speed, independent of whatever signal source is being employed to produce the picture, only during vertical flyback. 2.3. The circuit can be employed in television receivers to reproduce both broadcast television signals and video signals from a recorder connected to the receiver.

The invention concerns circuitry for automatically switching the control 
speed of a phase-control circuit in accordance with the playback of both 
broadcast television signals and recorded video signals. 
Playing back broadcast television signals demands a low control speed in 
order to allow smooth synchronization unimpeded by the interference that 
is present in noisy broadcast signals and would at a high control speed 
immediately affect synchronization to the extent that the image on the 
screen would become agitated. This malfunction is called "jitter" and 
appears on the screen in the form of a horizontal distortion of vertical 
lines. When recorded signals are processed in a videorecorder on the other 
hand, the control speed must be high in order to allow rapid correction of 
phase deviations between the pulses from the horizontal oscillator and the 
synchronization pulses deriving from the recorder. 
Extensive phase deviations occur in particular in the vicinity of the 
electron-beam flyback because that is where the heads that scan the tape 
are being switched over. If the control speed of the phase-control circuit 
is not increased in the presence of video signals from the tape, 
interference in line synchronization will become evident in the initial 
lines of the picture. A control speed is attained either by changing the 
time constant of the filter in the control circuit or by increasing the 
control current until a capacitor that supplies the control voltage 
charges or discharges. The control speed is usually changed by providing 
very definite program locations that can be subjected to a source of VCR 
signals to activate the change automatically. The procedure, however, is 
consequently strictly limited to those locations. What is called "AV" 
drive, whether by way of the SCART jack or the AV jack, currently only 
allows operation at a high control speed, which is, however, not always 
absolutely desirable or necessary. A known advantage of a low control 
speed is that is makes it essentially easier to eliminate interference. 
The low control speed is accordingly employed when a broadcast signal is 
being received. 
The object of the invention is to eliminate the need for expensive 
switching with no significant sacrifice in quality or other deleterious 
results. This object is attained by the invention recited in the major 
claim.

The FIGURE illustrates a phase-control circuit to be employed in 
conjunction with horizontal deflection in a television receiver. A 
phase-control circuit of this type has two sources 1 and 2 of constant 
current that are alternately activated by the edge of a signal that is 
derived from an oscillator 3 and that is to be regulated and by a 
synchronization signal SY as long as the current that charges a capacitor 
8 is in equilibrium with the current that discharges it. One possible 
circuit for controlling contacts 6 and 7 is constructed of 
logical-conjunction circuits in the form of an AND gate 9 and of a NOR 
gate 10 that has an inverting input terminal, to which the synchronization 
signals are applied. At the input terminals of gates 9 and 10 are the 
synchronization signals and oscillator signals. Their logical conjunctions 
generate the control voltages in control lines 4 and 5 that are needed for 
contacts 6 and 7. Sources 1 and 2 of constant current charge and discharge 
capacitor 8, to which is applied the voltage that controls oscillator 3. 
The control current, and hence the control speed, is at a prescribed level 
during the picture forward stroke. The level of the control current, and 
hence the control speed, can be changed. The switching occurs during the 
vertical flyback of the electron beams in the picture tube, in the 
invisible part of the picture, that is. The control pulse 11 for switching 
the control current is obtained in a practical way from a control circuit 
12 which is already present in the receiver. Control circuit 12 is known 
in the art and is, for this reason, not shown in further detail. The 
control circuit 12, generates pulse 11 by having a counter which sets a 
bistable switching circuit at a predetermined instant of time. After an 
interval of 1.5 msec, this bistable circuit is reset. This time during 
which pulse 11 lasts, corresponds to 22 line periods in a television 
picture. This procedure allows phase control to occur rapidly when it has 
to occur rapidly and slowly when a slow control speed is practical. The 
frequency response of the filter in the phase-control circuit must in this 
case be selected to avoid malfunctions due to switching the control speed 
during picture forward stroke. The control speed of the phase-control 
circuit can of course be changed by other means, by switching the value of 
the filter capacitor for example.