The present invention relates to a digital transmitting apparatus for a video signal, and more particularly to an inter-frame predictive coding apparatus for the video signal.
An inter-frame predictive coding apparatus has been conventionally well-known as a system of performing compression coding for a video signal such as a television signal and the like. In the inter-frame predictive coding apparatus, a difference in amplitude of picture elements (pixels) at a given location, between a past frame memorized in a memory and the present frame, is detected and quantized for transmission. This apparatus has the advantage that it is possible to perform extremely great compression when a stationary image or an image having a small moving region is coded. However, it has the disadvantage that the compression effect is reduced as the area of a moving region becomes larger. In order to remove this disadvantage, a motion compensation coding method has been employed. In this method, a motion vector which corresponds to the movement in the image is detected for each of the picture elements, or for one or more blocks which have been previously designated in an image screen, and the difference between the frames is calculated, with spatial correction in accordance with the motion vector. One example of this method is disclosed in U.S. Pat. No. 4,371,895 to Koga titled "CODED VIDEO SIGNAL TRANSMITTING AND RECEIVING SYSTEM".
When employing either the inter-frame predictive coding or the motion compensation coding, no information should be generated, theoretically, for a stationary input image or a stationary region in a moving image, but in practice, level fluctuation unavoidably occurs in the luminance amplitude value, under the influence of noise in the input video signal, or jitter in sampling pulses. As a result, a great deal of information is often generated undesirably in relation to a stationary image or a stationary region of a moving image. One possible method of reducing the amount of undesirable information is to provide coding control by using sub-sampling. This method is disadvantageous in that spatial resolution is reduced in the stationary region, that is, the image in the stationary region is blurred.