CA1158355A - Adaptive amplitude averaging for weighting quantizing noise - Google Patents
Adaptive amplitude averaging for weighting quantizing noiseInfo
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- CA1158355A CA1158355A CA000370023A CA370023A CA1158355A CA 1158355 A CA1158355 A CA 1158355A CA 000370023 A CA000370023 A CA 000370023A CA 370023 A CA370023 A CA 370023A CA 1158355 A CA1158355 A CA 1158355A
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- 238000012935 Averaging Methods 0.000 title claims abstract description 35
- 230000003044 adaptive effect Effects 0.000 title description 3
- 238000005070 sampling Methods 0.000 claims description 16
- 238000000034 method Methods 0.000 claims description 15
- 238000013139 quantization Methods 0.000 claims description 15
- 230000010363 phase shift Effects 0.000 claims description 7
- 230000000737 periodic effect Effects 0.000 claims description 4
- 230000001360 synchronised effect Effects 0.000 claims description 4
- 238000010586 diagram Methods 0.000 description 3
- 230000005540 biological transmission Effects 0.000 description 2
- 230000001934 delay Effects 0.000 description 2
- 238000001914 filtration Methods 0.000 description 2
- 229920000136 polysorbate Polymers 0.000 description 2
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- 230000010354 integration Effects 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N9/00—Details of colour television systems
- H04N9/64—Circuits for processing colour signals
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B14/00—Transmission systems not characterised by the medium used for transmission
- H04B14/02—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation
- H04B14/04—Transmission systems not characterised by the medium used for transmission characterised by the use of pulse modulation using pulse code modulation
- H04B14/046—Systems or methods for reducing noise or bandwidth
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/90—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using coding techniques not provided for in groups H04N19/10-H04N19/85, e.g. fractals
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- Multimedia (AREA)
- Computer Networks & Wireless Communication (AREA)
- Analogue/Digital Conversion (AREA)
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Abstract
-16- RCA 74,900 ABSTRACT OF THE INVENTION
An apparatus reduces quantizing noise by averaging picture information when only low frequency information is present. Thus, high frequency detail is not lost. This is determined by looking at proximate samples to see if their amplitudes are within a selected amount of each other. The averaging typically is of two or four picture samples depending upon how great an area has only low frequencies, but greater numbers of samples can be used.
An apparatus reduces quantizing noise by averaging picture information when only low frequency information is present. Thus, high frequency detail is not lost. This is determined by looking at proximate samples to see if their amplitudes are within a selected amount of each other. The averaging typically is of two or four picture samples depending upon how great an area has only low frequencies, but greater numbers of samples can be used.
Description
~15~3~
-1- RCA 74,900 ADAPTIVE AMPLITUDE AVERAGING FOR WEIGHTING Q~ANTIZING NOISE
1 The present inven-tion relates to digi-tal television, and more particularly, -to processing -that reduces quantizing noise.
In digital video, when too few bits are used to quantize each sample of the video waveform, quantiza-tion 5 noise is large and causes objec-tionable contouring or "puddling" of the displayed picture. This is most objection-able in areas of gradual intensity changes where the slope of the video waveform is small compared to the quantization step size. This is because in such slowly changing, low 10 frequency portions, of the picture, the eye is most sensitive to quantization noise. By increasing the number of bits per sample, more levels are available for quantization, and the gradual change is more closely represented. However, the use of more bits per sample results in the penalty of a corres-15 ponding increase in -the required data rate of the digital video. It is known from -the article en-titled "PCM Encoded NTSC Color Television Subjective Tests" by A.A~ Goldberg, JSMPTE, Augus-t, 1973, p.p. 649-654, to add ei-ther a square wave or a random signal -to -the video signal before quantiz-20 ation to reduce contouring, and then low pass filtering -the reproduced analog video signal to reduce the visibility of the added signal and the quantizing noise. However in this system, the low pass filter also reduces the high frequency video signal information.
It is therefore desirable to reduce the visibility of contouring, and quantizing noise wi-thout increasing the data rate and without reducing the high frequency video signal information.
According to one aspect of the invention, there is 30 provided a method for reducing quantizing noise in a sampled quantized signal, said method comprising adding a periodic multilevel offset signal to said sampled signal prior -to quantization, quantizing said signal, determining if only low frequency information is present, averaging a plurali-ty 35 of samples when only low frequency information is present, 1 ~5~55 -la- R~A 74,900 1 every plurali-ty of averaged samples having the same average value of offset signal amplitude, whereby additional quantizing levels be-tween the original levels are recovered in the averaged samples, thereby reducing quantizing noise.
According to another aspect of the invention, there 5 is provided an apparatus for reducing quantizing noise in a sampled quantized signal, said apparatus comprising means for adding a periodic multilevel offset signal to said signal prior to quantization, means for quantizing said signal, means for determining if only low frequency information is present, 10 mean~ for averaging a plurality of samples when only low frequency information is present, every plurality of averaged samples having the same average value of offset signal ampli-tude, whereby additional quantizing levels between the original levels are recovered in the averaged samples, thereby 15 reducing quantizing nolse.
t~5~
1 -2- RCA 74,900 In the Drawings:
FIGURE 1 shows a gradually changing video signal quantized using only a few levels;
FIGURE 2 shows the same signal with more quantizing levels;
FIGURE 3 shows a flat field with coarse quantization;
FIGURE 4A through D show va:rious waveforms concerning a flat field using amplitude averaging in accordance with the present invention;
FIGURE 5 shows in bloc~ diagram form an adaptive amplitude averaging system;
FIGURE 6 and 7 show some waveforms used in the system of FIGURE 5;
FIGIJRE 8 shows a diagram for use in explaining FIGURE 5;
FIGURE 9 shows the details of a delay buffer used in FIGURE 5;
FIGURE 10 shows the details of two point averaging logic circuit used i.n FIGURE 5;
FIGURE 11 shows the details of a four point averaging logic circuit used in FIGURE 5; and FIGURE 12 shows in more detail waveforms of the present invention.
FIGURE 1 shows a graph of an analog video signal 12 which is sampled at a fixed sampling frequency at sampling points of times 14 and quantized to the next lowest of various quantizing levels 16. The result is a digital waveform 18. The difference between analog signal 12 and digital signal 18 is the quantizing error, which can be as great as one quantizing level. Because -the slope of the waveform 12 changes slowly with respect -to the difference between quantizing levels 16, sharp edges 20 occur ln the digital waveform which are separated from each other by some considerable distance along the horizontal axis by a constant amplitude signal. The 4~
~ 1~8355 1 -3- RCA 74,900 result is highly visible contours in the picture display from signal 18.
FIGURE 2 shows a graph where the number of quantizing levels have been increased and where corresponding reference numerals have been applied to corresponding elements of the graph. I-t will be noted that the steps 20 are much smaller in amplitude and occur more frequently than is the case in FIGURE 1, resulting in a smaller quantizing error. However, increased quantizing levels require an increased data rate. Consider the situation in FIGURE 3 where again corresponding portions of the graph have been given corresponding reference numerals. It will be seen that due to the small number of the quantizing levels 16, there is a fixed quantizing error 22, if the analog video signal 12 is flat and occurs between quantizing levels as shown in FIGURE 3.
FIGURE 4A shows a signal which can help overcome these problems. An offset signal 24 is shown which has a square wave shape, a frequency equal to one-half the sampling frequency used, and a peak amplitude equal to one-half of the amplitude difference between adjacent quantizing levels. Note that center of the upper portion of the square wave 24a coincides with every other sampling j point 14. FIGURE 4B shows the results when this offset signal 24 is added to an analog video signal 12 having a constant amplitude of 3/4 o-f a quantizing level before the quantizing step. Note that in FIGURE 4B, the amplitude of adjacent samples alternates up and down by one-half of a quantizing level. FIGURE 4C shows the result of the signal of FIGURE 4B after quantization.
This signal 25, in the case shown, exis-ts only at the discrete quantizing levels Q and 1 and adjacent signals do not differ from each other by more than one quantizing level. This being -the case, and in accordance with -the invention, the signal 25 is integrated or averaged, either by the eye or electronically, to give the result 3 ~ ~
1 -4- RCA 74,900 shown in FIGURE 4D, if the averaging is done electronically. The result is a signal level half-way between two quantization levels. Thls holds true for any value of the analog video signal 12 lying in the upper halE of any quantizing range, e.g. between one-half and one, one and one-half and two, two and one-half and three, etc. For a value of signal in the lower half of a quantizing range, e.g. zero and one-half, one and one-half, two and two and one-half, etc., the combined video and offset signal will always be quantized at the lower end of the range, e.g. zero, one, two, respectively, e-tc.
In either case, after the integration or averaging is ' performed, the maximum quantizing error is one half of quantizing step, ins-tead of the maximum error of one step as discussed above in connection with FIGUR~ 1. This corresponds to an effective doubling of the number of quantization levels without adding an extra one bit per sample, which can be used for representing slowly changing information. For high frequency information, the maximum error is increased to one and one-half levels (one original level plus one-half level -from the offset signal) since these signals cannot be averaged. However, as explained above, noise in high frequency portions of the signal is less objectionable than in low frequency portions.
The concept described above can be extended to averaging more than two samples to obtain more low frequency amplitude resolution. For example, two line alternating synchronous offset signals each having two levels tha-t differ from -the levels in the other signal, for a total oE four levels (each level corresponding to one-quarter of a quantizing step) might be added to the analog video signal before quantiza-tion.
Under the conditions that four adjacen-t samples are all no more than one quantizing level apar-t, an averaging of the four would produce one of four possible amplitude levels, three intermediate amplitude levels e.g.
~ 3L5~3~5 1 -5- RCA 74,900 one-quarter, one-half, three-quarters of a quantizing step and one quantizing level. This is the equivalent of adding two bits per sample. If the condition is not met that the four adjacent samples differ by not more than one quantiziny level, two adjacent pixels can be examined.
If the two pixels are not more than one level apart, they can be averaged to provide one additional bit of amplitude resolution per sample. If the condition that two adjacent pixels differ by no more than one level is not met either, then the picture content is a high ! frequency signal, i.e. a sharp transition, where quantization error is not an important factor.
Although the aforementioned added signal is helpful in explaining the invention, when the quantizing steps are small, compared to the noise in the input video signal, no o-Efset signal is required. Basically, this application relates to a~aptive filtering or averaging of mutual proximate signal samples.
An e~ample of a system for carrying out the above operations is shown in FIGURE 5. An analog video signal is fed into one input of adder 26, while offset signals are applied in a line-alternate fashion to its other input. These offset signals are shown in FIGURE 6 and 7. During alternate lines, an offset signal 28, such as shown in FIGURE 6, is applied to the adder 26. It alternates between amplitudes of zero and three-quarters of the difference between adjacent quantization levels. During the remaining alternate lines, the signal 30 of FIGURE 7 is applied to adder 26. It has an amplitude that alterna-tes be-tween one-quarter and one-half that of the difference between adjacent quantizatlon levels.
These signals 28 and 30 occur at one-half the ra-te of the sampling frequency wi-th a phase shif-t of 90 therebetween.
As shown in FIGURES6 and 7, the sampling clock has a 180 phase shift from line to line, thus shifting the sample points 14 from line to line. Further, there is a ~0 3 ~ 5 1 -6- RCA 74,900 180 phase shift for alterna-te lines of each of the offset signals 28 and 30.
These phase shifts occur every time the respective signals start a new horizontal line. The portion of the output signal from adder 26 due to signals 28 and 30 is shown diagramatically in FIGURE 8 which shows a portion of a raster. The quantizing levels that have been added to the analog video signal by adder 26 are shown in FIGURE 8 expressed in quarters of a quantizing level. Note that these numbers represent the additional levels and not the absolute value of the digital signals coming ou-t of adder 26. In the following discussion, the sampling point 32 will be considered as the one currently under consideration, and reference will be made to the proceeding point 34 on the same line, a point 36 on the line above, and a point 38 on the line below. The analog video signal plus the additional quantizing levels are applied to an analog to digital converter 40, which in turn applies an 8 bit digital representation of the sum of said analog video signal and the additional levels through a ~ transmission path 41 to a delay buffer 42. Thls 8 bit buffer 42 supplies at each of its outputs 32a, 34a, 36a, 38a, 8 bit slgnals representing the amplitude of the signal occurring at the various sample points 32, 34, 36 and 38 respectively. l'he outputs are denominated i using corresponding reference numbers with the suffix "a" added to indicate which points appear at which outputs. All of the outputs of buffer 42 are applied to four point averaging logic circuit 44. This circuit supplies at an output 46 a flag signal when all four points 32, 34, 36, and 38 are not more than one quantizing step different in amplitude Erom each other. Ou-tput 46 supplies -the flag as a control signal to r~UX 48, which comprises a SPDT switch. The swi-tch is in the posi-tion shown, when -the above condi-tion is true, so tha-t an averaye of the amplitudes of the four poin-ts 32, 34, 36, 38 is supplied by ou-tput 50 of logic circuit 44 and is applied -to data output 52 by MUX 48. The signal at ~ 1583~
1 -7- RCA 74,900 outputs50 and 52 has the resolution of a 10 bit signal, which reduces contouring without increasing the data rate through transmission path 41. If all four of said points are not within one ~uantizing level difference of each other, then there is no four point average selection flag signal at output 46, and hence ~IUX 48 is switched to its lower position, and thus receives the output of r~ux 56, which also comprises a SPDT switch.
Signals representing points 32 and 34 from outputs 32a and 34a are applied to two point averaging logic circuit 54, and if these points are within one quantizing level step of each other, a two point average flag selection signal is applied from output 58 to MUX 56, so that it is in the position shown. In this case, output 60 supplies a signal representing the actual two point average of points 32 and 34 to MUX 48 by way of MUX 56, and hence to data output 52. This gives the resolution of a 9 bit signal. In the event that points 32 and 34 are not within one quantizing level step of each other, then no two point average selection flag signal is present at output 58, and hence ~5UX 56 is in the lower position (not shown, just the signal representing point 32 is applied 25 through MUX 56 and MUX 48 to data output 52, which signal is an 8 bit one.
The line alternate added offset signals 28 and 30 of FIGURES 6 and 7 are repeated in FIGURES 12a and b.
In operation on an analog signal 1212 as illustrated in FIGURE 12c having a relatively constant amplitude lying at a quantizing level 1216, the arrangement of FIGURE 5 produces during a first horizontal line ~ summed signal illustrated as 1214 of FIGURE 12d, representing the sum of signals 28 and 1212. During the next horizontal line, sum signal 1220 is produced representing the sum of signals 30 and 1212. Where quantized, slgnal 1214 will take on a digital value equal to digitizing level 1216 at each sample point as illustrated by signal 1222 of FIGURE 12f, for analog signal 1214 never reaches 3~
1 -8- RC~ 74,900 guantizing level 1218. Similarly, signal 1224 in FIGURE 12g represents the digital value resulting from the digitizlng of signal 1220. Signal 1224 also remains at quantizing level 1216, for signal 1220 does not reach the next quantizing level 1218. The time average of signals 1222 and 1224 equals quantizing level 1216, and consequently the digital value is a close approximation to the analog values.
FIGV~ES 12h-12O illustrate the conditions when the input analog signal takes on a value lying slightly above quantizing level 1216, as illustrated by signal 1226 of FIGURE 12h. Signal 1226 lies above quantizing level 1216 by 1/4 of a quantizing level. If signal 1226 were simply quantized as in the prior art, the quantizing error would be 1/'4 of a quantizing level. Signal 122& in FIGURE 12j illustrates the sum of offset signal 28 and analog signal 1226 as generated by the apparatus of FIGURE 5. It should be noted that signal 1228 reached the next higher quantizing level 1218 at alternate sampling points. Signal 1230 of ~GU~E 12k represents the sum of signal 30 and signal 1226. Since signal 1226 is only 1/4 of a quantizing level above level 1216 and the maximum amplitude of signal 30 with which it is summed is 1/2 quantizing level, signal 1230 does not reach next higher quantizing level 1218. The result of quantizing signal 1228 is illustrated as signal 1232 of FIGURE 12m and the result of quantizing signal 1230 isillustrated as signal 1234 in F IGURE 12n. The time ; average of the digital sum of digital signals lZ32 and 1234 produced by the arrangement of FIGURF 5 is 1/4 of a quantiZing level above level 1216, which is exactly the value of analoa signal 1226. qhus, in this case the quantiZing error has been reduced from 1/4 of a level to zero.
If the analog signal applied to the arrangement of FIGURE 5 lies half-way between quantizing levels 1216 and 1218, the sum signals are as illustrated by waveforms 1236 and 1238, illustrated together in FIGURE 12O. It will be apparent that sum waveform 1236 when digitized will be identical with signal 1232 but for a phase shift,with half its dwell time at level 3~
1 -9- RCA74,900 1216 and the other half at 1218, Signal 1238 similarly will, whèn digitized, assume a digital value identical to - signal 1232. When summed in the apparatus of FIGURE 5, the output signal will assume 2 time-average value half way between levels 1216 and 1218. This is eXactly equal to the value of the input analog signal, and results in zero digitizing error.
If the input signal has a magnitude lying 1/4 digitizing level below level 1218, the digitizing error would be expec'ted to be 3/4 of a digitizing level. When summed with signal 28, the sum dwells half the time above level 1218 and half the time below. The sum with signal 30 remains at all times at or above level 1218. ~hen digitized and time averaged, tne output signal will be 1/4 level below level 1218, whereby the digitizing error is reduced to zero.
FIGURE 9 shows an embodiment of the delay buffer 42.
The 8 bit input signal from quantizer 40 is applied to output 38a directly and to a 63.5 microseconds (one horizontal line) minus 70 nanosecond delay line 62. The output of delay line 62 comprises output 32a and is also applied to delay line 64, which has a delay of 140 nanoseconds. The output of delay line 64 is applied to output 34a and also to delay line 66 which has a delay of o3.5 microseconds minus 70 nanoseconds. The output of delay line 66 comprises output 36a. It should be noted that all of the above delays are for a 525 lines per rame, 30 frames per second system and for a sampling frequency of 7.16 MHz. The 70 nanosecond delays are needed to achieve a shift cf one half of a sample interval with said sampling frequency, which is needed due to the phase shift between signals 28 and 30. Other systems would use other values of delay for the delay llnes 62, 64 and 66.
FIG~RE 10 shows a detailed diagram of the two point averaging logic circuit 54. I'he signal from output 32a of delay buffer 42 is applied to an input of subtractor 68, while the signal at output 34a is applied .... .
~1 5B3~
1 -10- RCA 74,900 to another input of subtractor 68. A difference signal is present at the output of subtractor 68 and is applied to one input of a digital comparator 70, that has applied at another lnput a logic "1" signal present on line 71. The comparator 70 supplies at output 58 a logic "1" signal if the difference applied between its two inputs is less than or equal to one, and a logic "0" if otherwise. This signal is the beforementioned two point average selection flag.
Signals at outputs 32 and 34a are also applied to adder 72 and their sum, which comprises the two point a~erage is applied to output 60.
FIGURE 11 shows the details of the four pointi averaging logic 44. Signals from outputs 32a, 34a, 36a~and 38a are applied to the circuit 44. The signal from 32a is applied to all of the subtractors 74, 76 and 78. The signal from 36a is applied to subtractor 74, which applies the difference between that signal and that from output 32a to digital comparator 80. This comparator supplies the logic level "1", if the difference between its input signals is less than or equal to one, to AND gate 86. The signal from output 38a is applied to subtractor 76, which supplies the difference between that signal and that from ouput 32a to comparator 82~ Comparator 82 supplies an output signal, if this difference is less than or equal to one, to ~ID
~ gate 86. The signal from output 34a is applied to subtractor 78 and the difference between that signal and that from output 32a is applied to digital comparator 84.
If the difference is less than or equal to one, a logic one signal is applied to AND gate 86. It will be noted that a logic one signal is applied to the digital comparators 80, 82 and 84 from line 85 so they can make the proper comparison. If the difference is less than or equal -to one from all of the comparators 80, 82 and 84, AND gate 86 supplies a high signal, which comprises the four point average flag, at output 46. 'rhe signals from out~uts 32a and 36a are applied to adder 88, which in turn supplies their sum to one input of adder 92. The signals 3 ~ 5 RCA 74,900 From outputs 38a and 34a are applied to adder 9O, which in turn applies to their sum to another input of adder 92.
Thus output of adder 92 comprises the ten bit four point average signal which is present at output 50.
It will be appreciated that variations are possible within the scope of the invention. For example, the point to the right of point 32 can be used to derive the two point average, and points to the upper and lower right of point 32 can be used to derive the four point average. Other combinations of surrounding samples can be used. Further, the concept can be extended to 8 or more point averaging.
If the input video signal is already a digital signal, then converter 40 will be just a quantizer for requantization after the addition of offset signals by adder 26.
Still further, the use of 180 degree phase shift from line to line in the sampling frequency is not required. It was used in a preferred embodiment which is used together with the invention issued in U.S. Patent 4,323,916 on April 6/1982, to Reitmeier and Dischert.
The offset signal, if used, need not be synchronous with the sampling signal, but this may generate interference signals.
-1- RCA 74,900 ADAPTIVE AMPLITUDE AVERAGING FOR WEIGHTING Q~ANTIZING NOISE
1 The present inven-tion relates to digi-tal television, and more particularly, -to processing -that reduces quantizing noise.
In digital video, when too few bits are used to quantize each sample of the video waveform, quantiza-tion 5 noise is large and causes objec-tionable contouring or "puddling" of the displayed picture. This is most objection-able in areas of gradual intensity changes where the slope of the video waveform is small compared to the quantization step size. This is because in such slowly changing, low 10 frequency portions, of the picture, the eye is most sensitive to quantization noise. By increasing the number of bits per sample, more levels are available for quantization, and the gradual change is more closely represented. However, the use of more bits per sample results in the penalty of a corres-15 ponding increase in -the required data rate of the digital video. It is known from -the article en-titled "PCM Encoded NTSC Color Television Subjective Tests" by A.A~ Goldberg, JSMPTE, Augus-t, 1973, p.p. 649-654, to add ei-ther a square wave or a random signal -to -the video signal before quantiz-20 ation to reduce contouring, and then low pass filtering -the reproduced analog video signal to reduce the visibility of the added signal and the quantizing noise. However in this system, the low pass filter also reduces the high frequency video signal information.
It is therefore desirable to reduce the visibility of contouring, and quantizing noise wi-thout increasing the data rate and without reducing the high frequency video signal information.
According to one aspect of the invention, there is 30 provided a method for reducing quantizing noise in a sampled quantized signal, said method comprising adding a periodic multilevel offset signal to said sampled signal prior -to quantization, quantizing said signal, determining if only low frequency information is present, averaging a plurali-ty 35 of samples when only low frequency information is present, 1 ~5~55 -la- R~A 74,900 1 every plurali-ty of averaged samples having the same average value of offset signal amplitude, whereby additional quantizing levels be-tween the original levels are recovered in the averaged samples, thereby reducing quantizing noise.
According to another aspect of the invention, there 5 is provided an apparatus for reducing quantizing noise in a sampled quantized signal, said apparatus comprising means for adding a periodic multilevel offset signal to said signal prior to quantization, means for quantizing said signal, means for determining if only low frequency information is present, 10 mean~ for averaging a plurality of samples when only low frequency information is present, every plurality of averaged samples having the same average value of offset signal ampli-tude, whereby additional quantizing levels between the original levels are recovered in the averaged samples, thereby 15 reducing quantizing nolse.
t~5~
1 -2- RCA 74,900 In the Drawings:
FIGURE 1 shows a gradually changing video signal quantized using only a few levels;
FIGURE 2 shows the same signal with more quantizing levels;
FIGURE 3 shows a flat field with coarse quantization;
FIGURE 4A through D show va:rious waveforms concerning a flat field using amplitude averaging in accordance with the present invention;
FIGURE 5 shows in bloc~ diagram form an adaptive amplitude averaging system;
FIGURE 6 and 7 show some waveforms used in the system of FIGURE 5;
FIGIJRE 8 shows a diagram for use in explaining FIGURE 5;
FIGURE 9 shows the details of a delay buffer used in FIGURE 5;
FIGURE 10 shows the details of two point averaging logic circuit used i.n FIGURE 5;
FIGURE 11 shows the details of a four point averaging logic circuit used in FIGURE 5; and FIGURE 12 shows in more detail waveforms of the present invention.
FIGURE 1 shows a graph of an analog video signal 12 which is sampled at a fixed sampling frequency at sampling points of times 14 and quantized to the next lowest of various quantizing levels 16. The result is a digital waveform 18. The difference between analog signal 12 and digital signal 18 is the quantizing error, which can be as great as one quantizing level. Because -the slope of the waveform 12 changes slowly with respect -to the difference between quantizing levels 16, sharp edges 20 occur ln the digital waveform which are separated from each other by some considerable distance along the horizontal axis by a constant amplitude signal. The 4~
~ 1~8355 1 -3- RCA 74,900 result is highly visible contours in the picture display from signal 18.
FIGURE 2 shows a graph where the number of quantizing levels have been increased and where corresponding reference numerals have been applied to corresponding elements of the graph. I-t will be noted that the steps 20 are much smaller in amplitude and occur more frequently than is the case in FIGURE 1, resulting in a smaller quantizing error. However, increased quantizing levels require an increased data rate. Consider the situation in FIGURE 3 where again corresponding portions of the graph have been given corresponding reference numerals. It will be seen that due to the small number of the quantizing levels 16, there is a fixed quantizing error 22, if the analog video signal 12 is flat and occurs between quantizing levels as shown in FIGURE 3.
FIGURE 4A shows a signal which can help overcome these problems. An offset signal 24 is shown which has a square wave shape, a frequency equal to one-half the sampling frequency used, and a peak amplitude equal to one-half of the amplitude difference between adjacent quantizing levels. Note that center of the upper portion of the square wave 24a coincides with every other sampling j point 14. FIGURE 4B shows the results when this offset signal 24 is added to an analog video signal 12 having a constant amplitude of 3/4 o-f a quantizing level before the quantizing step. Note that in FIGURE 4B, the amplitude of adjacent samples alternates up and down by one-half of a quantizing level. FIGURE 4C shows the result of the signal of FIGURE 4B after quantization.
This signal 25, in the case shown, exis-ts only at the discrete quantizing levels Q and 1 and adjacent signals do not differ from each other by more than one quantizing level. This being -the case, and in accordance with -the invention, the signal 25 is integrated or averaged, either by the eye or electronically, to give the result 3 ~ ~
1 -4- RCA 74,900 shown in FIGURE 4D, if the averaging is done electronically. The result is a signal level half-way between two quantization levels. Thls holds true for any value of the analog video signal 12 lying in the upper halE of any quantizing range, e.g. between one-half and one, one and one-half and two, two and one-half and three, etc. For a value of signal in the lower half of a quantizing range, e.g. zero and one-half, one and one-half, two and two and one-half, etc., the combined video and offset signal will always be quantized at the lower end of the range, e.g. zero, one, two, respectively, e-tc.
In either case, after the integration or averaging is ' performed, the maximum quantizing error is one half of quantizing step, ins-tead of the maximum error of one step as discussed above in connection with FIGUR~ 1. This corresponds to an effective doubling of the number of quantization levels without adding an extra one bit per sample, which can be used for representing slowly changing information. For high frequency information, the maximum error is increased to one and one-half levels (one original level plus one-half level -from the offset signal) since these signals cannot be averaged. However, as explained above, noise in high frequency portions of the signal is less objectionable than in low frequency portions.
The concept described above can be extended to averaging more than two samples to obtain more low frequency amplitude resolution. For example, two line alternating synchronous offset signals each having two levels tha-t differ from -the levels in the other signal, for a total oE four levels (each level corresponding to one-quarter of a quantizing step) might be added to the analog video signal before quantiza-tion.
Under the conditions that four adjacen-t samples are all no more than one quantizing level apar-t, an averaging of the four would produce one of four possible amplitude levels, three intermediate amplitude levels e.g.
~ 3L5~3~5 1 -5- RCA 74,900 one-quarter, one-half, three-quarters of a quantizing step and one quantizing level. This is the equivalent of adding two bits per sample. If the condition is not met that the four adjacent samples differ by not more than one quantiziny level, two adjacent pixels can be examined.
If the two pixels are not more than one level apart, they can be averaged to provide one additional bit of amplitude resolution per sample. If the condition that two adjacent pixels differ by no more than one level is not met either, then the picture content is a high ! frequency signal, i.e. a sharp transition, where quantization error is not an important factor.
Although the aforementioned added signal is helpful in explaining the invention, when the quantizing steps are small, compared to the noise in the input video signal, no o-Efset signal is required. Basically, this application relates to a~aptive filtering or averaging of mutual proximate signal samples.
An e~ample of a system for carrying out the above operations is shown in FIGURE 5. An analog video signal is fed into one input of adder 26, while offset signals are applied in a line-alternate fashion to its other input. These offset signals are shown in FIGURE 6 and 7. During alternate lines, an offset signal 28, such as shown in FIGURE 6, is applied to the adder 26. It alternates between amplitudes of zero and three-quarters of the difference between adjacent quantization levels. During the remaining alternate lines, the signal 30 of FIGURE 7 is applied to adder 26. It has an amplitude that alterna-tes be-tween one-quarter and one-half that of the difference between adjacent quantizatlon levels.
These signals 28 and 30 occur at one-half the ra-te of the sampling frequency wi-th a phase shif-t of 90 therebetween.
As shown in FIGURES6 and 7, the sampling clock has a 180 phase shift from line to line, thus shifting the sample points 14 from line to line. Further, there is a ~0 3 ~ 5 1 -6- RCA 74,900 180 phase shift for alterna-te lines of each of the offset signals 28 and 30.
These phase shifts occur every time the respective signals start a new horizontal line. The portion of the output signal from adder 26 due to signals 28 and 30 is shown diagramatically in FIGURE 8 which shows a portion of a raster. The quantizing levels that have been added to the analog video signal by adder 26 are shown in FIGURE 8 expressed in quarters of a quantizing level. Note that these numbers represent the additional levels and not the absolute value of the digital signals coming ou-t of adder 26. In the following discussion, the sampling point 32 will be considered as the one currently under consideration, and reference will be made to the proceeding point 34 on the same line, a point 36 on the line above, and a point 38 on the line below. The analog video signal plus the additional quantizing levels are applied to an analog to digital converter 40, which in turn applies an 8 bit digital representation of the sum of said analog video signal and the additional levels through a ~ transmission path 41 to a delay buffer 42. Thls 8 bit buffer 42 supplies at each of its outputs 32a, 34a, 36a, 38a, 8 bit slgnals representing the amplitude of the signal occurring at the various sample points 32, 34, 36 and 38 respectively. l'he outputs are denominated i using corresponding reference numbers with the suffix "a" added to indicate which points appear at which outputs. All of the outputs of buffer 42 are applied to four point averaging logic circuit 44. This circuit supplies at an output 46 a flag signal when all four points 32, 34, 36, and 38 are not more than one quantizing step different in amplitude Erom each other. Ou-tput 46 supplies -the flag as a control signal to r~UX 48, which comprises a SPDT switch. The swi-tch is in the posi-tion shown, when -the above condi-tion is true, so tha-t an averaye of the amplitudes of the four poin-ts 32, 34, 36, 38 is supplied by ou-tput 50 of logic circuit 44 and is applied -to data output 52 by MUX 48. The signal at ~ 1583~
1 -7- RCA 74,900 outputs50 and 52 has the resolution of a 10 bit signal, which reduces contouring without increasing the data rate through transmission path 41. If all four of said points are not within one ~uantizing level difference of each other, then there is no four point average selection flag signal at output 46, and hence ~IUX 48 is switched to its lower position, and thus receives the output of r~ux 56, which also comprises a SPDT switch.
Signals representing points 32 and 34 from outputs 32a and 34a are applied to two point averaging logic circuit 54, and if these points are within one quantizing level step of each other, a two point average flag selection signal is applied from output 58 to MUX 56, so that it is in the position shown. In this case, output 60 supplies a signal representing the actual two point average of points 32 and 34 to MUX 48 by way of MUX 56, and hence to data output 52. This gives the resolution of a 9 bit signal. In the event that points 32 and 34 are not within one quantizing level step of each other, then no two point average selection flag signal is present at output 58, and hence ~5UX 56 is in the lower position (not shown, just the signal representing point 32 is applied 25 through MUX 56 and MUX 48 to data output 52, which signal is an 8 bit one.
The line alternate added offset signals 28 and 30 of FIGURES 6 and 7 are repeated in FIGURES 12a and b.
In operation on an analog signal 1212 as illustrated in FIGURE 12c having a relatively constant amplitude lying at a quantizing level 1216, the arrangement of FIGURE 5 produces during a first horizontal line ~ summed signal illustrated as 1214 of FIGURE 12d, representing the sum of signals 28 and 1212. During the next horizontal line, sum signal 1220 is produced representing the sum of signals 30 and 1212. Where quantized, slgnal 1214 will take on a digital value equal to digitizing level 1216 at each sample point as illustrated by signal 1222 of FIGURE 12f, for analog signal 1214 never reaches 3~
1 -8- RC~ 74,900 guantizing level 1218. Similarly, signal 1224 in FIGURE 12g represents the digital value resulting from the digitizlng of signal 1220. Signal 1224 also remains at quantizing level 1216, for signal 1220 does not reach the next quantizing level 1218. The time average of signals 1222 and 1224 equals quantizing level 1216, and consequently the digital value is a close approximation to the analog values.
FIGV~ES 12h-12O illustrate the conditions when the input analog signal takes on a value lying slightly above quantizing level 1216, as illustrated by signal 1226 of FIGURE 12h. Signal 1226 lies above quantizing level 1216 by 1/4 of a quantizing level. If signal 1226 were simply quantized as in the prior art, the quantizing error would be 1/'4 of a quantizing level. Signal 122& in FIGURE 12j illustrates the sum of offset signal 28 and analog signal 1226 as generated by the apparatus of FIGURE 5. It should be noted that signal 1228 reached the next higher quantizing level 1218 at alternate sampling points. Signal 1230 of ~GU~E 12k represents the sum of signal 30 and signal 1226. Since signal 1226 is only 1/4 of a quantizing level above level 1216 and the maximum amplitude of signal 30 with which it is summed is 1/2 quantizing level, signal 1230 does not reach next higher quantizing level 1218. The result of quantizing signal 1228 is illustrated as signal 1232 of FIGURE 12m and the result of quantizing signal 1230 isillustrated as signal 1234 in F IGURE 12n. The time ; average of the digital sum of digital signals lZ32 and 1234 produced by the arrangement of FIGURF 5 is 1/4 of a quantiZing level above level 1216, which is exactly the value of analoa signal 1226. qhus, in this case the quantiZing error has been reduced from 1/4 of a level to zero.
If the analog signal applied to the arrangement of FIGURE 5 lies half-way between quantizing levels 1216 and 1218, the sum signals are as illustrated by waveforms 1236 and 1238, illustrated together in FIGURE 12O. It will be apparent that sum waveform 1236 when digitized will be identical with signal 1232 but for a phase shift,with half its dwell time at level 3~
1 -9- RCA74,900 1216 and the other half at 1218, Signal 1238 similarly will, whèn digitized, assume a digital value identical to - signal 1232. When summed in the apparatus of FIGURE 5, the output signal will assume 2 time-average value half way between levels 1216 and 1218. This is eXactly equal to the value of the input analog signal, and results in zero digitizing error.
If the input signal has a magnitude lying 1/4 digitizing level below level 1218, the digitizing error would be expec'ted to be 3/4 of a digitizing level. When summed with signal 28, the sum dwells half the time above level 1218 and half the time below. The sum with signal 30 remains at all times at or above level 1218. ~hen digitized and time averaged, tne output signal will be 1/4 level below level 1218, whereby the digitizing error is reduced to zero.
FIGURE 9 shows an embodiment of the delay buffer 42.
The 8 bit input signal from quantizer 40 is applied to output 38a directly and to a 63.5 microseconds (one horizontal line) minus 70 nanosecond delay line 62. The output of delay line 62 comprises output 32a and is also applied to delay line 64, which has a delay of 140 nanoseconds. The output of delay line 64 is applied to output 34a and also to delay line 66 which has a delay of o3.5 microseconds minus 70 nanoseconds. The output of delay line 66 comprises output 36a. It should be noted that all of the above delays are for a 525 lines per rame, 30 frames per second system and for a sampling frequency of 7.16 MHz. The 70 nanosecond delays are needed to achieve a shift cf one half of a sample interval with said sampling frequency, which is needed due to the phase shift between signals 28 and 30. Other systems would use other values of delay for the delay llnes 62, 64 and 66.
FIG~RE 10 shows a detailed diagram of the two point averaging logic circuit 54. I'he signal from output 32a of delay buffer 42 is applied to an input of subtractor 68, while the signal at output 34a is applied .... .
~1 5B3~
1 -10- RCA 74,900 to another input of subtractor 68. A difference signal is present at the output of subtractor 68 and is applied to one input of a digital comparator 70, that has applied at another lnput a logic "1" signal present on line 71. The comparator 70 supplies at output 58 a logic "1" signal if the difference applied between its two inputs is less than or equal to one, and a logic "0" if otherwise. This signal is the beforementioned two point average selection flag.
Signals at outputs 32 and 34a are also applied to adder 72 and their sum, which comprises the two point a~erage is applied to output 60.
FIGURE 11 shows the details of the four pointi averaging logic 44. Signals from outputs 32a, 34a, 36a~and 38a are applied to the circuit 44. The signal from 32a is applied to all of the subtractors 74, 76 and 78. The signal from 36a is applied to subtractor 74, which applies the difference between that signal and that from output 32a to digital comparator 80. This comparator supplies the logic level "1", if the difference between its input signals is less than or equal to one, to AND gate 86. The signal from output 38a is applied to subtractor 76, which supplies the difference between that signal and that from ouput 32a to comparator 82~ Comparator 82 supplies an output signal, if this difference is less than or equal to one, to ~ID
~ gate 86. The signal from output 34a is applied to subtractor 78 and the difference between that signal and that from output 32a is applied to digital comparator 84.
If the difference is less than or equal to one, a logic one signal is applied to AND gate 86. It will be noted that a logic one signal is applied to the digital comparators 80, 82 and 84 from line 85 so they can make the proper comparison. If the difference is less than or equal -to one from all of the comparators 80, 82 and 84, AND gate 86 supplies a high signal, which comprises the four point average flag, at output 46. 'rhe signals from out~uts 32a and 36a are applied to adder 88, which in turn supplies their sum to one input of adder 92. The signals 3 ~ 5 RCA 74,900 From outputs 38a and 34a are applied to adder 9O, which in turn applies to their sum to another input of adder 92.
Thus output of adder 92 comprises the ten bit four point average signal which is present at output 50.
It will be appreciated that variations are possible within the scope of the invention. For example, the point to the right of point 32 can be used to derive the two point average, and points to the upper and lower right of point 32 can be used to derive the four point average. Other combinations of surrounding samples can be used. Further, the concept can be extended to 8 or more point averaging.
If the input video signal is already a digital signal, then converter 40 will be just a quantizer for requantization after the addition of offset signals by adder 26.
Still further, the use of 180 degree phase shift from line to line in the sampling frequency is not required. It was used in a preferred embodiment which is used together with the invention issued in U.S. Patent 4,323,916 on April 6/1982, to Reitmeier and Dischert.
The offset signal, if used, need not be synchronous with the sampling signal, but this may generate interference signals.
Claims (24)
1. A method for reducing quantizing noise in a sampled quantized signal, said method comprising adding a periodic multilevel offset signal to said sampled signal prior to quantization, quantizing said signal, determining if only low frequency information is present, averaging a plurality of samples when only low frequency information is present, every plurality of averaged samples having the same average value of offset signal amplitude, whereby additional quantizing levels between the original levels are recovered in the averaged samples, thereby reducing quantizing noise.
2. A method as claimed in claim 1 wherein said determining step comprises comparing at least two proximate samples to determine if their amplitudes are within a selected amount of each other.
3. A method as claimed in claim 2 wherein said comparing step comprises comparing four proximate samples.
4. A method as claimed in claim 3 wherein said averaging step comprises averaging at least four proximate samples if said four samples are within said selected amount of each other and if not, then averaging two proximate samples if said two samples are within said selected amount of each other.
5. A method as claimed in any of claims 2, 3 or 4, wherein said selected amount comprises one quantizing step.
6. A method as claims in claim 4 wherein said averaging step comprises averaging four proximate samples.
7. A method as claimed in claim 1 wherein said 5 averaging step comprises averaging at least two proximate RCA 74,90 CANADA
8. A method as claimed in claim 1 wherein said offset signal is synchronous with the sampling frequency.
9. A method as claimed in claim 8 wherein said offset signal comprises a square wave having a frequency equal to one half of said sampling frequency.
10. A method as claimed in claim 9 wherein said sampled signal comprises a television signal having scanning lines and said offset signal has a 180° phase shift from line to line.
11. A method as claimed in claim 10 wherein said offset signal has line alternating peak to peak amplitudes of one quarter and three quarters of the quantizing steps.
12. A method as claims in claim 1, wherein said sampled signal comprises a television signal having scanning lines and said offset singal comprises a square wave having different peak amplitudes on adjacent lines and the same average amplitude on adjacent lines.
13. An apparatus for reducing quantizing noise in asampled quantized signal, said apparatus comprising means for adding a periodic multilevel offset signal to said signal prior to quantization, means for quantizing said signal, means for determining if only low frequency information is present, means for averaging a plurality of samples when only low frequency information is present, every plurality of averaged samples having the same average value of offset signal amplitude, whereby additional quantizing levels between the original levels are recovered in the averaged samples, thereby reducing quantizing noise.
14. An apparatus as claimed in claim 13 wherein RCA 74,900 CANADA
said determining means comprises means for comparing at least two proximate samples to determine if their amplitudes are within a selected amount of each other.
said determining means comprises means for comparing at least two proximate samples to determine if their amplitudes are within a selected amount of each other.
15. An apparatus as claimed in claim 14 wherein said comparing means compares four proximate samples.
16. An apparatus as claimed in claim 15 wherein said averaging means comprises means for averaging at least four proximate samples if said four samples are within said selected amount of each other.
17. An apparatus as claimed in any of claims 14, 15, or 16, wherein said selected amount comprises one quantizing step.
18. An apparatus as claimed in claim 13 wherein said averaging means comprises means for averaging at least two proximate samples.
19. An appartus as claimed in claim 18 wherein said averaging means comprises means for averaging four proximate samples.
20. An apparatus as claimed in claim 13 wherein said offset signal is synchronous with the sampling frequency.
21. An appartus as claimed in claim 20 wherein said offset signal comprises a square wave having a frequency equal to one half of said sampling frequency.
22. An apparatus as claimed in claim 21 wherein said sampled signal comprises a television signal having scanning lines, and said offset signal has a 180° phase RCA 74,900 CANADA
shift form line to line.
shift form line to line.
23. An appartus as claimed in claim 22 wherein said offset signal has line alternating peak to peak amplitudes of one quarter and three quarter of the quantizing setps.
24. An appartus as claimed in claim 13, wherein said sampled signal comprises a television signal having scanning lines and said offset signal comprises a square wave having different peak amplitudes on adjacent lines and the same average amplitude on adjacent lines.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8004196 | 1980-02-07 | ||
| GB8004196 | 1980-02-07 | ||
| US06/149,998 US4334237A (en) | 1980-02-07 | 1980-05-15 | Adaptive amplitude averaging for weighting quantizing noise |
| US149,998 | 1980-05-15 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1158355A true CA1158355A (en) | 1983-12-06 |
Family
ID=26274431
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000370023A Expired CA1158355A (en) | 1980-02-07 | 1981-02-04 | Adaptive amplitude averaging for weighting quantizing noise |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA1158355A (en) |
| DE (1) | DE3104247A1 (en) |
| FR (1) | FR2475834B1 (en) |
| GB (1) | GB2070385B (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2505579A1 (en) * | 1981-05-08 | 1982-11-12 | Agence France Presse | METHOD FOR THE FILTER RECONSTRUCTION OF AN ANALOGUE SIGNAL FROM A PSEUDOANALOGIC SIGNAL OBTAINED BY DOUBLE DIGITAL / DIGITAL ANALOGUE / DIGITAL CONVERSION OF THE ANALOGUE SIGNAL WITH EVENTUALLY PROCESSING THE DIGITAL SIGNAL |
| FR2529043B1 (en) * | 1982-06-18 | 1986-04-25 | Thomson Csf | METHOD AND DEVICE FOR ANALOG-TO-DIGITAL CONVERSION OF A TELEVISION SIGNAL, APPLIED TO A TELEVISION SIGNAL TRANSMISSION SYSTEM |
| FR2529414B1 (en) * | 1982-06-25 | 1986-04-11 | Thomson Csf | METHOD AND SYSTEM FOR COMPRESSION OF DATA RATE TRANSMITTED BETWEEN A TRANSMITTER AND A RECEIVER, APPLIED TO A TELEVISION IMAGE TRANSMISSION SYSTEM |
| KR890004853B1 (en) * | 1985-01-28 | 1989-11-29 | 미쓰비시전기 주식회사 | Circuits for processing video signals |
| DE3504762A1 (en) * | 1985-02-13 | 1986-08-14 | Robert Bosch Gmbh, 7000 Stuttgart | METHOD AND CIRCUIT ARRANGEMENT FOR SUPPRESSING QUANTIZATION UNCERTAINTIES |
| DE3744132A1 (en) * | 1987-12-24 | 1989-07-06 | Asea Brown Boveri | Method and circuit for suppressing the quantisation noise |
| CA2014935C (en) * | 1989-05-04 | 1996-02-06 | James D. Johnston | Perceptually-adapted image coding system |
| DE4211315C2 (en) * | 1992-04-04 | 1994-05-19 | Ant Nachrichtentech | Method for increasing the useful-to-noise signal distance in systems for digital signal processing |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3009016A (en) * | 1959-10-06 | 1961-11-14 | Bell Telephone Labor Inc | Noise suppressing video circuit |
| US3244808A (en) * | 1962-01-12 | 1966-04-05 | Massachusetts Inst Technology | Pulse code modulation with few amplitude steps |
| GB1218015A (en) * | 1967-03-13 | 1971-01-06 | Nat Res Dev | Improvements in or relating to systems for transmitting television signals |
| GB1415520A (en) * | 1970-06-15 | 1975-11-26 | British Broadcasting Corp | Transmission of digital information |
-
1981
- 1981-02-04 CA CA000370023A patent/CA1158355A/en not_active Expired
- 1981-02-06 GB GB8103686A patent/GB2070385B/en not_active Expired
- 1981-02-06 FR FR8102389A patent/FR2475834B1/en not_active Expired
- 1981-02-06 DE DE19813104247 patent/DE3104247A1/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| FR2475834A1 (en) | 1981-08-14 |
| DE3104247A1 (en) | 1981-12-03 |
| FR2475834B1 (en) | 1986-07-18 |
| GB2070385A (en) | 1981-09-03 |
| GB2070385B (en) | 1984-08-08 |
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