JP2585219B2 - Filter device for adaptive subsample - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sub-Nyquist sampling filter device for digitally encoding a television signal and reducing its sampling frequency, and more particularly to switching of a post-filter to improve image quality. It is intended to improve.

[Conventional technology]

First, FIG. 3 shows a configuration diagram of the sub-Nyquist sampling. In the figure, reference numeral 1 denotes an A / D (Analog to Dig) image signal.
ital) a digital video input terminal for inputting the converted signal; 110, a two-dimensional pre-filter for reducing the diagonal component of the signal from the digital video input terminal 1; 11, a thinning out of the output signal of the two-dimensional pre-filter 110 for each pixel Is a sub-sample switch for sub-sampling. 12 is a communication path for transmitting a signal from the sub-sample switch 11, and 111 is a communication path 1.
A two-dimensional interpolation filter 44 for interpolating the signal from 2 is a digital video signal output terminal for outputting the output signal of the two-dimensional interpolation filter 111 to the outside.

Next, an example of a conventional configuration of the two-dimensional pre-filter 110 and the two-dimensional interpolation filter 111 is shown in FIG.

First, in the two-dimensional pre-filter 110, reference numeral 2 denotes a one-line delay unit (hereinafter referred to as 1H delay unit) for delaying the signal input from the digital video input terminal 1 by one line, and reference numeral 3 denotes an output signal of the 1H delay unit 2 further. 1H delayer for delaying one line, 4 is a one-pixel delayer (hereinafter referred to as 1D delayer) for delaying the output signal of 1H delayer 1 by one pixel, and 5 is further delaying the output signal of 1D delayer 4 by one pixel. A 1D delay unit, 9 is a 1D delay unit for delaying the output signal of the 1H delay unit 3 by one pixel, and 10 is a 1D delay unit for delaying the signal input from the digital video input terminal 1 by one pixel. 101 is 1H delay 2 and 1D delay 5,
An adder for obtaining the sum of the output signals of 9, 10 is provided, 102 is a divider for dividing the output signal of the 1D delay unit 4 by 2, and 103 is an adder 101.
Is an adder that divides the output of the divider by 8, and 104 is an adder that adds the outputs of the dividers 102 and 103.

Further, in the two-dimensional interpolation filter 111, 13 is the communication path 1
A 1H delay device for delaying the signal from 2 by one line, 14 a 1H delay device for further delaying the output signal of the 1H delay device 13 by one line, 19 a 1D delay device for delaying the output signal of the 1H delay device 14 by one pixel, Reference numeral 20 denotes a 1D delay unit that delays the signal from the communication path 12 by one pixel, and 15 denotes a 1D delay unit that delays the output signal of the 1H delay unit 13 by one pixel.
The delay unit 16 is a 1D delay unit that further delays the output signal of the 1D delay unit 15 by one pixel. 105 is 1H delay unit 13 and 1D delay unit 19,
An adder for adding the output signals of 20, 16 is added. 106 is a divider for dividing the output signal of the adder 105 by 4. 107 is an adder for adding the output signal of the 1D delay unit 15 and the output signal of the divider 106. It is an adder.

 Next, the operation will be described.

First, sub-sampling will be described with reference to FIG. In FIG. 3, a signal 3a obtained by sampling an image at a sampling frequency of 2fs is input to a digital video input terminal 1. The signal 3a is as shown in FIG. 6 when represented by a pixel arrangement, and as shown in FIG. 8 when represented by a two-dimensional space spectrum sampled in the x and y directions. In FIG. 6, Tx indicates one pixel interval, and Ty indicates one line interval. In FIG. 8, since the sampling frequency exists on a grid point having a fundamental period of 1 / Ty, 1 / Tx, a two-dimensional spatial spectrum region in which an image can be reproduced without aliasing noise has a horizontal spatial frequency of 1 / 2Tx, This is a rectangular area having a vertical spatial frequency of 1/2 Ty. Normally, in subsampling, a PASS (Phase Alternative) in which the sampling frequency is shifted by 180 ° for each line
Sub-Nyquist Sampling) is adopted. Third
The signal 3c after the sub-sampling in the figure is a staggered sample point in FIG. 7 when represented by the pixel arrangement, and is represented as in FIG. 9 in the two-dimensional spatial spectrum sampled in the x and y directions. In FIG. 9, the sampling frequency is represented by a staggered grid point, and the video can be reproduced without aliasing noise.
The dimensional space spectrum region is a triangular region formed by connecting a horizontal space frequency of 1 / 2Tx and a vertical space frequency of 1 / 2Ty by a straight line. When a thin oblique line exists on an image, aliasing noise occurs. For this reason, it is important to reduce oblique components in the sub-sample filter.

In FIG. 3, a signal 3a input from the digital video input terminal 1 has a basic clock 2fs to reduce oblique components.
Is input to the two-dimensional pre-filter 110 that operates. The signal 3b that has passed through the two-dimensional pre-filter 110 becomes a signal with a diagonal component dropped, and is sub-sampled by the sub-sample switch 11 to become a signal 3c. Signal 3c is the sample clock fs
Since the signal is resampled every time, the image information is reduced by half. The signal 3c is transmitted using the communication path 12, and the transmitted signal is the sample clock fs.
It is a signal for each. Next, the sample clock is set to 2fs on the receiving side.
In FIG. 7, the missing pixels indicated by crosses in FIG. 7 are interpolated by the two-dimensional interpolation filter 111 and the oblique components are reduced. Then, the interpolated signal 3d is output to the digital video output terminal 44 as a signal whose sample clock has become 2 fs.

The importance of the filtering in the sub-sampling has been described above with reference to FIG. 3. Next, a specific example of the conventional filtering will be described with reference to FIG. By the time the signal 10a input from the video input terminal 1 becomes the input signal 10b of the sub-sample switch 11, the two-dimensional pre-filter 110 realizing the transfer characteristic of the following equation (1) is obtained.
As a result, the oblique component is dropped.

Z ^-L : One line delay on the image Z ^-1 : One pixel delay on the image Since the signal 10b is processed by the sampling clock of 2fs, the sub-sample switch 11 performs the sub-switching by the clock of fs which inverts the phase by 180 ° for each line. Sampled, and when this is represented by pixel arrangement, the staggered grid sampling shown in FIG. 7 is obtained.
The sub-sampled signal 10c is transmitted by the communication path 12 at the transmission clock fs. The signal transmitted in this manner is a 2 fs clock signal with 0 inserted at the missing sample point in FIG. On the receiving side to which a signal from the communication path 12 is input, the input signal is converted to a digital video output terminal.
Missing sample points are interpolated by the interpolation filter 111 that has realized the transfer characteristic of the above equation (1) until the output video signal 10d becomes 44.

The above filters are both filters for reducing the diagonal components in the two-dimensional pre-filter 110 and the two-dimensional interpolation filter 111.

[Problems to be solved by the invention]

The conventional sub-sample filter is configured as described above, and performs filtering in the oblique direction unconditionally even when there is no oblique high-frequency component in the image information. When a component is included, the image quality of that part is deteriorated. Therefore, in order to prevent this, it is necessary to use a filter having a high order of filters, that is, a filter having complicated hardware, and there is a disadvantage that the hardware scale becomes large.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and can achieve an image quality having a higher horizontal and vertical resolution than the conventional one with the same hardware scale as the conventional one, and furthermore, has an adaptive type with less erroneous detection. It is intended to obtain a filter device for sub-sampling.

[Means for solving the problem]

The adaptive sub-sample filter device according to the present invention includes a horizontal low-pass filter (Low
Pass Filter: LPF) and vertical direction LPF, comparison means for detecting and comparing local horizontal and vertical changes in the image, a target pixel and four missing sample points adjacent to the target pixel And switching means for selecting any one of the output values of the horizontal LPF and the vertical LPF based on the comparison result in the above.

[Action]

In the present invention, a local change in the horizontal direction and a change in the vertical direction of an image are detected, and a vertical LPF is applied to an image having many high-frequency components in the horizontal direction and an image having many high-frequency components in the vertical direction. Is applied with a horizontal LPF so that high-resolution image quality can be obtained with less erroneous detection.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a post-filter (interpolation filter) of an adaptive sub-sampling filter device according to an embodiment of the present invention, that is, a configuration of a receiving side. In the figure, 12 is a communication path, 13
Is a 1H delay unit that delays the signal from the communication path 12 by one line, 15 is a 1D delay unit that delays the output signal of the 1H delay unit 13 by one pixel, and 16 is an additional 1 pixel delay of the output signal of the 1D delay unit 15. 1D delay unit, 17 and 18 are 1H delay unit 13 and 1D delay unit 16, respectively.
, The adder 23 adds the two output signals of the dividers 17 and 18, 31 is a 1H delay unit that delays the output signal of the adder 23 by one line, and 35 is This is a 1D delay unit that delays the output signal of the 1H delay unit 31 by one pixel. These components form a horizontal digital filter (horizontal LPF).

Reference numeral 14 denotes a 1H delay unit that further delays the output signal of the 1H delay unit 13 by one line, 19 denotes a 1D delay unit that delays the output signal of the 1H delay unit 14 by one pixel, and 20 denotes a delay of the signal from the communication path 12 by one pixel. 1D delay devices, 21 and 22 are dividers for dividing output signals of 1D delay devices 20 and 19 by 2, 25 is an adder for adding two output signals of dividers 21 and 22, and 32 is an adder A 1H delay unit for delaying the output signal of 25 by one line, and a 1D delay unit for delaying the output signal of 1H delay unit 32 by one pixel. And from channel 12
A vertical LPF is configured for each component up to the 1D delay device 36.

Further, 24 is a subtractor for subtracting the output signal of the 1D delay unit 19 from the output signal of the 1D delay unit 20, 26 is a subtractor for subtracting the output signal of the 1D delay unit 16 from the output signal of the 1H delay unit 13, 27, Numeral 28 is an absolute value circuit which takes the absolute value of the output signal of the subtracters 26 and 24, respectively. 29 is a comparator for comparing the two output signals of the absolute value circuits 27 and 28, 33 is a 1H delay unit that delays the output signal of the comparator 29 by one line, and 37 is a one-pixel delay of the output signal of the 1H delay unit 33. 1D delay device, 38 is a 2D delay device that delays the output signal of the comparator 29 by two pixels, 39 is a 2H delay device that delays the output signal of the comparator 29 by two lines, and 40 is an output signal of the 2H delay device 39 that is two pixels. This is a 2D delay device for delaying. 41 is a decision circuit composed of a group of gate circuits, 42 is a 1D
It is a changeover switch for selecting either the output signal of the delay unit 35 or the output signal of the 1D delay unit 36, that is, one of the horizontal LPF output signal and the vertical LPF output signal.

Reference numeral 30 denotes a 1H delay unit that delays the output signal of the 1D delay unit 15 by one line, and 34 denotes a 1D delay unit that delays the output signal of the 1H delay unit 30 by one pixel.
Delay device, 43 is the output signal of switch 42 and 1D delay device
An adder for adding the output signal of the adder 34 is a digital video output terminal for outputting the output signal of the adder 43 to the outside.

Next, the operation of the two-dimensional post filter, that is, the operation of the receiving side will be described with reference to FIG.

The signal 1a input from the communication path 12 is a signal in which 0 data is inserted at the missing sample point in FIG. This input signal
1a is delayed by one line by a 1H delay unit 13, and further delayed by one pixel by 1D delay units 15 and 16, respectively. The output signal of the 1H delay unit 13 is divided by 2 by the divider 17, and the output signal of the 1D delay unit 16 is divided by 2 by the divider 18. Dividers 17, 18
Are added by the adder 23, further delayed by one line by the 1H delay unit 31, and delayed by one pixel by the 1D delay unit 35 to become the output signal 1b. Where the input signals 1a to 1D
The transfer characteristic of the horizontal LPF up to the output signal 1b of the delay unit 35 is represented by H (Z) = Z ^-1 · (1 + Z ^-2 ) · Z ^-2L / 2. This transfer characteristic is equivalent to performing the calculation of E = (A + B) / 2 to obtain the point E in FIG. 2 as the calculation on the pixel arrangement. At this time, since the signal 1a is a signal in which 0 data is inserted for each pixel, the signal 1b is
When the point is 0 insertion data, an output value in the horizontal direction LPF is obtained, and when the point E is not 0 insertion data, it becomes 0.

On the other hand, the signal 1a input from the communication path 12 is delayed by one pixel by the 1D delay unit 20, and is further divided by 2 by the divider 21. The output signal of the 1H delay unit 13 is further
, And the output is delayed by one pixel by the 1D delay unit 19. The output signal of the 1D delay unit 19 is divided by a divider 22
The output signals of the dividers 21 and 22 are added by an adder 25, further delayed by one line by a 1H delay unit 32, delayed by one pixel by a 1D delay unit 36, and output signal 1c. Become. Here, the input signal 1a is converted to a 1D delay 36
The transfer characteristic of the vertical LPF up to the output signal 1c is expressed by H (Z) = Z− ^L · (1 + Z− ^2L ) · Z− ² / 2. This transfer characteristic is calculated as
In FIG. 2, calculating the point E is equivalent to performing the calculation of E = (C + D) / 2. At this time, since the signal 1a is a signal in which 0 data is inserted for each pixel, the signal 1c obtains an output value in the vertical direction LPF when the point E is 0 insertion data, and when the E point is not 0 insertion data. It becomes 0.

The output signals of the two LPFs described above are selected by the logic described below.

First, the output signal of the 1D delay unit 16 is subtracted from the output signal of the 1H delay unit 13 by the subtracter 26, and the absolute value of this output signal is obtained as the signal 1d by the absolute value circuit 27. On the other hand, the output signal of the 1D delay unit 19 is subtracted from the output signal of the 1D delay unit 20 by the subtractor 24, and the absolute value of this output signal is signaled by the absolute value circuit 28.
Get as 1e. Comparator 29 compares signal 1d with signal 1e,
Outputs a 1-bit signal. The output signal of the comparator 29 is delayed by one line by a 1H delay unit 33, and further delayed by one pixel by a 1D delay unit 37. The output signal of the comparator 29 is delayed by two pixels by the 2D delay unit 38. The output signal of the comparator 29 is delayed by two lines by a 2H delay unit 39, and further a 2D delay unit
40 delays two pixels. The judgment circuit 41 is composed of a group of gate circuits as described above, and includes a 1D delay device 37, a comparator 2
Five output signals from the 9, 2D delay units 38, 40, and 2H delay unit 39 are input, and an output 1f is obtained according to the criterion shown in FIG.

The flow of the signal from the output of the comparator 29 to the signal 1f is determined by interpolating the missing sample point of interest, as shown in FIG. Which of the horizontal LPF and the vertical LPF was selected at the missing sample point γ of the target point α, and which of the missing sample point δ at the lower left of one pixel on the space of the point of interest α and the Whether the horizontal direction LPF or the vertical direction LPF is used at the point of interest α is determined depending on whether the LPF is selected.

In FIG. 5, a switch 4 is set as a signal when the vertical LPF is used, and a signal when the horizontal LPF is used as 0.
7 shows the selection criteria. In the figure, α, β, γ, δ, and ε indicate output signals of the comparator 29 when the missing sample points α, β, γ, δ, and ε are set as the target pixels. The signal 1g selected based on such a selection criterion is added to the output 1D delay unit 15 by the adder 43 and the signal 1h further delayed by one line and one pixel by the 1H delay unit 30 and the 1D delay unit 34, and interpolated. The digital video output signal 1i is output from the digital output terminal 44. Here, the transfer characteristic from the input signal 1a to the output signal 1h of the 1D delay unit 34 is represented by H (Z) = Z− ² · Z− ^2L . Further, since 0 is inserted by the sub-sample, one of the signals 1h and 1g is a signal which becomes 0 alternately. Therefore, the transfer characteristic including the insertion of 0 from the input signal 1a to the output signal 1i of the digital video output terminal 44 (when ε is the target pixel) is as follows: When the horizontal LPF is selected, H (Z) = Z ^-1 · (1 + Z ⁻¹ ) ² · Z ^−2L / 2 When the vertical LPF is selected, H (Z) = Z− ^L · (1 + Z− ^L ) ² · Z ⁻² / 2. According to the selection logic, as an operation on the pixel arrangement in FIG. 2, the signal 1d corresponds to | A−B | and the signal 1e corresponds to | C−D | Assuming that the outputs of the comparator 29 at points β, γ, δ, and ε are Sα, Sβ, Sγ, Sδ, and Sε, Sα = 0, | A− when | A−B | <| C−D | The case of B | ≧ | C−D | corresponds to Sα = 1. Therefore, when Sβ = 0, Sγ = 0, Sδ = 0, Sε = 0 or when Sα = 0 and at least one of Sβ, Sγ, Sδ, Sε is not 0, E = (A + 2E + B) / 2 Sβ = 1 , Sγ = 1, Sδ = 1, Sε = 0 or when Sα = 0 and at least one of Sβ, Sγ, Sδ, Sε is not 1, the selection is made as E = (C + 2E + D) / 2. As described above, there is a risk of erroneous detection if the determination is made based only on the target pixel α point. Therefore, when the LPF is switched based on the comprehensive determination of the target pixel and the peripheral pixels, the erroneous detection is reduced. With this, the horizontal LPF is selected for the image with little horizontal change depending on the image, and the vertical LPF is selected with little misjudgment for the image with little vertical change, and the missing pixels are interpolated, and the high precision adaptive Interpolation filtering can be realized.

In such an apparatus of the present embodiment, a vertical LPF is applied to an image having many high-frequency components in the horizontal direction, and a horizontal LPF is applied to an image having many high-frequency components in the vertical direction. With this circuit scale, it is possible to faithfully reproduce image quality with higher horizontal and vertical resolutions than before.

〔The invention's effect〕

As described above, according to the sub-sample filter device according to the present invention, the horizontal filter and the vertical LPF are provided in the post-filter, and the local horizontal change and vertical change of the image are detected with less erroneous detection. Detects and outputs the output of the vertical LPF for images with large changes in the horizontal direction, and outputs the output of the horizontal LPF for images with many changes in the vertical direction. The effect is obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a post-filter of an adaptive sub-sampling filter device according to an embodiment of the present invention.
FIG. 3 is a layout diagram on a pixel for explaining the present invention and the conventional example as an operation on the pixel, FIG. 3 is a block diagram of a PASS system for explaining a PASS system, and FIG. FIG. 5 is a layout diagram on a pixel for explaining a selection logic of two LPFs, FIG. 5 is a diagram showing selection criteria of two LPFs in a post-filter of the present invention, and FIG. 6 is a drawing showing sampling points before sub-sampling. FIG. 7 is a pixel arrangement diagram showing sampling points after sub-sampling, and FIG.
The figure shows the two-dimensional space spectrum of the sampling points shown in FIG. 6, FIG. 9 shows the two-dimensional space spectrum of the sampling points shown in FIG. 7, and FIG. It is a block diagram which shows a pre-filter and an interpolation filter. 11 ... Subsample switch, 12 ... Communication path, 13,14,
30-33 ... 1 line delay unit, 15,16,19,20,34-37 ... 1
Pixel delay unit 17, 18, 21, 22… Divider, 24, 26… Subtractor, 23, 25, 43… Adder, 27, 28… Absolute value circuit, 29…
... Comparator, 38,40 ... 2-pixel delay unit, 39 ... 2-line delay unit, 41 ... Judgment circuit, 42 ... Switch, 110
... two-dimensional pre-filter, 111 ... two-dimensional post-filter. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

(57) [Claims] A PASS system (Phase Alte) for reducing the sampling frequency of a digitized television signal on a communication channel.
A digital filter device used for rnative sub-nyquist sampling, wherein an interpolation filter provided on the receiving side for interpolating missing pixels by interpolation has a transfer characteristic of H (Z) = (1 + Z ⁻¹ ) ² • Z- ^L / 2, where Z- ^L : spatial one-line delay, Z- ¹ : spatial one-pixel delay, and the transfer characteristic is H (Z) = (1 + Z- ^L ) ^2. A vertical digital filter that is Z ^-1/2 , a vertical difference absolute value V ₀ between pixel values of a pixel above and below one line in the space of the pixel of interest to be interpolated, and one pixel in the space of the pixel of interest Horizontal difference absolute value H ₀ between pixel values of previous and subsequent pixels
And a comparing means for obtaining the difference absolute values V _{−1, −1} and H when the missing sample point at the upper left of one pixel in the space of the target pixel is set as the target pixel.
_{−1, −1} , each absolute difference value V _{1, −1} , H _{1, −1} when a missing sample point at the upper right of one pixel on the space of the noted pixel is set as the noted pixel, 1 pixel on the space of the noted pixel The absolute value of each difference V _−1,1 , H _−1,1 when the lower left missing sample point is set as the target pixel, and the difference absolute value V− _1,1 and H− _1,1 when the lower right missing sample point is set as the target pixel. From the absolute difference values V _1,1 and H _1,1 , Vx, y> Hx, y (x = 1, -1, y = 1, -1), or x = −1,1, y = −1 , 1, one or more combinations satisfy Vx, y ≦ Hx, y, and when V ₀ > H ₀ , the output value of the horizontal digital filter is calculated as Vx, y ≦ Hx, In the case of y (x = -1,1, y = -1,1) or one or more of the above four combinations are Vx, y
> Hx, y, and switching means for selecting and outputting the output value of the vertical digital filter when V ₀ ≤H _0. .