FIG. 4 shows a conventional transmission path for video signals without band compression. In FIG. 4 the reference numerals 1, 2, and 3 designate R, G, and B signal digital video input terminals, respectively. The reference numerals 13, 30, and 31 designate data transmission paths for transmitting the signals from the input terminals 1, 2, and 3, respectively. The reference numerals 22, 23, and 24 designate R, G, and B signal digital video output terminals, respectively, for outputting the signal from the data transmission paths 13, 30, and 31 to the outside.
FIG. 5 shows a conventional band compression apparatus for video signals using the PASS (Phase Alternative Sub-Nyquist Sampling) system as a band compression method. In FIG. 5, the same reference numerals designate the same or corresponding elements as those shown in FIG. 4. The reference numeral 4 designates a sub-sampling prefilter for restricting the band of the signal from the R signal digital video input terminal 1 before the sub-sampling. The reference numeral 6 designates a sub-sampling prefilter for restricting the band of the signal from the B signal digital video input terminal 3 before the sub-sampling. The reference numeral 50 designates a switch for switching the output between the sub-sampling prefilter 4 and the output of the sub-sampling prefilter 6 at each one picture element. The reference numeral 13 designates a data transmission path for transmitting the output of the switch 50 to the receiver's side. The reference numeral 51 designates a switch for switching the output from the data transmission path 13 at each one picture element. The reference numeral 19 designates a sub-sampling interpolating filter for generating an interpolating value from the output of the switch 51. The reference numeral 21 designates a sub-sampling interpolating filter for generating an interpolating value from the output of the switch 51. The reference numeral 30 designates a transmission path for transmitting the signal from the G signal digital video input terminal 2. The reference numeral 22 designates an R signal digital video output terminal for outputting the output from the sub-sampling interpolating filter 19 to the outside. The reference numeral 23 designates a G signal digital video output terminal for outputting the output from the transmission path 30 to the outside. The reference numeral 24 designates a B signal digital video output terminal for outputting the output from the sub-sampling interpolating filter 21 to the outside.
FIGS. 7 and 8 show examples of a sub-sampling prefilter and a sub-sample interpolating filter of FIG. 5, respectively.
In FIG. 7, the reference numeral 71 designates a digital video input terminal, the numeral 72 designates a 1H delay line for delaying the output from the digital video input terminal 71 by one line. The numeral 73 designates a 1H delay line for delaying the output of the 1H delay line 72 by one line. The numeral 74 designates a one picture element delay line for delaying the output of the 1H delay line 73 by one picture element. The numeral 75 designates a one picture element delay line for delaying the signal from the digital video input terminal 71 by one picture element. The numeral 76 designates a one picture element delay line for delaying the output of the 1H delay line 72 by one picture element. The numeral 77 designates a one picture element delay line for delaying the output of the one picture element delay line 76 by one picture element. The numeral 78 designates an adder for adding the output of the 1H delay line 72, the output of the one picture element delay line 77, the output of the one picture element delay line 74, and the output of the one picture element delay line 75. The reference numeral 79 designates a divider for dividing the output of the one picture element delay line 76 by 2 (or a multiplier for multiplying the output of the one picture element delay line 76 by 1/2). The numeral 80 designates a divider for dividing the output of the adder 78 by 8 (or a multiplier for multiplying the output of the adder 78 by 1/8). The numeral 81 designates an adder for adding the output of the divider 79 and that of the divider 80. The reference numeral 82 designates a digital video output terminal for outputting the output of the adder 81 to the outside.
In FIG. 8, the reference numeral 83 designates a digital video input terminal, the numeral 84 designates a 1H delay line for delaying the output of the digital video input terminal 83 by one line, the numeral 85 designates a 1H delay line for delaying the output of the 1H delay line 84 by one line. The numeral 87 designates a one picture element delay line for delaying the signal from the digital video input terminal 83 by one picture element. The numeral 86 designates a one picture element delay line for delaying the output of the 1H delay line 85 by one picture element. The numeral 88 designates a one picture element delay line for delaying the output of the 1H delay line 84 by one picture element. The numeral 89 designates a one picture element delay line for delaying the output of the one picture element delay line 88 by one picture element. The numeral 90 designates an adder for adding the output of the 1H delay line 84, the output of the one picture element delay line 86, the output of the one picture element delay line 87, and the output of the one picture element delay line 89. The numeral 91 designates a divider for dividing the output of the adder 90 by 4 (or a multiplier for multiplying the output of the adder 90 by 1/4). The numeral 92 designates an adder for adding the output of the one picture element delay line 88 and the output of the divider 91. The numeral 93 designates a digital video output terminal for outputting the output of the adder 92 to the outside.
The device operates as follows.
First of all, the operation of the conventional system shown in FIG. 4 will be described. A color video signal is usually processed and transmitted by being decomposed into three original colors of light, that is, an R signal (red), a G signal (green), and a B signal (blue). In FIG. 4, the R signal video information input to the R signal digital video input terminal 1 is transmitted by the transmission path 13 to the receiver's side, and is output from the R signal digital video output terminal 22. Similarly as above, the G signal video information input to the G signal digital video input terminal 2 is transmitted by the transmission path 30 to the receiver's side, and is output from the G signal digital video output terminal 23. Similarly as above the B signal video information input to the B signal digital video input terminal 3 is transmitted by the transmission path 31 to the receiver's side, and is output from the B signal digital video output terminal 24. Now suppose that a video signal is sampled with a gradation of 8 bits per picture element and a sampling frequency of 2Fs=10 MHz, three 80 MBPS (Bit Per Second) transmission paths are required in this transmission system.
A band compression is conducted so as to reduce the number of the required transmission paths. The conventional system utilizing the PASS method as a band compression method will be described with reference to FIGS. 5 to 8.
First of all, the R signal video information input to the R signal digital video input terminal 1 is band restricted by the sub-sampling prefilter 4. On the other hand, the B signal video information input to the B signal digital video input terminal 3 is band restricted by the sub-sampling prefilter 6. The R signal and B signal video information, both band restricted, are switched by the switch 50 at each one picture element to be sub-sampled as shown in FIG. 6. Herein, the mark O designates the position on the screen of the resampling point of the R signal, and the mark X designates the position on the screen of the resampling point of the B signal. Accordingly, the transmission information quantity of the R signal and the B signal become 1/2, and the output of the switch 50 can be transmitted to the switch 51 at the receiver's side by only a transmission path 13. The signal transmitted to the switch 51 is switched at one picture element, and is decoded to the R signal and the G signal. The decoded R signal is input to the sub-sampling interpolating filter 19. In this filter 19, an interpolating value is generated at the non-sampling point shown by the mark X in FIG. 6 by that data 0 inserted at the non-sampling point. Thus, an R signal digital output is output from the R signal digital video output terminal 22 to the outside.
Similarly as above the decoded B signal is input to the sub-sampling interpolating filter 21. In this filter 21, data 0 is inserted at the non-sampling point, an interpolating value is generated at the non-sampling point shown by the mark O in FIG. 6 by that data 0 inserted at the non-sampling point. Thus, a B signal digital output is output from the B signal digital video output terminal 24 to the outside.
On the other hand, the G signal is input to the G signal digital video input terminal 2, and is transmitted to the receiver's side by the transmission path 30. The transmitted G signal is output as a G signal digital video output from the G signal digital video output terminal 23 to the outside. Two 80 MBPS transmission paths are required in the above described PASS system.
The sub-sampling prefilters 4 and 6 shown in FIG. 5 will be described in a greater detail with reference to FIG. 7. When a zigzag-grid shaped sub-sampling shown in FIG. 6 is conducted, the characteristics in a two dimensional space spectrum as shown in FIG. 9 is obtained. In FIG. 9, the alias centers formed by the zigzag-grid shaped sub-sampling appear at positions designated by double circles, and therefore alias noises are usually generated at the regions where the alias signal originated from the alias center and the base-band signal overlap with each other on the spectrum plain. Therefore, it is possible to reproduce picture images accurately on a screen by band restricting the base-band signals into rhombus shaped regions without the alias signals being overlapped with base-band signals. The video signal input to the digital video input terminal 71 of the sub-sampling prefilter shown in FIG. 7 is transmitted in accordance with the transmission characteristics represented by the following formula (1) before it reaches the digital video output terminal 82, whereby the base-band signals are band restricted into the hatched regions shown in FIG. 9 without overlapping with the alias signals originating from the alias centers. ##EQU1## Herein Z.sup.-l : two line delay on the screen (1H delay of field video signal)
Z.sup.-1 : one picture element delay on the screen
Next, in the sub-sampling interpolating filter shown in FIG. 8, a video signal into which the data 0 is inserted at the dropped sampling point (non-sampling point) is input to the digital video input terminal 83. Accordingly, the dropped sampling point is interleaved by the sub-sampling interpolating filter which realizes the characteristics of the formula (1) before it reaches the digital video output terminal 93.
In this conventional band compression system for a video signal using the PASS method, the compression ratio is 1/2 because the video signals are compressed at a frequency of 1/2 of the sampling frequency, and for example, number of the transmission paths is only reduced to two from three which corresponds to the number of R, G, and B signals obtained by decomposing the video information.