Circuit Arrangement for Processing Video Components

This invention relates to circuitry for processing video signal components.

It is know from the journal "Funkschau", volume 25, page 53, 1985, to process and reproduce a composite video signal digitally. In this system the composite video signal is converted to digital form in an analog-to-digital converter and then split into the components of the luminance signal Y and the chrominance components U and V in a digital color decoder. In order to avoid an excessive number of connections the signals are transmitted in time multiplexed fashion and subsequently stored in picture memories.

In picture-in-picture systems wherein images from one video source are to be displayed within images from another video source, the video signals from the first source are typically split into components parts and then applied to analog-to-digital converters. Desirably only a single analog-to-digital converter is used by multiplexing the analog video components thereto. However, problems arise in multiplexing the components to a single converter. For example the analog-to-digital converter must operate at a high sampling rate in order to produce a picture of acceptable resolution.

There are several solutions to appropriately multiplexing the component video signals to a single converter. For example, the analog video component signals may be sampled and multiplexed to form horizontal lines of signal in the form of YUYV sampled data where Y is the luminance signal and U and V are the chrominance signal parts (U=B-Y, V=R-Y). These sequences of sampled data are applied to the analog-to-digital converter which generates their digital counterparts. The digital values are written to memory at a first frequency rate, and read out at a higher frequency rate to achieve horizontal compression of the image. For vertical compression only one out of every set of three or four horizontal lines of the video input signal is analyzed. The reduction ratio is selected dependent on the desired result. If necessary, the signal is applied to a vertical filter.

The signal read from memory is applied to three digital-to-analog converters to reproduce the compressed Y, U and V signals. Subsequently, the compressed signals are substituted for portions of the main video signal.

The concepts developed to date provide picture-in-picture processing for TV signals requiring a reduced number of circuit connections. However, a relatively high multiplexing frequency is used in order to obtain a correct signal bandwidth for each video component.

It is an object of the present invention to provide a circuit for processing a picture-in-picture video signal at reduced cost.

The present invention provides for subsampling the input video signal in order to reduce the amount of data to be processed. Subsampling results in the loss of picture information to be displayed, however this loss of information is acceptable because the size of the reproduced image is reduced. For example, the inset image may be reduced to a quarter of its original size. In processing the video signal the luminance and color difference components may be selected. Both parts of the signal (luminance and chrominance) may be applied to the same horizontal filter structure and a single analog-to-digital converter.

In the present invention, separated chrominance components (U, V) are coupled to a first multiplexer which alternately selects the respective chrominance components at a rate of F.sub.H /N, where F.sub.H is the horizontal line frequency and N is an integer greater than 2. The output of the first multiplexer and the luminance component are coupled to respective signal input terminals of a second multiplexer. The second multiplexer is conditioned to alternately provide the luminance and chrominance components at one half the vertical frequency. That is, the second multiplexer provides fields of luminance samples alternating with fields of chrominance samples.

FIG. 1 is a block diagram of video signal multiplexing circuitry embodying the present invention.

FIG. 2 is a waveform diagram useful in describing the invention.

Referring to FIG. 1, a video signal is applied from a source (not shown) to an input terminal, IN. It is fed to the input of a color decoder 1, a low pass filter 5, a sync separator circuit and central clock pulse generator 6. At the output of the color decoder 1 separated chrominance signal components U, V are provided which are applied to a 2-in-1 multiplexer 2. In the exemplary embodiment the 2-in-1 multiplexer is switched by a square wave signal at a frequency FH/8 which is equivalent to one eighth of the line frequency. The multiplexer 2 outputs successive groups of four lines of the component U alternating with groups of four lines of the component V. The output of the 2-in-1 multiplexer 2 and the output of the low pass filter 5, at which the luminance signal is present, are coupled to a 2-in-1 multiplexer 3, which is switched at a frequency FV/2. The frequency FV/2 is equivalent to one half of the picture frequency. Thus, the inputs of the multiplexer 3 are switched at each field interval. The switching signals FH/8, FV/2 for controlling the multiplexers 2, 3 are generated by the sync separator circuit and central clock pulse generator 6 and by dividers 7, 8. The output of the multiplexer 3 is coupled to an analog-to-digital converter 4 which provides digital representations of the multiplexed video signal.

FIG. 2a illustrates the horizontal timing of a video signal and FIG. 2b shows the switching frequency FH/8 generated from the horizontal pulse by the sync separator circuit and central clock pulse generator 6 and divider 8. FIG. 2c shows the picture vertical timing pulses, and FIG. 2d shows the switching frequency FV/2 generated from the vertical pulses by the sync separator circuit and central clock pulse generator 6 and the divider 7.

According to the invention, out of a full picture the fields are processed by means of the multiplexers 2, 3 in such a way, for example, that in the first field the luminance signal Y and in the other field the color difference signals U and V are used.

Thus, from the incoming picture signals, for the first field only the luminance signal Y is fed to the analog-to-digital converter 4. The next field contains only the information of the two color difference signals U or V wherein the first four successive lines contain the first color component U of the field and the next four successive lines contain the second component V. This process is repeated throughout the second field.

Table I illustrates in part the line format of the video components of two successive fields.