This invention relates generally to television receivers and is specifically directed to a network for use in improving the apparent resolution of images displayed by television CRT's (cathode ray tubes) by modulating the scan velocity of the CRT electron beam.
The sharpness and crispness of a television image is dependent on factors such as the bandwidth of the transmitted signal, the bandwidth of the receiver circuitry, and the resolution capabilities of the image reproducing device. In the case of television receivers using cathode ray tubes as image reproducing devices, the spot size of the scanning electron beam is an important parameter in determining overall resolution capability.
In most cathode ray tubes, the spot size of the electron beam increases significantly as the beam current is increased. Therefore, when a large black-to-white video transition occurs, i.e., when a TV image includes a white area immediately following a black area, the spot size of the beam grows concurrently with the increase in the luminance signal. To a television viewer, the overall effect is that, in the case of a white stripe on a black field, for example, the edges of the stripe will appear to be blurred because of the fact that the electron beam spot is scanning and varying in size simultaneously. The white area will generally appear expanded and the black area correspondingly reduced in size. In the case of commercial color television programming, the net effect of spot size variance with luminance transitions is a television image which is less crisp than is desirable. This is particularly true where the picture contains many highlights, in which case the spot size of the scanning electron beam may grow to be as large as 1/4 inch and completely obscure some video detail.
In an attempt to improve the crispness of television images, the prior art has improved the bandwidth of television circuitry, improved electron guns so as to produce electron beams having smaller spot size, included "peaking" circuitry in the luminance channels of the receivers to generate steeper luminance transitions, and developed image enhancement systems for increasing the sharpness of television images by modulating the scan velocity of the CRT electron beam.
Examples of such image edge enhancement systems are disclosed in U.S. Pat. Nos. 2,678,964 and 3,830,958, for example. Briefly, one of the methods disclosed therein consists of processing the luminance signal to develop therefrom a control signal which corresponds to a derivative of the luminance signal. The control signal is then used to modulate the scan velocity of the CRT electron beam. The variance in the scan velocity of the beam can, as pointed out in the above-mentioned patents, result in a reproduced video image which has sharper edges, particularly on large black-to-white or white-to-black transitions.
As shown in the U.S. Pat. No. 3,830,958, for example, a control signal which corresponds to the derivative of the video signal, can be generated with a network which includes two delay lines. Such a network also provides a delayed video signal for use in intensity modulating the CRT electron beam with picture information. Delaying the video signal before applying it to the CRT is necessary to provide the correct timing relationship between the control signal and the video signal used to generate the video image.
Although the two delay line network is an effective means for generating the required signals, the use of two delay lines makes it an expensive network, particularly in view of the fact that most television receivers already include circuitry (video peaking circuitry) for enhancing the sharpness of the television images. Consequently, the improvement in resolution which is afforded by modulating the scan velocity of the CRT electron beam has not been incorporated into consumer television receivers. A network capable of generating the control signal and the delayed video signal and which does not require two delay lines would lower the cost of the network and hasten the entry of this form of image edge enhancement into the consumer market.