GB2240002A - Transmission of DATV through letterbox PAL channel - Google Patents

Abstract

In order to transmit a digital assistance signal with a PAL television picture signal in which a plurality of lines forming the picture signal are re-ordered so that some are received sequentially to form an image on a portion of a receiver display (letterbox presentation) and the remaining lines are received before and/or after the lines for display as an image, the digital assistance signal on line 200 is transmitted in the PAL chrominance channel of the remaining lines. Preferably, switching means 215 are provided to select between the digital assistance signal and the chrominance component of the picture signal at the input of the PAL coder 135 dependent on whether lines for display as an image or, alternatively, the remaining lines are to be transmitted. <IMAGE>

Description

TRANSMISSION OF DATV THROUGH LETTESBOX PAL CHANNEL The present invention relates to a method and apparatus for transmitting a digital assistance signal with a PAL television picture signal.

It is, on occasion, necessary to transmit an HDTV signal, which typically has 1000 lines per picture and an aspect ratio of 16:9, using a conventional 625/50 television channel. At the receiver, the signal is up-converted, possibly using information from a subsidiary channel, for display on HDTV apparatus. It is, however, desirable that it be possible to display the intermediate 625/50 signal on conventional television apparatus using a 625/50 display with an aspect ratio of 4:3.

Our copending application, 8925996, discloses a method for doing this in which the 4:3 display comprises a letterbox presentation wherein the picture occupies only about 432, that is three-quarters or (4/3)/(16/9), of the 576 active lines, centrally disposed. These lines carry the appropriate proportion of the vertical detail, the remaining lines in the letterbox border being used to carry the remaining proportion of the vertical detail. A block diagram of these processes, applied to a single-component television signal, is shown in Fig. 1.

As the conventional channel is assumed to carry a PAL colour television signal, the principle just described could be applied to the red, green or blue components or, preferably, to the luminance (Y) and chrominance (U,V) components of the PAL signal which are PAL coded and decoded for transmission, as shown in Fig. 2. High definition R, G and B signals, appearing on lines 100 are transformed into Y, U and V signals by a matrix 105 and the U and V signals are halved in bandwidth by filters 110 so that they represent, in total, a signal of the same bandwidth as the Y signal.

They may then optionally be combined into a single chrominance signal in circuit 115 for convenience in subsequent processing by, for example, sub-sampling by a factor of two and sample-by-sample multiplexing. The vertical downconverter 120 operates separately on both luminance and chroninance to produce signals having nominally half the number of lines and the rearranger 125 operates so as to produce the required letterbox format. The vertically rearranged chraminance is then optionally split into U and V signals in circuit 130 and, along with the rearranged luminance, coded to a PAL signal in coder 135 for transmission.

At the receiver, decoder 140 separates the PAL signal into Y, U and V components. The latter are then optionally combined into a single chrominance signal in circuit 145 and, along with the lunanance, vertically rearranged by processor 150 so as to undo the letterbox format. The vertical upconverter 155 operates separately on both luminance and chrominance to produce signals having the same number of lines as the original signal. Finally, optional processor 160 separates the chrominance signal into U and V signals for transformation, along with the luminance, in matrix 165 into R, G and B signals for display.It will be appreciated that the PAL coder 135 and PAL decoder 140 may be of non-standard form so as to suppress interaction between luminance and chrominance signals and also to permit the transmission of horizontal resolution greater than normal within the constraints of the conventional channel bandwidth. Both of these aspects could be provided by, for example, a coder and decoder constructed according to principles described in our patents GB 2 044 577 and GB 2 113 037.

Now, just as it is possible to reduce the horizontal resolution of the chrominance signal below that of the luminance signal by a factor of at least two without significant perceptible degradation, so it is also possible to reduce the vertical resolution by a factor. This means that the vertical resolution conveyed by the chrominance component of the central picture occupying the "letterbox" area of the signal in the arrangement of Fig. 2 is adequate for display. Consequently, the capacity of the PAL chrominance channel in the letterbox border is available for other uses.

In accordance with this invention there is provided a method of transmitting a digital assistance signal with a PAL television picture signal in which a plurality of lines forming the picture signal are re-ordered so that some are received sequentially to form an image on a portion of a receiver display, the remaining lines being received before and/or after the lines for display as an image; the method being characterized in that the digital assistance signal is transmitted in the PAL ohraminance channel of the said remaining lines. This method has the advantage that it requires no further spectral space beyond that needed for the conventional PAL signal.

Two methods in accordance with the invention will now be described in detail, by way of example, with reference to the drawings, in which: Figure 1 is a block diagram illustrating a known method of transmitting an HDTV signal through a conventional television channel; Figure 2 is a block diagram showing the method of Figure 1 applied to a PAL colour television signal; Figure 3 is a block diagram illustrating a first method in accordance with the invention; and Figure 4 is a block diagram illustrating a modified method in accordance with the invention.

The system shown in Figure 3 is, insofar as the picture signal processing circuitry is concerned broadly similar to that shown in Figure 2 and those elements which perform the same function have been given the same reference numbers as in Figure 2.

A digital assistance signal to be transmitted with the picture information, representative of one or more properties of the picture being transmitted, for example, the motion vector at each point in the picture appears on a line 200. This signal is assumed to be a continuous sequence of binary values and is buffered in circuit 205 so that it emerges only at periods corresponding to the active border regions of the letterbox format. The intermittent bit stream is then coded, in circuit 210, into two analogue signals for multilevel quadrature amplitude modulation (QNM) of the colour suboarrier. The two analogue signals are then fed to one input of a switch 215 whose other input takes a feed of the U and V signals emerging from the optional circuit 130.The output of switch 215 is taken to the U and V inputs of the PAL coder 135 and the switch operates so as to select the coded assistance signals during the letterbox border periods and the rearranged U and V signals during the letterbox period. For this purpose the switch control signal is derived in a circuit 220 which takes a feed of the incoming synchronising signals S appearing on lines 100.

The buffering of the data in circuit 205 has the effect of increasing its instantaneous bit rate by a factor of approximately 16/3, being the ratio of full to active periods multiplied by the ratio of letterbox plus border to border heights. The rate can be arranged to be a multiple of the line frequency or related to the subcarrier frequency by a ratio of small integers to aid clock recovery in the following QAM decoder. The form of the coding introduced by QAM coder 210 is governed by the bit rate of the DATV signal and the required ruggedness. The simplest form is 4 wherein the two analogue signals have only two states and the modulated carrier four phases of equal amplitude, corresponding to the four combinations of the two two-state signals. The analogue signals could be generated by demultiplexing the bit stream emerging from buffer 205 into alternate bits to give two two-level signals at half the clock rate and low pass filtering.

At the receiver, the U and V signals obtained by PAL decoding are taken to a switch 225 one of whose outputs feeds the rearranger 150 via optional combining circuit 145 and the other of whose outputs is taken to a buffer 235 via a circuit 230 which decodes the two multi-level analogue signals into a single bit stream. Decoder 230 includes means for data clock recovery, based either on input synchronising pulses or the subcarrier frequency. The switch 225 operates so as to direct the U and V signals to the rearranger during the letterbox period and the coded assistance signals to the buffer during the letterbox border. The switch control signal is derived in a circuit 240 which takes a feed of the synchronising signals derived from the received PAL signal.The buffer 235 operates so as to produce a continuous digital assistance signal from the intermittent signal in the borders which can be used in the manner appropriate to the nature of the signal.

It will be appreciated that the PAL coder 135 and decoder 140 may contain complex processing in the chraminance path in order to avoid luminance and chrominance interaction whilst permitting enhanced resolution. With such coders and decoders, the digital assistance signal may suffer impairment which renders it undecodable after passage through the coder and decoder. In these circumstances, the alternative arrangement, shown in Fig. 4, may be preferable.

In this arrangement an alternative PAL coder 300 and decoder 315 are used for the periods during the letterbox border. The alternative coder 300 and decoder 315 are such that the digital assistance signal does not undergo any processing which renders it undecodable at the expense of obtaining enhanced resolution. For example, the coder 300 and decoder 315 could be based on simple video frequency partitioning of lurnance and chrominance in which the U and V signals are subject only to video frequency filtering and quadrature modulation and demodulation at the colour subcarrier frequency.

In Fig. 4 the alternative PAL coder 300 takes a feed of the Y signal emerging from the rearranger 125 and a feed of the coded assistance signals emerging from the QAM coder 210. The output of coder 300 feeds one input of a switch 305 whose other input takes a feed of the PAL signal from coder 135. Switch 305 operates so as to select the output of coder 135 during the letterbox period and the output of cpder 300 during the letterbox border periods. For this purpose the switch control signal is derived in a circuit 310 which takes a feed of the incoming synchronising signals S appearing on lines 100.

At the receiver, the incoming PAL signal feeds decoder 140 and the alternative PAL decoder 315. The Y output of decoder 140 is taken to one input of a switch 320 whose other input takes a feed of the Y output of decoder 315. The output of switch 320 feeds the rearranger 150 and it operates so as to select the Y signal from decoder 140 during the letterbox period and the Y signal from decoder 315 during the letterbox border. In addition, the U and V outputs of decoder 140 are taken to one input of a switch 325 whose other input takes a feed of a null signal, corresponding to zero values of U and V. The output of switch 325 feeds the rearranger 150 via he optional combiner 145 and it operates in synchronism with switch 320 so as to select the chrominance output of decoder 140 during the letterbox period.The control signal for switches 320 and 325 is derived in a circuit 330 which takes a feed of the synchronising signals derived from the received PAL signal. The U and V outputs of decoder 315 are taken to buffer 235 via QAM decoder 230 as before. Buffer 235 operates so as to ignore from the input signal during the letterbox period, corresponding to normal chrominance, thereby rendering switch 225 unnecessary.

Because the upper and lower borders of the compatible picture now contain high level chrcminance information caused by the presence of the digital assistance signal in the chrominance channel, it is preferred that steps be taken to minimise the distraction caused to viewers by apparently random colours in the areas above and below the picture. These could include reducing the amplitude of the digital assistance signal when the mean level of the associated Y signal in the border, which is assumed to be the picture dependent as mentioned in our copending application 8925996, is high.

Claims (8)

1. A method of transmitting a digital assistance signal with a PAL television picture signal in which a plurality of lines forming the picture signal are re-ordered so that some are received sequentially to form an image on a portion of a receiver display, the remaining lines being received before and/or after the lines for display as an image; the method being characterized in that the digital assistance signal is transmitted in the PAL chrominance channel of the said retraining lines.

2. A method according to claim 1 in which switching means are provided to select between the digital assistance signal and the chrominance component of the picture signal at the input of the PAL coder in dependence on whether lines for display as an image or the said remaining lines, are to be transmitted.

3. A method according to claim 1 in which two different PAL coders are provided for coding the picture signal and the digital assistance signal respectively, the output from the coders being transmitted selectively dependent on whether lines for display as an image or said remaining lines are being transmitted.

4. A method of transmitting a digital assistance signal with a PAL television signal, the method being substantially as hereinbefore described.

5. Apparatus for transmitting a digital assistance signal with a PAL television picture signal including means for re-ordering a plurality of lines forming the picture signal so that some are received sequentially to form an image on a portion of a receiver display, the remaining lines being received before and/or after the lines for display as an image; and means for transmitting the digital assistance signal in the PAL chrce ance channel of the said remaining lines.

6. Apparatus according to claim 5 including switching means for selecting between the digital assistance signal and the chraminance component of the picture signal at the input of the PAL coder in dependence on whether lines for display as an image or the said remaining lines are to be transmitted.

7. Apparatus according to claim 5 including two different PAL coders for coding the picture signal and the digital assistance signal respectively, the output fran the coders being transmitted selectively dependent on whether lines for display as an image or said remaining lines, are being transmitted.

8. Apparatus for transmitting a digital assistance signal with a PAL television signal, the apparatus being substantially as hereinbefore described.