JP2504266B2 - Adaptive coding control method for video signals - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a moving picture coding technique for digital transmission at a reduced bit rate such as video conferencing, video telephone, and digital television transmission.

(Prior Art) There are many digital image communication services such as videophones, videoconferences, and digital television transmissions that encode moving image signals and reduce the bit rate for transmission. Generally, in video communication, inter-frame coding is used to significantly reduce the bit rate, but this does not work effectively at the time of scene change or when complicated and fast motion is present in the coded output. As the amount of data increases, block-shaped distortion occurs, which significantly deteriorates the image quality.

On the other hand, in the past, in order to maintain real-time communication, on the encoding side, a post-buffer for temporarily storing output data of the encoder as shown in FIGS. 2A and 2B, or an encoding The amount of data in the pre-buffer that temporarily stores the input data to the device is monitored, and if the amount of data is greater than a certain value, the frame rate is lowered, that is, the amount of data is reduced by the method of dropping frames, and at the same time. For block-like distortion, a distortion removal filter was used on the decoding side to improve the decoded image quality.

(Problems to be Solved by the Invention) However, if frame dropping is frequently used, temporal resolution becomes low, resulting in discontinuous motion and awkward images. Further, the block distortion removal only on the decoding side cannot be said to be sufficient processing, and there is a problem in that both have a great visual adverse effect.

An object of the present invention is to perform coding control with less dropped frames and block distortion.

(Means for Solving the Problem) A feature of the present invention is that a conventional encoder can be connected in advance without modification, and the encoder receives signals of output information generation amount and quantization step size. By adaptively controlling the passband to be narrow when the amount of information is large and wide when it is small, the coding rate can be reduced without lowering the temporal resolution, and block-shaped This is an adaptive coding control system equipped with a low-pass filter that enables coding with little distortion.

(Embodiment) FIGS. 1 (a) and 1 (b) are embodiments according to the present invention.

The input moving image digital signal is passed through a pre-filter 11 which operates by receiving an encoding control signal for performing adaptive encoding control from an encoder 12 and an encoding generated information amount monitoring unit 13, and an encoder 12 to go into.

The pre-filter 11 is composed of a filter control unit 15 and a low pass type filter unit 14 as shown in FIG.

The coding control signal that the filter control unit 15 receives from the coding occurrence information monitoring unit 13 is the product of the coding occurrence information amount and the quantization step size, the quantization step size, and the integral value of the quantization step size in this embodiment. is there.

The ON / OFF determination of the operation of the low-pass filter unit 14 by the filter control unit 15 uses the method of controlling by the hysteresis operation of the quantization step size in this embodiment.

When the operation is turned on as described above, the filter control unit 15 sets the value of the band pass width determinant (K) of the low pass filter unit 14 to the product of the coded information amount and the quantization step size and its threshold value, or the quantum value. Determined and changed according to the relationship between the integration step size and its threshold value.

In this embodiment, the filter control with the band pass width determining factor (K) is performed by a mask type low pass filter of 3 × 3 matrix as shown in FIG. 1 (c).

As described above, the product of the encoded generation information amount from the output information generation amount monitoring unit 13 and the quantization step size from the encoder 12, or the integrated value signal of the quantization step size from the output information generation amount monitoring unit 13 According to the above, the pass band width of the filter is adaptively switched by changing the value of K, and the spatial high frequency component of the input pixel is reduced.

The encoder 12 performs interframe prediction error conversion coding using motion compensation, intraframe conversion coding, and the like, and the output result is stored in a postbuffer for output rate smoothing. At this time, when the spatial high frequency component of the input image is reduced by the above pre-filter, the amount of output information is reduced due to the effect that the inter-frame prediction error signal becomes small, the high frequency component coefficient of the converted signal becomes zero, and the like. At the same time, since the fine quantization is performed by the coding control based on the small amount of output information generated, the block-shaped distortion also becomes small.

Encoding generated information amount monitoring unit 13 placed after the encoder 12
Is a pre-filter 11 and an encoder 12 for the encoded control signal.
And feed-back control.

Here, for the encoding control in the encoder 12, for example, a conventional method such as switching of the quantization characteristic can be applied. However, the control by the pre-filter is performed instead of the control for dropping frames.

(Effect of the invention) When 100 frames of the moving image "salesman" used as a test image in CCITT TV telephone / conference system standard work are encoded at 384 Kbit / s, 15 Hz, the average encoded bit frame is filtered. It was 321 Kbit / s (excluding stuff bits) without the filter, but with the filter, it was 276 Kbit / s (same as above), which was an effect of about 15% reduction.

As described above, according to the present invention, the image input unit of the encoder is provided with the low-pass filter whose pass bandwidth can be controlled from the outside, and the input image signal is passed through the low-pass filter to the encoder. , The product of the amount of coded information output from the encoder and the quantization step size is monitored, and the passband width of the low-pass filter is calculated when the numerical value obtained by monitoring is large. Is controlled to be narrow, and when the value is small, it is controlled to be wide, thereby controlling the amount of coded information output from the encoder.

Since the passband width is controlled for the input image signal in this way, the prediction error signal does not change, and therefore the coding efficiency does not decrease. Even when applied to hybrid block coding, artificial distortion such as block distortion does not occur. Noise does not occur. Not only that, since the high frequency component of the input image signal is removed, the image is easily coded, and the artificial coding noise such as block distortion is significantly reduced.

In particular, according to the present invention, a low-pass filter is provided on the image input side of the encoder, and the product of the generated information output from the encoder and the quantization step size is supplied to this filter as a control signal. Since there is no need to newly provide a configuration for controlling the control information, it is possible to connect to
Almost no modification of the encoder and its peripheral circuits is required, and the encoder of any configuration can be applied. Furthermore, since the passband width of this filter is controlled by using the generated information output from the encoder as a control signal, it is not only necessary to transmit at a fixed bit rate, but also when transmitting at a variable bit rate. Can be applied.

Therefore, the present invention is applicable to complicated and fast moving images and sheet changes in services such as video conferencing, video telephone, digital television transmission, etc., in which digital moving image signals are encoded and compressed at a constant bit rate for transmission. It is possible to perform coding without block-like distortion while maintaining natural movement, and this effect is great.

[Brief description of drawings]

FIG. 1 (a) shows a block diagram of a moving picture coding apparatus using the present invention. 1 (b) is a block diagram of the pre-filter shown in FIG. 1 (a), and FIG. 1 (c) is 3 × 3.
Determinants (K) of pass band of mask type low pass filter
An example of arrangement is shown. FIG. 2A shows a conventional method of moving picture coding control using a post buffer, and FIG. 2B shows a conventional method of moving picture coding control using a prebuffer. 11 …… Pre-filter, 12,22,26 …… Encoder, 13 …… Encoding generated information amount monitoring unit, 14 …… Low-pass filter, 15
...... Low-pass filter control section, 21,25 ...... Frame dropping processing section, 23 ...... Post buffer, 24 ...... Pre-buffer.

Claims (2)

(57) [Claims] 1. An adaptive encoding control method for encoding and transmitting a digital moving image signal at a constant bit rate, the low encoding range having a pass band width externally controllable at an image input unit of an encoder. A pass filter is provided so that the input image signal is input to the encoder through the low pass filter, and the product of the encoding generation information amount and the quantization step size output from the encoder is calculated. By monitoring and controlling the pass bandwidth of the low-pass filter so as to be narrow when the numerical value obtained by the monitoring is large, and to be wide when the numerical value is small, Adaptive coding of a moving picture signal, characterized in that the amount of coded information output from the coder is controlled to carry out moving picture coding without visually degrading the coded image quality. control method. 2. The adaptive value according to claim 1, wherein an integral value of a quantization step size is used as the generation information output from the encoder, which is control information of the low pass filter. Coding control method.