JPH0832042B2 - Quasi-variable-length code DPCM encoder - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DPCM coding device that codes and quantizes a video signal with high efficiency.

2. Description of the Related Art FIG. 2 shows a simple example of a quasi-variable-length code DPCM coder. In this configuration, the luminance signal y _i of the prediction pixel is
As shown in FIG. 6A, using the past pixels b _2y and b _0y which have already been encoded in the previous line and the previous value a _{2y of the} current line, y _i = a _2y
The color signal c _i is predicted to be + b _0y −b _2y, and the color signal c _i is predicted to be c _i = −b _0y because the phase is inverted between the previous line and the current line as shown in FIG. 3B. . Assuming that the luminance signal / color signal separation is performed by a second-order linear digital filter, the luminance signal is a low-pass filter Z · {(1 + Z ⁻¹ ) / 2} ² , and the color signal is a high-pass filter −Z. Since they are separated using {(1-Z ^-1 ) / 2} ² , the final predicted value x _i is x _i = y _i + c _i = (a ₁ + 2a ₂ + a ₃ + 2b _-1 + b ₁ − 2b ₂ −b ₃ ) / 4 (1), which requires 4 pixels in the previous line and 3 pixels in the current line. This is shown in FIG.

In the above, the prediction error is small because the video signal has a planar correlation, but if the previous value or the correlation between the previous line pixel and the predicted pixel is lost due to the boundary of the image, this prediction is Otherwise, it may cause deterioration of the quality of the transmitted image. The quasi-variable length quantization circuit normally quantizes into a short code, and only when the prediction error of the predicted pixel is large,
This is quantized into a long code to shorten the average code length. In this case, after a predetermined number of long codes are generated in one line, the quantization in that line is limited to the fixed length code. Therefore, it can be said that the image quality tends to be inferior in the latter half of the one line than in the latter half.

Problems to be Solved by the Invention Since no control has been conventionally added to the preceding line prediction operation or the long code generation of the quasi-variable length quantization circuit, the following problems occur. First, in the previous line prediction, the prediction point is selected as shown in FIG. 4 (a), but as shown in FIGS. 4 (b) and 4 (c), the front end / end of the effective screen, or At the time of prediction for the pixels of the first line of the effective screen, the required previous line prediction point may be located in the horizontal blanking period or the vertical blanking period. Since the signal in the horizontal / vertical blanking period has no correlation with the effective screen, the prediction error at this time is larger than the prediction error due to only the current line. Furthermore, this error uses the plane prediction. In nature, it can spread to the central portion of the effective screen of the subsequent line and cause a great deterioration in transmission quality. Secondly, the error generated in the peripheral part of this effective screen is likely to be quantized by the long code by the quasi-variable length quantization circuit during transmission according to its size. Since it is the peripheral portion, the advantage of long code quantization is not visually utilized. Especially, when the number of long codes that can be used is limited depending on the transmission rate, the use of such long codes is extremely disadvantageous.

The present invention solves the above-mentioned problems and provides a DPCM coding apparatus which can effectively use a long code and improve the transmission quality.

Means for Solving the Problems The present invention performs plane prediction of a pixel signal that appears next from a plurality of pixels before the current line and the previous line of a digitized video signal, and lengthens prediction error of the predicted pixel. Quantize to a code or short code, and if a certain number of long codes occur on a line, then quantize by limiting to a fixed length code on that line, and further predict the current line prediction in the peripheral area of the effective screen. It is controlled to change, and the quasi-variable length quantization is limited to a fixed length code in the peripheral area of the effective screen.

By the above method, it is possible to improve the transmission quality by preventing the error from increasing in the peripheral portion of the effective screen of the quasi-variable-length code DPCM encoding device and by effectively using the limited number of long codes.

Embodiment An embodiment of the DPCM encoding device according to the present invention will be shown in FIG. 1 together with its circuit configuration. This is a quasi-variable length quantization circuit 14, and previous line / current line prediction circuits 18 and 17,
Long code counter 12 and fixed length code selection signal output block
13, a pixel counter 24, a preceding line prediction stop signal output block 22, a preceding line prediction stop gate 21, a fixed length code forced signal output block 23, and the like. Here, as an example, the sampling frequency is 8.91 MHz, so that there are 566 sample points on one line and the number of pixels on the effective screen is 460 pixels. When the transmission speed is 32.06 Mbps, up to 46 pixels of the 460 pixels can be transmitted as an 8-bit long code, and the rest can be transmitted by a 4-bit short code. The fixed-length code selection signal output block 13 outputs the fixed-length code (short code) selection signal to the quasi-variable-length quantization circuit 14 immediately after the long-code counter 12 counts the number of long codes by 46.

The prediction formula of this circuit is already shown in the formula (1). According to FIG. 4, a maximum of 3 pixels in the forward direction and a maximum of 1 pixel in the backward direction are required for the prediction of the pixel x _i . Therefore, when these belong to the horizontal blanking period, if the signals of the front end 3 pixels and the rear end 1 pixel of the effective screen are masked in the front line prediction stop gate 21 as shown in FIG. 5, It is possible to remove an adverse effect on the current line prediction point that may occur when the previous line pixel required for prediction belongs to the horizontal blanking period.

The previous line prediction result passes through two delays (in the current line prediction circuit) up to the subtracter 11 for detecting an error with the input PCM signal, and the pixel counter 24 uses the quasi variable length quantization circuit.
From the two points of counting one clock earlier than the pixel signal at the time of 14 inputs, the mask signal given to the preceding line prediction stop gate 21 by the preceding line prediction stop signal output block 22 is as shown in FIG.

Similarly, if the preceding line pixel belongs to the vertical blanking period, that is, the prediction signal is masked by the preceding line prediction stop gate 21 even when the first line of the effective screen is predicted, the adverse effect of the preceding line prediction can be eliminated.

In FIG. 5, the vertical blanking period and the first line of the effective screen are shown as the previous line prediction stop period. The long code quantization prohibited area, that is, the fixed length code forced area is
As shown in the figure, if the horizontal blanking period and the period of 5 pixels at the front end of the effective screen are set, the pixel counter 24 is one clock earlier than the pixel signal at the time of inputting the quasi-variable length quantization circuit 14. The signal supplied from the output block 23 to the quasi-variable length quantization circuit 14 is as shown in FIG. 6 (b).

As described above, according to the present invention, in the DPCM coding apparatus using the quasi-variable-length code, it is possible to prevent the deterioration of the transmission quality due to the large prediction error that occurs in the peripheral portion of the effective screen, and to effectively use the long code. By using it, the transmission quality can be improved.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a quasi-variable length DPCM encoding device according to an embodiment of the present invention, FIG. 2 is a configuration diagram showing a simple example of a conventional quasi-variable length DPCM encoding device, FIG. 3 and FIG. FIG. 4 is a predicted pixel state diagram in the conventional example, FIG. 5 is a predicted pixel state diagram of the device in FIG. 1, and FIG. 6 is an operation waveform diagram of the device. 10,25,26 …… Delayer, 11 …… Subtractor, 12 …… Long code counter, 13 …… Fixed length code selection signal output block, 14 ……
Quasi-variable-length quantization circuit, 15, 16 ... Adder, 17 ... Current line prediction circuit, 18 ... Previous line prediction circuit, 20 ... AND gate, 21 ... Previous line prediction stop gate, 22 ... Previous Line prediction stop signal output block, 23 ... fixed length code forced signal output block, 24 ... pixel counter, 27 ... NAND gate.

Claims (1)

[Claims] 1. A prediction circuit for planarly predicting a pixel signal that appears next from a plurality of sample pixels before the current line and the previous line of the digitized video signal, and a prediction error of the predicted pixel by a long code or Quantizing into a short code, when a predetermined number of long codes occur in a certain line, after that, a quasi-variable length quantizing circuit that quantizes by limiting to a fixed length code in that line is a component, and the prediction circuit is A pre-line prediction stop circuit that changes only the current line prediction in the peripheral area of the effective screen, and a compulsory circuit that forcibly limits the quasi-variable-length quantization circuit to a fixed-length code in the peripheral area of the effective screen. Quasi-variable length DPCM encoder.