Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
1999-03-02
2003-09-16
Kelley, Chris (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
Reexamination Certificate
active
06621868
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a video communication system wherein a picture is sent via a transmission line on which errors can be generated during sending through a wireless transmission line, a LAN, etc., a video communication method, and computer readable recording media on which a video communication program is recorded.
This application is based on Japanese Patent Application, No. Hei 10-49094 and Japanese Patent Application, No. Hei 10-49095, the contents of which are incorporated herein by reference.
2. Description of the related Art
In the communication of video data, the inter-frame prediction encoding method is a representative video encoding method for increasing compressibility. In this inter-frame prediction encoding method, at the sending side, the predictive error between the latest previously encoded picture and the present picture is encoded, and the only this part is sent as encoded data via the transmission line to the receiving side. Then, at the receiving side, using a picture previously received, decoded and then stored as a reference picture, the present picture is decoded from this reference picture and the received data.
When encoded data using this inter-frame prediction encoding is transmitted, if an error is carried into the encoded data over the transmission line, the picture will be damaged and moreover the error will propagate to the picture of the later encoded data. That is, when the receiving side uses erred picture as a reference picture, even if encoded data of a subsequent picture does not have an error, the error will propagate to the decoded picture because the area damaged by the error is referenced.
One technology for resolving this problem is a method wherein the sending side refers only to a picture which can be correctly decoded at the receiving side, as in the Annex N of the picture encoding method ITU—T H. 263. In this method, a reference picture memory for a plurality of pictures is provided at the sending side and receiving side, and furthermore, the sending side decodes the reference picture by using a picture designated at the receiving side. The encoding—decoding sequence according to the this method is as follows.
First, the sending side sends a signal designating a picture to be used as the reference picture at the same time as the encoded data, and the receiving side uses as the reference picture the picture designated by the signal designating the reference picture, and the received encoded data is decoded. In addition, the pictures which could be correctly decoded are sequentially stored in the reference picture memory at the receiving side. Next, the receiving side sends to the sending side the information concerning whether the encoded data could be decoded and information designating the newest picture in the reference picture memory. In addition, at the sending side, by encoding using a correctly decodable picture designated by the receiving side as the reference picture, the receiving side uses the picture stored in the reference picture memory as the reference picture, and it can be decoded without error.
Here, in the above-described “information concerning whether the encoded data can be decoded” sent to the sending side by the receiving side, there are two types of signal: the ACK signal and the NACK signal. First, the ACK signal is sent to the sending side by the receiving side when the picture is correctly decoded, and includes the picture designation data which designates a correctly decoded picture. In addition, the NACK signal is sent to the sending side by the receiving side when the picture cannot be correctly decoded, and includes picture designation data which designates the newest picture among pictures stored in the receiving side's reference picture memory.
In addition, at the sending side, when the above-described ACK signal or NACK signal is received, the methods of changing the reference picture used in the encoding can be roughly divided into two types.
The first method switches from the reference picture used in the previous encoding to the picture designated by the picture designation data of the correctly decoded picture on the receiver side included in the ACK signal, or by the picture designation data of the most recent picture among the pictures stored in the reference picture memory on the receiver's side included in the NACK signal. In this method, encoding is carried out by changing the reference picture so that the sending side refers to the picture which could be correctly decoded at the receiving side for every picture. This is called the ACK method.
In normal inter-frame prediction encoding, because the previous picture is always made the reference picture, when one picture has an error, the error will necessarily propagate to the next picture. However, in the ACK method, there is no propagation of subsequent pictures irrespective of whether or not the previous received picture is decoded without error because the reference picture is changed every picture. However, for each picture, the encoding efficiency is lower than normal inter-frame encoding.
The second method switches the reference picture used in the previous encoding to the designated picture by the picture designation data, included in the NACK signal, of the newest picture among the pictures stored in the reference picture receiving side memory. In addition, in this method, when a NACK signal is not received, encoding using the latest previous picture as the reference is carried out. This is called the NACK method.
In this NACK method, if there is an error in one picture, until the reference picture is switched at the sending side by sending the NACK signal, the error propagates like a normal inter-frame prediction encoding. When the picture with the error is not displayed, the propagation of this error will cause a lowering of the number of pictures that can be correctly decoded. However, the reference picture is changed only when a NACK signal is received, and thus the encoding efficiency is higher than the above-described ACK method.
The differences between the ways these two above-described methods switch the reference picture lies in the switching method of the reference picture at the sending side when receiving an ACK signal sent when the receiving side has correctly decoded the picture. Thus, there is a difference in whether to change the reference picture according to the picture designation data of the picture which has been correctly decoded at the receiving side included in the ACK signal, or to make the latest picture previously encoded at the sending side the reference picture.
In any case, depending on the aim of the communication, either the ACK method, which has the effect of decreasing the propagation of errors, or the NACK method, which has the effect of reducing the decrease in encoding efficiency, can be chosen, and this choice can be assumed before the commencement of communication and during communication.
Concerning the designating of the ACK method or the NACK method, when the designation is made at the beginning of the communication, it is possible for the receiving side to designate to the sending side in advance whether the ACK method or NACK method is being used. However, when the designation occurs during a communication, in the conventional methods, the sending side has been able to choose whether to use the ACK method or the NACK method, while the receiving side could not do so.
This is caused by the fact that what the receiving side designates to the sending side is only which specific picture is to be made the reference picture, and during communication, clearly, there was no method for the receiving side to designate to the sending side that the latest previously encoded picture is to be made the reference picture, and thus during communication the receiving side could not designate the NACK method, and there was no means to change from the ACK method to the NACK method.
Because of this, it was not possible to control the
Kimata Hideaki
Tomita Yasuhiro
Burns Doane Swecker & Mathis L.L.P
Kelley Chris
Nippon Telegraph and Telephone Corporation
Wong Allen
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