System for Digital Transmission of Color Television Signals

1. Field of the Invention

The invention relates to a system for digital transmission of color television signals according to the difference-pulse-code modulation (dpcm) method.

2. Description of the Prior Art

Various methods are known for digital transmission of color television signals. In one of these methods, the complete color television signal is coded according to the pcm method. Because the complete color television signal is coded, a large channel capacity is required. By this system, data flow at a transmission quality that corresponds approximately to television broadcasting would require about 100 Mbit/s.

A separate coding of the luminance and chrominance components of the color television signal allows further reduction of the data flow. For this purpose, known systems employ a separate a dpcm coder for the luminance signal and for each of the two color-difference signals. Such dpcm coders are quite costly for such multiple use.

The proposed system for digital transmission of color television signals according to the dpcm method involves a smaller equipment cost while meeting the desired quality standards.

In the proposed system, a coder and a decoder are provided for several components of the color television signals. In the coder and the decoder for these components, storage elements are provided which are triggered according to a time-division multiplex method.

While retaining a favorable data reduction factor, the proposed system achieves a substantial decrease in expense since some relatively large electronic structural components of the dpcm coder, such as analog-to-digital converters and quantizers, may be used jointly for several color television signal components.

By means of the system of the invention, it is possible to code and decode, for example, the three primary signals--red, green and blue. The difference between the various scanning rates of the luminance signal and the color-difference signals is favorable to a further development in the coding of these signals according to the invention. In this development, a first coder and a first decoder are provided for the luminance signal. Both color-difference signals are fed to a second coder, and a second decoder decodes the coded color-difference signals.

The invention will be explained in greater detail with the aid of the figures. These figures show the following:

FIG. 1 illustrates a known arrangement for digital coding in diagrammatical representation.

FIG. 2 likewise illustrates in diagrammatical representation, an embodiment of a coder according to the invention.

FIG. 3 illustrates an embodiment of a known dpcm coder.

FIG. 4 illustrates an embodiment of a coder according to the invention as block diagram.

FIG. 5 illustrates an embodiment of a decoder according to the invention.

FIG. 6 illustrates in diagrammatical representation an embodiment of a decoder according to the invention.

Similar elements in the figures are provided with the same reference symbols.

In the arrangement of FIG. 1, the luminance signal Y, as well as the color difference signals R--Y and B--Y, are fed to dpcm coders 1, 2, and 3. At the output of these coders, digital dpcm signals are present which are combined in a circuit 4, for example, by time-multiplex, and are ready for transmission.

In contrast to the known arrangement according to FIG. 1, in the system of the invention as illustrated in FIG. 2, the color-difference signals R--Y and B--Y are fed into one coder 5. The luminance signal Y is coded, in the same manner as in the known arrangement, in a separate dpcm coder 1. The output signals of the coders can then again be combined and transmitted in a circuit 6 similar to circuit 4. Circuits 4 and 6 are known per se and are not described in detail since this is not necessary for the understanding of the present invention.

As an example of a known dpcm coder, a circuit is indicated in FIG. 3. The signals to be coded, which may be the luminance signal of a color television signal, are fed into the coder at terminal 7. First, in an analog-to-digital converter 8, the analog signal is converted into a corresponding digital signal. A scanning train of clock pulses T is fed into the analog-digital converter. This train of pulses ordinarily has a frequency twice that of the maximum transmitting frequency. By way of a subtraction element 9, the digital signals reach a quantizer 10. This quantizer 10 assigns the many possible amplitude levels of the transmitting signal to a few pre-selected amplitude levels, which is necessary for a bandwidth-saving transmission. The output signal of the quantizer is fed both to the output 11 of the coder and to the input of an addition circuit 12. This addition circuit 12 and a store 13 constitute a so-called predictor at whose output 14 the most probable value for the next scanning is present. This value is deducted, by means of subtraction element 9, from the input signal, so that only the difference between the true scanning value and the predicted value needs to be transmitted, which is the basic concept of the dpcm.

FIG. 4 shows an embodiment of the invention. Color-difference signals R--Y and B--Y are fed to inputs 20 and 21. An electronic switch 22 feeds the color-difference signal alternatingly, in periods T, to the analog-to-digital converter 23. The converter 23 corresponds essentially to the analog-to-digital converter 8 of the arrangement of FIG. 3. It is, however, triggered with a frequency T.sub.2, which is twice as high as the frequency T. The digital color-difference signals reach a quantizer 25 via a subtraction element 24. Subtraction element 24 and quantizer 25, as well as addition circuit 26, correspond to the corresponding components of the arrangement of FIG. 3.

In contrast to the known arrangement, however, two stores 27 and 28 are connected to the output of addition circuit 26, namely, one for color-difference signal R--Y, and one for color-difference signal B--Y. The outlets of stores 27 and 28 are alternatingly connected, by way of an electronic switch 29, to point 30 and thus to the inverting input of subtraction element 24, and to the second input of addition circuit 26. Switch 29 is likewise triggered in period T. The stores themselves can be successfully established with the aid of edge-controlled D-flip-flops to which T on the one hand and T on the other hand are fed as clock pulse.

The clock pulses T and T.sub.2 are produced in pulse generators known per se which do not need to be described in detail within the scope of this specification.

The invention was explained with the aid of a simple predictor. However, predictors are known wherein more than one scanning value is used for prediction. In this system, too, the invention can be used successfully, when all scanning value stores are provided in each case for one of the color-difference signals.

FIG. 5 shows diagrammatically a decoding circuit for decoding dpcm signals which are obtained, for example, by means of the circuit according to FIG. 4, and which are fed in at terminal 31. The circuit constitutes a dpcm coder, known per se, wherein, by way of a return circuit 32, pcm signals are added to the input signals in an addition circuit 33.

In contrast to the traditional circuits, only two stores 34 and 35 are provided whose outputs are triggered with a clock frequency by a switch 36, similarly to stores 27 and 28 in FIG. 4. The pcm signals thus produced are converted, in the digital-to-analog converter 37, into time-multiplex pulse amplitude modulated (PAM) signals, which in turn are converted, by means of the time demultiplexer 38, into the two color-difference signals R--Y and B--Y. The time demultiplexer 38 consists essentially of an electronic switch which, like switch 36, is triggered by a clock (or synchronous impulse) frequency T.