Patent GB-827604-A (GB)
Title: Television receiver
IPC: H04N5/08
Filed: 1956-03-14
Abstract: 827,604. Television. SYLVANIA ELECTRIC PRODUCTS Inc. March 14, 1956 [March 14, 1955], No. 8023/56. Class 40 (3). [Also in Group XL (c)] Relates to a circuit for reducing the effects of transient noise in a television receiver and is based on the fact that there is in general little variation of brightness level in adjacent picture elements in either the vertical or horizontal direction, or in corresponding picture elements in successive pictures. By employing circuits that limit the amount of change in brightness in adjacent or corresponding elements to that range expected for signals without noise, better picture quality is obtained through a corresponding reduction of random changes in signal levels. In the embodiment described the signal amplitude of corresponding elements separated by one frame peiiod is compared, a memory device being provided for the storage of a signal from the last frame for comparison with the incoming corresponding signal. If the difference between the two signals lies within the predetermined range, the incoming signal is passed unchanged and is then stored for subsequent comparison with the next incoming signal. If the incoming signal lies outside the predetermined range, it will be modified so that the resultant signal falls at either the upper or lower limit of the range. The modified signal is then stored for comparison with the next incoming signal. As shown in Fig. 2, the output v(t) from the video detector is applied to subtracting circuits 116, 118, 120, and the output v<1>(t) from the circuit 120 is passed to subsequent circuits of the receiver, and is also fed back via a storage and delay device 104 to an input circuit of an adding circuit 112 and a subtracting circuit 114 respectively. A potential C, which determines the permissible variation between corresponding signal elements is applied to a second input terminal of the circuits 112, 114, the circuit 112 providing an output corresponding to the upper limit and the circuit 114 an output corresponding to the lower limit. The output signals from circuits 112, 114 are compared at 116, 118, with the incoming signal v(t) and if v(t) is too high, a positive potential is applied via diode 122 as a correcting voltage to the subtracting circuit 120, and if too low, a negative correcting voltage is passed via diode 124 to the circuit 120. If the input signal lies within the permissible range no potential appears on the lead 120b and the input signal is passed unchanged by the circuit 120. In this arrangement the synchronization pulses are also compared with corresponding preceding pulses and if a pulse is missing from the incoming signal the previously stored and modified pulse will be repeated. In a modification, Fig. 3, the incoming signal v(t) is fed to a subtracting circuit 206 and also to a subtracting circuit 214. The output of the circuit 214, v<1>(t), is fed back via a delay and storage circuit 204 to the circuit 206 and the difference of the two signals applied to the input of 206 is fed to a subtracting circuit 210 and an adding circuit 212. The potential C is also fed to the circuits 210 and 212, and a positive potential is produced by circuit 210 and fed via diode 216 to the subtracting circuit 214, or a negative potential is produced by circuit 212 and fed via diode 218 to the circuit 214, if the difference between the input and stored signals is outside the predetermined range, the input signal being passed unchanged if it is within the range.