Apparatus and method for reducing quantizing distortion
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of reducing quantizing distortion of previously quantized electronic signals in systems which do not pass the quantized signal with full quantizing accuracy. The invention will be found to be useful for previously quantized analog signals, either in digital form, such as digital pulse amplitude modulation which is commonly known, or in analog form. Analog forms include multilevel pulse amplitude modulation, where the analog signal is constrained to a given number of predetermined values. Such multilevel pulse amplitude modulation is used for example to transmit analog signals in the vertical and horizontal blanking interval areas of a video signal, wherein each given analog level corresponds to a particular binary pattern representation of the original analog signal. While the invention is described by way of example for use with video signals, one skilled in the art will recognize that it may also be used with other types of signals, such as audio or telemetry signals. In the description of the preferred embodiment provided herein, reference is made to timing signals such as horizontal sync, etc., which are somewhat restricted to video type signals, one skilled in the art will recognize that the descriptions herein referencing these intervals may be adapted to fit other timing signals, such as blocks of audio data, radar sweep periods, telemetry scan periods, etc. 
     The invention finds particular usefulness in applications where a video signal which has been previously digitized with 10  bit accuracy is passed through equipment having only 8 bit capability. The quantizing distortion introduced by the truncation of the lower two bits of resolution is reduced by offsetting the 8 bit truncated version by a small amount, preferably 1 LSB in a regular and predictable pattern, with the pattern being controlled in accordance with the value of the truncated bits. The invention which is particularly useful for digitized video signals and associated equipment will also find use for other types of quantized signals and with other levels of quantizing precision and truncation, so long as a clocking signal, related to the quantizing of the continuous analog version, or representing a continuous analog version, is available. 
     2. Description of the Prior Art 
     The prior art contains circuits for masking the quantizing distortion of video signals. Many of these circuits add random noise to the reduced precision signal, or use a variety of rounding schemes to achieve the reduction in distortion. One of these rounding schemes is described in the book D·I·G·I·T·L Television Edited by C. P. Sandbank, published by John Wiley &amp; Sons, New York, N.Y. 10158. Pages 553 through 555 describe a method whereby a 12 bit digitized video signal is added to itself in an error feedback circuit to provide an 8 bit video signal. This method works well for the linear ramp signal shown in the book, however it is not nearly as effective on real video signals. 
     SUMMARY OF THE INVENTION 
     This invention is a method and apparatus for passing quantized electronic signals of a given quantizing resolution through processing systems which have a quantizing accuracy less than that which the electronic signal is quantized to. The invention includes truncating the quantized signal to a given precision matching the precision of the processing system, which truncation is effected by simply discarding the lower significance portions of the quantized signal to provide a truncated quantized signal, offsetting means to offset the truncated quantized signal by a predetermined amount, normally equal to the level of the lowest significance portion of the quantized signal, and a pattern means to control the offsetting means to perform said offset in a predetermined and predictable manner. In digital video versions of the invention, the pattern means generates one of 2 T  offsetting patterns in response to the T truncated lower significant bits of the digitized video signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the encoder means of the invention. 
     FIG. 2 is a schematic diagram of the encoder means of the invention. 
     FIG. 3 is a diagram of an embodiment of the decoder means of the invention. 
     FIG. 4 is a diagram of a second embodiment of the decoder of the invention. 
     FIG. 5 is a diagram of a third embodiment of the decoder of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In digital video systems which use quantized values that represent analog video levels, this invention will find considerable use. The CCIR-601 and CCIR-656 standard specifications, and the Society of Motion Picture and Television Engineers (SMPTE) recommendation RP125 specify an 8 bit standard. This standard is incorporated in the D1 standard in digital video recorders. An alternate specification is available for a 10 bit standard because 8 bits of quantizing causes visible degradation in the video signal. Present digital video recorders are capable of recording only the 8 bit standard. Typically, a 10 bit digitized signal is truncated to 8 bits for use by the 8 bit systems. The preferred invention embodiment includes a pattern means which will then generate 6 patterns which the offsetting means uses to add 1 LSB to the resultant 8 bit truncated video signal, thereby dithering the truncated video signal in a predetermined pattern. 
     Objects of the invention include: 
     Filling the signal in between the allowed quantizing steps so as to minimize the effects of quantizing errors, 
     Dithering the reduced quantizing accuracy signal in a predetermined pattern which pattern changes according to the truncated information of the quantized signal, 
     Filling in between the quantizing steps of a video signal so as to minimize the visible effects of the quantizing error to the human eye, 
     Minimizing the quantizing error effects in a fashion that does not introduce further noise or artifacts in the analog signal, 
     Allowing multiple passes of the quantized signal through the inventive process without introducing additional additive filling or dither, 
     Allowing the truncated signal to be recovered to the original accuracy as much as possible, 
     Providing an encoding means for dithering a quantized signal, which requires no further to processing when the quantized signal is converted back to a continuous analog signal. 
     These and other objects of the invention will become apparent to one skilled in the art from the description herein, taken in conjunction with the attached drawings. 
     SUMMARY DESCRIPTION OF THE ENCODING OPERATION 
     The encoding technique encodes the Least Significant Bits (LSBs) from a 10 bit signal into the 8 bit path by adding one bit in a pre-determined dithering scheme that is invisible to the eye. A one is added to the upper eight bits (the truncated version) 25% of the time when the two LSBs are Binary 01. One is added 50% of the time when the LSBs are Binary 10. And one is added 75% of the time when the LSBs are Binary 11. One skilled in the art will recognize that this concept is not restricted to video signals, but generally applicable to most quantized signals. 
     FIG. 1 shows a block diagram of the preferred embodiment of the encoder having video input 1, PROM 2 which receives the truncated input video which has been truncated by discarding significance portions thereof, PAL 3 which receives quantizing clock 8 and generates divided clock signals 9, 10, 11, and 12, to produce dither rate signals which are internally used in the PAL, and which PAL 3 also receives the discarded significance portions of the signal 1 and which generates the dither signal 5 which is coupled to the PROM 2 to selectively cause the PROM to output a dithered truncated signal, which is achieved by the addition of a 1 LSB amount to the truncated video signal. 
     FIG. 2 shows a schematic diagram of the invention, as used with digital video signals. In the preferred form, the digitized video has been separated into Luma (L) and chroma (C) components as is well known in the art. The schematic of FIG. 2 contains parallel paths for the L and C portions of the video signal. 
     Input luma 1L is coupled to PROM 2L which operates to blank the video signal at the proper time in response to blanking signals BL0 and BL1. When BL0 and BL1 are both low, the input is passed directly to the output. If BL0 is High and BL1 is Low, the output is one half the input. If BL0 is Low and BL1 is High, the output is one fourth the input. IF BL0 and BL1 are both High, the output is blanked to 0. This action allows the video to be blanked to 0 over a three clock or more period to provide some shaping of the video envelope. The blanked video out of the PROM is latched in latch 6, due to speed limitation of the PROM. 
     The Programmable Array Logic 1C (PAL) 3 receives the two LSBs of the input 1L signal, as well as the sampling clock 8. The PAL divides the sampling clock by 2, 4, and 8 and provides these divided signals at 11, 9, and 12 respectively. In addition, a horizontal rate signal 13, is coupled to PAL 3 which divides this signal by two, making it available at 10. A control signal 14 is also coupled to 3 to turn the dithering on or off under operator control. The divided signals are also available internally. The PAL generates the dither signals 5L and 5C in response to the 2 LSBs and the divided clock signals as described below. 
     The dither signal 5Y is coupled to the adder made up of 7a and 7b where it is added to the blanked and truncated signal from latch 6. Similarly, the chroma dither signal 5C is coupled to PROM 3. Because the chroma signal is clocked at a slower rate, the PROM also serves as an adder, in addition to blanking the chroma signal in response to the chroma blanking signals BL3 and BL4. The blanking action of the Chroma PROM is similar to that of the Luma PROM, however the blanked value is mid scale. The PAL 3 also receives the two LSBs of the chroma signal and provides the chroma dither signal 5C in response to the chroma signal LSBs and the sampling clock. Because of the lower speed of the chroma signal, the PROM 2C is capable of adding the chroma dither signal to the truncated chroma signal, thus outputting a blanked and dithered chroma signal. 
     The following TABLE 1 is a program listing of the LUMA PROM 2L, the TABLE 2 is a program listing of the CHROMA PROM 2C, and the TABLE 3 is a listing of the PAL 3 program. One skilled in the art will recognize and be able to use this listing to copy and program the PROMS 2Y and 2C and PAL 3 for their own use. 
     
                                           TABLE 1                                 
__________________________________________________________________________
LUMA PROM                                                                 
ADDRESS                                                                   
      DATA                                                                
__________________________________________________________________________
OOO   00 01 02 03 04                                                      
                    05 06 07 08 09 0A 0B 0C 0D 0E 0F                      
O1O   10 11 12 13 14                                                      
                    15 16 17 18 19 1A 1B 1C 1D 1E 1F                      
O2O   20 21 22 23 24                                                      
                    25 26 27 28 29 2A 2B 2C 2D 2E 2F                      
O3O   30 31 32 33 34                                                      
                    35 36 37 38 39 3A 3B 3C 3D 3E 3F                      
O4O   40 41 42 43 44                                                      
                    45 46 47 48 49 4A 4B 4C 4D 4E 4F                      
O5O   50 51 52 53 54                                                      
                    55 56 57 58 59 5A 5B 5C 5D 5E 5F                      
O6O   60 61 62 63 64                                                      
                    65 66 67 68 69 6A 6B 6C 6D 6E 6F                      
O7O   70 71 72 73 74                                                      
                    75 76 77 78 79 7A 7B 7C 7D 7E 7F                      
O8O   80 81 82 83 84                                                      
                    85 86 87 88 89 8A 8B 8C 8D 8E 8F                      
O9O   90 91 92 93 94                                                      
                    95 96 97 98 99 9A 9B 9C 9D 9E 9F                      
OAO   A0 A1 A2 A3 A4                                                      
                    A5 A6 A7 A8 A9 AA AB AC AD AE AF                      
OBO   B0 B1 B2 B3 B4                                                      
                    B5 B6 B7 B8 B9 BA BB BC BD BE BF                      
OCO   C0 C1 C2 C3 C4                                                      
                    C5 C6 C7 C8 C9 CA CB CC CD CE CF                      
ODO   D0 D1 D2 D3 D4                                                      
                    D5 D6 D7 D8 D9 DA DB DC DD DE DF                      
OEO   E0 E1 E2 E3 E4                                                      
                    E5 E6 E7 E8 E9 EA EB EC ED EE EF                      
OFO   F0 F1 F2 F3 F4                                                      
                    F5 F6 F7 F8 F9 FA FB FC FD FE FF                      
1OO   00 00 01 01 02                                                      
                    02 03 03 04 04 05 05 06 06 07 07                      
11O   08 08 09 09 0A                                                      
                    0A 0B 0B 0C 0C 0D 0D 0E 0E 0F 0F                      
12O   10 10 11 11 12                                                      
                    12 13 13 14 14 15 15 16 16 17 17                      
13O   18 18 19 19 1A                                                      
                    1A 1B 1B 1C 1C 1D 1D 1E 1E 1F 1F                      
14O   20 20 21 21 22                                                      
                    22 23 23 24 24 25 25 26 26 27 27                      
15O   28 28 29 29 2A                                                      
                    2A 2B 2B 2C 2C 2D 2D 2E 2E 2F 2F                      
16O   30 30 31 31 32                                                      
                    32 33 33 34 34 35 35 36 36 37 37                      
17O   38 38 39 39 3A                                                      
                    3A 3B 3B 3C 3C 3D 3D 3E 3E 3F 3F                      
18O   40 40 41 41 42                                                      
                    42 43 43 44 44 45 45 46 46 47 47                      
19O   48 48 49 49 4A                                                      
                    4A 4B 4B 4C 4C 4D 4D 4E 4E 4F 4F                      
1AO   50 50 51 51 52                                                      
                    52 53 53 54 54 55 55 56 56 57 57                      
1BO   58 58 59 59 5A                                                      
                    5A 5B 5B 5C 5C 5D 5D 5E 5E 5F 5F                      
1CO   60 60 61 61 62                                                      
                    62 63 63 64 64 65 65 66 66 67 67                      
1DO   68 68 69 69 6A                                                      
                    6A 6B 6B 6C 6C 6D 6D 6E 6E 6F 6F                      
1EO   70 70 71 71 72                                                      
                    72 73 73 74 74 75 75 76 76 77 77                      
1FO   78 78 79 79 7A                                                      
                    7A 7B 7B 7C 7C 7D 7D 7E 7E 7F 7F                      
2OO   00 00 00 00 01                                                      
                    01 01 01 02 02 02 02 03 03 03 03                      
21O   04 04 04 04 05                                                      
                    05 05 05 06 06 06 06 07 07 07 07                      
22O   08 08 08 08 09                                                      
                    09 09 09 0A 0A 0A 0A 0B 0B 0B 0B                      
23O   0C 0C 0C 0C 0D                                                      
                    0D 0D 0D 0E 0E 0E 0E 0F 0F 0F 0F                      
24O   10 10 10 10 11                                                      
                    11 11 11 12 12 12 12 13 13 13 13                      
25O   14 14 14 14 15                                                      
                    15 15 15 16 16 16 16 17 17 17 17                      
26O   18 18 18 18 19                                                      
                    19 19 19 1A 1A 1A 1A 1B 1B 1B 1B                      
27O   1C 1C 1C 1C 1D                                                      
                    1D 1D 1D 1E 1E 1E 1E 1F 1F 1F 1F                      
28O   20 20 20 20 21                                                      
                    21 21 21 22 22 22 22 23 23 23 23                      
29O   24 24 24 24 25                                                      
                    25 25 25 26 26 26 26 27 27 27 27                      
2AO   28 28 28 28 29                                                      
                    29 29 29 2A 2A 2A 2A 2B 2B 2B 2B                      
2BO   2C 2C 2C 2C 2D                                                      
                    2D 2D 2D 2E 2E 2E 2E 2F 2F 2F 2F                      
2CO   30 30 30 30 31                                                      
                    31 31 31 32 32 32 32 33 33 33 33                      
2DO   34 34 34 34 35                                                      
                    35 35 35 36 36 36 36 37 37 37 37                      
2EO   38 38 38 38 39                                                      
                    39 39 39 3A 3A 3A 3A 3B 3B 3B 3B                      
2FO   3C 3C 3C 3C 3D                                                      
                    3D 3D 3D 3E 3E 3E 3E 3F 3F 3F 3F                      
31O   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
32O   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
33O   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
34O   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
35O   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
36O   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
370   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
380   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
390   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
3A0   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
3B0   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
3C0   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
3D0   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
3E0   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
3F0   00 00 00 00 00                                                      
                    00 00 00 00 00 00 00 00 00 00 00                      
__________________________________________________________________________
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
CHROMA PROM                                                               
__________________________________________________________________________
OOO 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F                       
O1O 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F                       
O2O 20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F                       
O3O 30 31 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F                       
O4O 40 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F                       
O5O 50 51 52 53 54 55 56 57 58 59 5A 5B 5C 5D 5E 5F                       
O6O 60 61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F                       
O7O 70 71 72 73 74 75 76 77 78 79 7A 7B 7C 7D 7E 7F                       
O8O 80 81 82 83 84 85 86 87 88 89 8A 8B 8C 8D 8E 8F                       
O9O 90 91 92 93 94 95 96 97 98 99 9A 9B 9C 9D 9E 9F                       
OAO A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 AA AB AC AD AE AF                       
OBO B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 BA BB BC BD BE BF                       
OCO C0 C1 C2 C3 C4 C5 C6 C7 C8 C9 CA CB CC CD CE CF                       
ODO D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 DA DB DC DD DE DF                       
OEO E0 E1 E2 E3 E4 E5 E6 E7 E8 E9 EA EB EC ED EE EF                       
OFO F0 F1 F2 F3 F4 F5 F6 F7 F8 F9 FA FB FC FD FE FF                       
1OO 40 40 41 41 42 42 43 43 44 44 45 45 46 46 47 47                       
11O 48 48 49 49 4A 4A 4B 4B 4C 4C 4D 4D 4E 4E 4F 4F                       
12O 50 50 51 51 52 52 53 53 54 54 55 55 56 56 57 57                       
13O 58 58 59 59 5A 5A 5B 5B 5C 5C 5D 5D 5E 5E 5F 5F                       
14O 60 60 61 61 62 62 63 63 64 64 65 65 66 66 67 67                       
15O 68 68 69 69 6A 6A 6B 6B 6C 6C 6D 6D 6E 6E 6F 6F                       
16O 70 70 71 71 72 72 73 73 74 74 75 75 76 76 77 77                       
17O 78 78 79 79 7A 7A 7B 7B 7C 7C 7D 7D 7E 7E 7F 7F                       
18O 80 80 81 81 82 82 83 83 84 84 85 85 86 86 87 87                       
19O 88 88 89 89 8A 8A 8B 8B 8C 8C 8D 8D 8E 8E 8F 8F                       
1AO 90 90 91 91 92 92 93 93 94 94 95 95 96 96 97 97                       
1BO 98 98 99 99 9A 9A 9B 9B 9C 9C 9D 9D 9E 9E 9F 9F                       
1CO A0 A0 A1 A1 A2 A2 A3 A3 A4 A4 A5 A5 A6 A6 A7 A7                       
1DO A8 A8 A9 A9 AA AA AB AB AC AC AD AD AE AE AF AF                       
1EO B0 B0 B1 B1 B2 B2 B3 B3 B4 B4 B5 B5 B6 B6 B7 B7                       
1FO B8 B8 B9 B9 BA BA BB BB BC BC BD BD BE BE BF BF                       
2OO 60 60 60 60 61 61 61 61 62 62 62 62 63 63 63 63                       
21O 64 64 64 64 65 65 65 65 66 66 66 66 67 67 67 67                       
22O 68 68 68 68 69 69 69 69 6A 6A 6A 6A 6B 6B 6B 6B                       
23O 6C 6C 6C 6C 6D 6D 6D 6D 6E 6E 6E 6E 6F 6F 6F 6F                       
24O 70 70 70 70 71 71 71 71 72 72 72 72 73 73 73 73                       
25O 74 74 74 74 75 75 75 75 76 76 76 76 77 77 77 77                       
26O 78 78 78 78 79 79 79 79 7A 7A 7A 7A 7B 7B 7B 7B                       
27O 7C 7C 7C 7C 7D 7D 7D 7D 7E 7E 7E 7E 7F 7F 7F 7F                       
28O 80 80 80 80 81 81 81 81 82 82 82 82 83 83 83 83                       
29O 84 84 84 84 85 85 85 85 86 86 86 86 87 87 87 87                       
2AO 88 88 88 88 89 89 89 89 8A 8A 8A 8A 8B 8B 8B 8B                       
2BO 8C 8C 8C 8C 8D 8D 8D 8D 8E 8E 8E 8E 8F 8F 8F 8F                       
2CO 90 90 90 90 91 91 91 91 92 92 92 92 93 93 93 93                       
2DO 94 94 94 94 95 95 95 95 96 96 96 96 97 97 97 97                       
2EO 98 98 98 98 99 99 99 99 9A 9A 9A 9A 9B 9B 9B 9B                       
2FO 9C 9C 9C 9C 9D 9D 9D 9D 9E 9E 9E 9E 9F 9F 9F 9F                       
31O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
32O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
33O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
34O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
35O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
36O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
370 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
380 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
390 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
3A0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
3B0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
3C0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
3D0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
3E0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
3F0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
4OO 00 0O 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 1E                       
41O 0F 1O 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 2E                       
42O 1F 2O 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 3E                       
43O 2F 3O 31 32 33 34 35 36 37 38 39 3A 3B 3C 3D 4E                       
44O 3F 4O 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 5E                       
45O 4F 5O 51 52 53 54 55 56 57 58 59 5A 5B 5C 5D 6E                       
46O 5F 6O 61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 7E                       
47O 6F 7O 71 72 73 74 75 76 77 78 79 7A 7B 7C 7D 7E                       
48O 80 82 83 84 85 86 87 88 89 8A 8B 8C 8D 8E 8F 90                       
49O 91 92 93 94 95 96 97 98 99 9A 9B 9C 9D 9E 9F A0                       
4AO A1 A2 A3 A4 A5 A6 A7 A8 A9 AA AB AC AD AE AF B0                       
4BO B1 B2 B3 B4 B5 B6 B7 B8 B9 BA BB BC BD BE BF C0                       
4CO C1 C2 C3 C4 C5 C6 C7 C8 C9 CA CB CC CD CE CF D0                       
4DO D1 D2 D3 D4 D5 D6 D7 D8 D9 DA DB DC DD DE DF E0                       
4EO E1 E2 E3 E4 E5 E6 E7 E8 E9 EA EB EC ED EE EF F0                       
4FO F1 F2 F3 F4 F5 F6 F7 F8 F9 FA FB FC FD FE FF FF                       
5OO 40 40 41 41 42 42 43 43 44 44 45 45 46 46 47 47                       
51O 48 48 49 49 4A 4A 4B 4B 4C 4C 4D 4D 4E 4E 4F 4F                       
52O 50 50 51 51 52 52 53 53 54 54 55 55 56 56 57 57                       
53O 58 58 59 59 5A 5A 5B 5B 5C 5C 5D 5D 5E 5E 5F 5F                       
54O 60 60 61 61 62 62 63 63 64 64 65 65 66 66 67 67                       
55O 68 68 69 69 6A 6A 6B 6B 6C 6C 6D 6D 6E 6E 6F 6F                       
56O 70 70 71 71 72 72 73 73 74 74 75 75 76 76 77 77                       
57O 78 78 79 79 7A 7A 7B 7B 7C 7C 7D 7D 7E 7E 7F 7F                       
58O 80 80 81 81 82 82 83 83 84 84 85 85 86 86 87 87                       
59O 88 88 89 89 8A 8A 8B 8B 8C 8C 8D 8D 8E 8E 8F 8F                       
5AO 90 90 91 91 92 92 93 93 94 94 95 95 96 96 97 97                       
5BO 98 98 99 99 9A 9A 9B 9B 9C 9C 9D 9D 9E 9E 9F 9F                       
5CO A0 A0 A1 A1 A2 A2 A3 A3 A4 A4 A5 A5 A6 A6 A7 A7                       
5DO A8 A8 A9 A9 AA AA AB AB AC AC AD AD AE AE AF AF                       
5EO B0 B0 B1 B1 B2 B2 B3 B3 B4 B4 B5 B5 B6 B6 B7 B7                       
5FO B8 B8 B9 B9 BA BA BB BB BC BC BD BD BE BE BF BF                       
6OO 60 60 60 60 61 61 61 61 62 62 62 62 63 63 63 63                       
61O 64 64 64 64 65 65 65 65 66 66 66 66 67 67 67 67                       
62O 68 68 68 68 69 69 69 69 6A 6A 6A 6A 6B 6B 6B 6B                       
63O 6C 6C 6C 6C 6D 6D 6D 6D 6E 6E 6E 6E 6F 6F 6F 6F                       
64O 70 70 70 70 71 71 71 71 72 72 72 72 73 73 73 73                       
65O 74 74 74 74 75 75 75 75 76 76 76 76 77 77 77 77                       
66O 78 78 78 78 79 79 79 79 7A 7A 7A 7A 7B 7B 7B 7B                       
67O 7C 7C 7C 7C 7D 7D 7D 7D 7E 7E 7E 7E 7F 7F 7F 7F                       
68O 80 80 80 80 81 81 81 81 82 82 82 82 83 83 83 83                       
69O 84 84 84 84 85 85 85 85 86 86 86 86 87 87 87 87                       
6AO 88 88 88 88 89 89 89 89 8A 8A 8A 8A 8B 8B 8B 8B                       
6BO 8C 8C 8C 8C 8D 8D 8D 8D 8E 8E 8E 8E 8F 8F 8F 8F                       
6CO 90 90 90 90 91 91 91 91 92 92 92 92 93 93 93 93                       
6DO 94 94 94 94 95 95 95 95 96 96 96 96 97 97 97 97                       
6EO 98 98 98 98 99 99 99 99 9A 9A 9A 9A 9B 9B 9B 9B                       
6FO 9C 9C 9C 9C 9D 9D 9D 9D 9E 9E 9E 9E 9F 9F 9F 9F                       
71O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
72O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
73O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
74O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
75O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
76O 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
770 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
780 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
790 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
7A0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
7H0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
7C0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
7D0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
7E0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
7F0 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80 80                       
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                       TABLE 3                                                     
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Module pla.sub.-- dthr05                                                  
flag `-r3`                                                                
title `dither05` SPG burst smpl                                           
VATEK UNITY project                                                       
                 03-8-90 V LOWE                                           
.sub.-- dthr05 device `p16v8r`;                                           
&#34; 01 V. Lowe creation - 04-26-91                                          
&#34; 02 add 1.5 clk                                                          
&#34; 03 add control bit                                                      
&#34; 04 make output combinatorial                                            
&#34; 05 mod CADD equation                                                    
&#34;***NOTE*** This device is used to add dither to the blanking             
&#34; PAL to minimize the effects of rounding. The blanking PAL               
&#34; adds 1 to the input value when YADD is high instead of only             
&#34; dithering by changing state of the LSB. Note that YADD is               
&#34; dithered at 1/2 clock rate when the 9th LSB is high and the             
&#34; 10th LSB is low. YADD is high 3/4th of the time when both are           
&#34; high (11) and high 1/4th of the time when the 9th LSB is low and        
&#34; the 10th is high (01). CADD is the chroma dither control and is         
&#34; dithered at 1/2 the luma rate because the chroma is sampled at 1/2      
&#34; the luma rate in the system.                                            
&#34;0UTPUTS                                                                  
SPO      PIN 19 =`pos,reg`;                                               
                       &#34; SPARE                                            
CADD     PIN 18 =`pos,com`;                                               
                       &#34; C ADD LSB                                        
YADD     PIN 17 =`pos,com`;                                               
                       &#34; Y ADD LSB                                        
SP1      PIN 16 =`pos,reg`;                                               
                       &#34; SPARE                                            
CK3      PIN 15 =`pos,reg`;                                               
                       &#34; CLOCK DIVIDED BY 4                               
                        (3.375 MHz)                                       
HSQ      PIN 14 =`pos,reg`;                                               
                       &#34; H SQUARE WAVE                                    
CK6      PIN 13 =`pos,reg`;                                               
                       &#34; CLOCK DIVIDED BY 2                               
                        (6.75 MHz)                                        
CK1.sub.-- 5                                                              
         PIN 12 =`pos,reg`;                                               
                       &#34; CLOCK DIVIDED BY 8                               
                        (1.6875 MHz)                                      
&#34;INPUTS                                                                   
CK       PIN 1;        &#34; 13.5 MHZ INPUT CLOCK                             
YX       PIN 2;        &#34; Y LSB -1                                         
YY       PIN 3;        &#34; Y LSB -2                                         
CX       PIN 4;        &#34; C LSB -1                                         
CY       PIN 5;        &#34; C LSB -2                                         
HRST     PIN 6;        &#34; H RESET FROM                                     
                        COUNTER                                           
CONTROL  PIN 7;        &#34; EXTERNAL DITHER                                  
                         ON CONTROL. HIGH =                               
                        DITHER ON                                         
S1       PIN 8;        &#34; SPARE INPUTS                                     
S2       PIN 9;          &#34;                                                
S3       PIN 10;         &#34;                                                
S4       PIN 11;         &#34;                                                
&#34;CONSTANTS                                                                
c = .c.;                                                                  
x = .x.;                                                                  
z = .z.;                                                                  
equations                                                                 
HSQ := (-HSQ 7 -HRST.sub.--) # (HSQ 7 HRST.sub.--);                       
CK6 := (-CK6);                                                            
CK3 := (-CK3 &amp; CK6) # (CK3 &amp; -CK6);                                       
CK1.sub.-- 5 := (CK6 &amp; CK3 &amp; -CK1.sub.-- 5) # (CK1.sub.-- 5 &amp; -(CK6 &amp;     
CK3));                                                                    
YADD = (YX &amp; -YY &amp; CK6 &amp; HSQ) &amp; CONTROL                                   
# (YX &amp; -YY &amp; -CK6 &amp; -HSQ) &amp; CONTROL                                      
# (YX &amp; YY &amp; (CK6 # CK3)) &amp; CONTROL                                       
# (-YX &amp; YY &amp; CK6 &amp; CK3) &amp; CONTROL;                                       
CADD = (CX &amp; CK3 &amp; CONTROL)                                               
# (CX &amp; CY &amp; (CK3 # CK1.sub.-- 5)) &amp; CONTROL                              
# (-CX &amp; CY &amp; CK3 &amp; CK1.sub.-- 5) &amp; CONTROL                               
end;                                                                      
______________________________________                                    
 
    
     The following is a description of the technique applied to a digital video signal using CCIR-601 quantizing and clock rates but the invention is not limited to this digital format. The 9th bit is encoded, in this scheme, by adding one to the upper 8 bits on alternate clocks of the sampling clock, 13.5 MHz in the present example, making the encoded data frequency 6.75 MHz which is 1/2of 13.5 MHz. The addition is alternated on alternate signal periods, in the present example horizontal lines, by using to the opposite phase of the 6.75 MHz clock. This means that there are is period to period coherent dither. In video there are no vertical stripes visible because the brighter pixels on one line are offset from the lines above and below. The line alternation also causes a natural frame to frame interleaving of the pixels that are brightened. Interlaced 525 and 625 line video has an odd number of lines per frame so that the first line of one frame is an odd number of inversions from the line in the previous frame. Therefore, the phase of encoding, on any particular line, is inverted from one frame to the next. 
     The 10th bit is encoded by using a clock derived by dividing the sample clock by four. The phase is also reversed in alternate periods. This division produces a natural phase alternation from line to line in 525 line video with a 13.5 MHz clock. 858 divided by 4=214.5. The 1/2 cycle remainder causes a line alternation. In 625 video systems 864 samples are created because of the slightly longer line length. This means that the 1/4 clock rate divides evenly so the encoding clock must be forced to alternate to achieve the desired concealment effect. 
     If an 11th bit were to be decoded, it would be encoded at the sample clock divided by 8, and a 12th bit would be encoded at the sample clock divided by 16, etc. Alternatively, but less preferred, encoding at rates other than binary can be used, for example, the sample clock ÷2, 3, 4, and 5 for the 9th, 10th, 11th, and 12th bits respectively. This method is less preferred however because of added complexity in deriving the clock and in ensuring the proper period to period phase reversal. 
     The binary encoding method is described in the following table. A 1 indicates that 1 is to be added to the upper 8 bits on that pixel and a 0 indicates that nothing is added. 
     
         ______________________________________                                    
           repeating addition factor across the line                      
        bits odd field    even field                                      
        9 10 time→ time→                                    
______________________________________                                    
line odd  0 1    010001000100 000100010001                                
line even 0 1    000100010001 010001000100                                
line odd  1 0    010101010101 101010101010                                
line even 1 0    101010101010 010101010101                                
line odd  1 1    011101110111 110111011101                                
line even 1 1    110111011101 011101110111                                
______________________________________                                    
 
    
     Three factors work together to make this encoding method appear invisible for video: 
     1. The lowpass filter prescribed for the D1 digital standard is 6 db down at 6.75 MHz. 
     2. The fine pitch of the encoding is too small for the eye to resolve at normal viewing distances. 
     3. The eye integrates the 30 Hertz frame rate in the low level intensity changes in the same pixel location to make the encoding appear invisible. 
     In systems where it is desired to pass the truncated signal through the invention a plurality of times, the encoding phase is to be reversed where two inventions are cascaded or where multiple generations of production through one invention are required. The encoding phase can be predetermined by the system control computer. When the signal paths include multipliers or otherwise can present a full scale truncated video signal, that is the multiplication coefficient is 1111 1111, or the video is allowed to achieve a value of 1111 1111, a different technique is used to create a lossless pass through the system. When two multipliers are in series and less than full scale output is expected, or the path otherwise is not expected to pass full value video, the invention will both encode the 9th and 10th bits using the above scheme. The second correction stage, or second pass through the encoding is caused to use the odd line encoding for even line encoding to interleave the corrections instead of adding another LSBs at the same point on each line. 
     Multiple generation dither encoding would possibly degrade the picture if the encoding always affected the same pixels on each line. Therefore, a detections scheme will be incorporated to determine the presence of previous encoding and the dominant phase of that encoding. Once information of the encoding and the dominant phase has been determined, that information will be used to force the new encoding to interleave with previous encoding. The first encoding detection scheme is to place a synchronization code in the horizontal or vertical blanking of the video signal. The second encoding detection scheme is to bury a flag signal at a vertical and/or horizontal rate that defines the last encoding phase. This can be done by forcing a 1 on the LSB of the first active pixel of each line that uses odd encoding and a 0 on the first active pixel of each line using even encoding. When used with video effects systems, this technique properly identifies full field and vertical split wipes but misidentifies the second half of horizontally split wipes. A second technique uses a phase comparison technique to compare the amplitude of each pixel to the preceding pixel to achieve a greater than, less than, or equal to comparison and then comparing to the 1/2 clock and 1/4 clock to determine encoding phase. This technique will only be effective part of the time but will give accurate phase information on a long term average and may be used solely, or in conjunction with the buried flag to give an indication in the absence of a flag reference. 
     SUMMARY OF RECOVERY OF ENCODED LSB 
     The primary purpose of the decoder is to generate additional LSBs in a data path in response to the preinserted dither, so as to convey more bits of information to the eye without creating visible artifacts. Another benefit of using the predetermined encoding algorithm that alternates from line to line and produce 10 bits of resultant data by using horizontal, vertical, field and/or frame rate recovery filters. It should be noted that in many applications, the action of the reconstruction LPF which follows the D-A convertor, or alternatively which follows the resampling section of the system, and which produces the continuous analog output, will provide sufficient recovery filtering action to make it unnecessary to use any additional recovery filtering circuit, and therefore, the invention may be practiced with the encoder circuit alone. 
     The preferred embodiment of the recovery filter is to combine the LSB from surrounding pixels, with a central pixel. These filtering techniques are most useful when the data is not changing in one of the dimensions from which the pixels are selected, or changing at a very small rate. Adjacent pixels in the H, V, and time dimensions are preferred for the filtering, however diagonally adjacent or other surrounding but not adjacent pixels can also be used, either alone or in conjunction with the adjacent pixels. 
     FIG. 3 shows the preferred embodiment of the decoder section of the invention, having dithered signal input 15a, delays 16a and 16b coupled to the input dithered signal to provide a plurality of surrounding elements, including a central element. The range of delay for 16 is preferred to be one or two sample periods, however larger periods may be utilized as well. FIG. 3 further includes difference detector 17 coupled to at least a pair of the plurality of surrounding versions, preferably two outer versions to determine the difference thereof, and comparator 18 responsive to at least one threshold value 19 and alternative to other threshold values 20 and to the difference detector 17 to determine if the difference of the surrounding versions of dithered signal input to the difference detector is greater than a threshold, and adder 21 responsive to some of the LSBs of the outer surrounding versions to provide the sum thereof, switch mechanism 22 responsive to 18 to select the output of 21, or a signal equivalent to zero to be combined with twice the central version in adder 25, thereby creating the higher precision version of said dithered input signal 15a as output signal 23a. For clarity, a scaler 24 is shown which scales the central element by 2X, however in practice, this is preferred be accomplished by simple bit shifting. Shown as an alternate embodiment, the central element may also be coupled to 17, thereby producing two differences, each of which is compared to a corresponding threshold 19 and 20. If both differences are less than the threshold, 22 is caused to select the output of 21. 
     In the digital video version of the preferred embodiment of FIG. 3, the delays 16a and 16b may range from one to a few sample periods, preferably one, thereby providing a central and two outer surrounding pixels. The outer pixels are preferred to be coupled to the difference detector 17, which computes the difference thereof, and comparator 18 compares said difference to the threshold 19. If the difference is less than the threshold, switch 22 is caused to select the output of 21, and if the difference is greater than the threshold, the switch 22 is caused to select a zero for input to 20. The value output from switch 22 is then added to the central pixel by adder 25 and this sum is output at 23a as the improved resolution version of the dithered input 15a. Essentially, the precision recovery is achieved by adding the two LSBs on either side of the center pixel and adding the result to twice the center pixel. The two additions gives a 10 bit result. That is the sum of the two adjacent LSBs is added to twice the center pixel. This type of recovery works best if limited to picture areas with very little horizontal change. 
     Vertical recovery may be achieved by the same method, except that the pixels from which the LSBs are taken are adjacent in the V direction. Vertical recovery is preferred to by achieved by adding the two pixels from the lines above and/or below the center pixel and adding the result to twice the center pixel. This would be accomplished by changing delays 16a and 16b each to one horizontal line. Again, the two additions have a 10 bit result. Vertical recovery is preferred to be limited to areas of the picture with very little vertical change by using subtraction and a threshold detector shown by 17 and 18. Similarly, time recovery may utilize pixels in the time dimension, that is from the previous and next field or frame, by changing delays 16a and b to field or frame delays. Similarly it is preferred to be limited to picture areas having very little time change by using a subtraction and a threshold detector. 
     The type of recovery, that is the direction(s) from which the pixels containing the LSBs which are selected is preferred to be limited to picture areas with very little change. This can be done by subtracting adjacent pixels and using a threshold detector to inhibit the addition of the adjacent pixels, as is commonly done in the art to control noise reduction averaging. Such a system is shown by Stratton in U.S. Pat. No. 4,803,547, in which horizontal and vertical differences are compared and used to switch between horizontal and vertical averaging. While Stratton is used to implement a comb filter for chroma separation, one skilled in the art will be capable of using the teachings of Stratton, in conjunction with the present disclosure, to properly construct a recovery circuit to implement the present invention. 
     A recovery circuit architecture which utilizes 9 surrounding elements to allow comparison of differences in horizontal, vertical and diagonal directions is shown in FIG. 4. The dithered input at 15b is coupled to comparison and processing circuit 18A, as well as to delays 16c through 16h, and 17a and 17b, in order to provide three elements each of three consecutive scans of the input signal to the comparison and processing circuit. If the delays 16 are one sample clock, and delays 17 are one scan line, the 9 elements will be a central and 8 surrounding elements. The comparison and processing 18a will then compute which direction has the smallest gradient, and process the central element and LSBs of the surrounding elements in that direction to produce the increased precision output 23b. 
     FIG. 5 shows a recovery circuit similar to FIG. 4 which allows the 9 elements to be taken from two frames, or from two fields. The dithered input at 15c is coupled to comparison and processing circuit 18b, as well as to delays 16i through 16n, and field or frame delay 26, and 1H delay 17c, in order to provide three elements each of three scans of the input signal to the comparison and processing circuit. One scan will be the present scan, and the other two from the previous field or frame depending on the length of 26. Note that 26 is offset by 1H as necessary to provide the scan above and scan below the input scan If the delays 16 are one sample clock, and delay 17c is one scan line, the 9 elements will be a central and 8 surrounding elements as with FIG. 4. The comparison and processing 18b will then compute which direction has the smallest gradient, and process the central element and LSBs of the surrounding elements in that direction to produce the increased precision output 23c. 
     Note in FIG. 5 that the comparison may be made between the input and previously output elements by enabling the delay 26 connection 19 in place of the connection to 15c. This will provide a recursive process capable achieving of even greater precision. 
     One skilled in the art will further realize that the addition may be selectively performed in either the H, V or time direction, with the selection being determined in response to the gradient or other measure of difference in that direction. The calculation of gradients which may be used to control such selection of addition is well known in the art, for example as discussed by Graham in U.S. Pat. No. 3,009,016. Alternatively, a ranking of the differences could be used to determine the direction of selection of pixels for combination, such as the ranking shown in U.S. Pat. No. 4,573,070. 
     As previously indicated, any combination of the three types of filtering can be used at any time or all at the same time to give the best decoding result, providing the change in the directions being used at a given instant is relatively small. In other words, it is preferred however that any dimension of filtering have little or no change in that dimension. These adaptive techniques that limit the filtering to small threshold changes in any particular dimension are used to limit any softening artifacts in the picture. These techniques can recover the 9th and 10th bits in the picture areas that need the most gray scale resolution. That is because, in most picture material, 8 bits of resolution are adequate in rapidly changing areas and less resolution only becomes noticeable in flat or slowly changing areas. It is generally preferred that only the last LSB of the dithered signal be used, however is possible to use the last two or last 3, or even the entire surrounding element with sufficient scaling of the central element to which they are added. The use of the last LSB is however quite imple to implement, and the use of additional bits brings about little or no improvement thereon. 
     The recovery can further be adaptively modified in response to the differences in a plurality of directions, in a fashion such that a portion of the sum of the LSBs in each of the directions is added to a central pixel, with the portion being responsive to the relative difference in magnitude of the differences for each direction. For example, if the differences in the H and V direction are within 20% of equality, then half of the H LSB sum and half of the V LSB sum would be added to the central pixel. If the H direction difference were less than 80% of the V direction sum, then all of the H LSB sum and none of the V LSB sum would be added to the central pixel. Conversely, of the V direction difference were less than 80% of the H direction sum, then all of the V LSB sum and none of the H LSB sum would be added to the central pixel. This operation could be conveniently carried out in a PROM look up table, where the portion of the directional sum to be combined with the central pixel is output in response to the difference of the direction differences, with the outputs then being added to the central pixel. Of course, a multiplicity of curves of output VS input VS difference of direction differences can be implemented in the PROM to suit the particular needs of the system. 
     As previously noted, the same inventive principals can be applied to any width of data path and reductions to any width of truncated data path. For example, 12 bits can be encoded in a 10 bit data path or an 8 bit data path, 14 bits in a 12 bit path and so forth. Similarly, recovery can be achieved to support any of the various reductions.