1. Field of the Invention
This invention relates to a data communication apparatus for communication using a plurality of kinds of data, such as image data, audio data, and code data (hereinafter referred to simply as data).
2. Related Background Art
With the recent development of communication technology, semiconductor technology and optical technology, digital communication lines have been provided to enable high-speed large-capacity data transmission.
Specifically, digital transmission is free from reduction in data quality during transmission and ensures that data can be uniformly maintained at a suitable quality level. Also, digital transmission enables integration of media since it does not require selection of data transmission paths in accordance with characteristics of media. Use of digital transmission has therefore enabled transmission between composite media, and telephone terminals capable of simultaneously transmitting images together with speech have been developed from conventional telephone terminals transmitting speech alone.
Under these circumstances, international standardization for enabling communication between different kinds of composite terminals is being forwarded by CCITT or other organizations, and service rules for audio-visual (AV) services such as TV telephone or TV meeting system with digital lines, protocol rules and multi-media multiplexed frame construction rules have been made public in the CCITT recommendations (drafts) H.320, H.242, H.221, etc.
In H.221, exchange between frame constructions and between terminal capabilities, frame alignment signals (FAS) for communication modes, and code assignment for bit allocation signal (BAS) with respect to AV service from 64 to 1,920 kbps are defined. In H.242, protocols for capability exchange and communication code changeover between AV terminals using BAS are defined. In H.320, system aspects for overall AV service are defined.
These recommendations or drafts of recommendation provide a method for performing, after setting an end-to-end physical connection and establishing synchronization by FAS during in-channel, multi-media communication between terminals using images, voice and data based on procedures such as sequence of operations for terminal capability exchange using BAS during in-channel and sequence of operations for mode changeover by communication mode designation. However, changing the self terminal capability at each terminal according to situations and selection of the communication mode within the range of exchanged capability are not included in the scope of the rules. The rate of information transfer on each of media in multi-media communication is determined in such a manner that the audio information transfer rate is determined by designating an audio encoding method, the data information transfer rate is determined by designating whether or not data is used and a transfer rate in the case of use, and the image information transfer rate is determined as the remainder of subtraction of the audio information transfer rate and the data information transfer rate from the overall information transfer rate of the set communication path.
In the system of conventional multi-media terminals defined as described above, however, communication between terminals using image data is performed as described below. Ordinarily, when a terminal is connected to a communication partner's terminal, the display is changed over from a picture showing the operation of the multi-media terminal to a received image. Thereafter, displaying received images is continued unless the communication is completed or the operator changes the display by intention. However, the display cannot be changed also when the partner's terminal stops the image data transmission or changes the transmitted information for information through other media. In such a case, the display continues showing meaningless images or shows a disturbed image.
According to the CCITT recommendation (or recommendation draft) H.261, there are generally two modes of image signal encoding:
an intra-mode in which one-frame picture is divided into 12.times.33 blocks or 3.times.33 blocks (macro-blocks), the difference between each of these blocks and the original picture is calculated and image data on the difference alone is transmitted; and PA1 an inter-mode in which data on the true images of macro-blocks is transmitted without using the above difference. This recommendation also provides that transmission is performed in the inter-mode at least one time every 132nd transmission with respect to macro-blocks to control accumulation of errors.
The CCITT recommendation (or recommendation draft) H.221 provides that when a picture renewal request BAS command is received, data for one-frame picture is transmitted in the inter-mode.
In this conventional system, however, macro-blocks are renewed at predetermined intervals and there is therefore a need to renewing the picture based on user's judgment by transmitting a picture renewal command to the partner's terminal each time an error occurs in a received image signal during image transmission/reception.
On the other hand, processing at the time of occurrence of a frame synchronization error is described in H.242. According to this description, when a frame synchronization error occurs and it is detected on the receiving side, a timer T3 (10 sec.) is set to recover frame synchronization. If the frame synchronization is recovered before timer termination, the ordinary operation is restarted. If the frame synchronization is not recovered before timer termination, synchronization is performed from the beginning.
In the above-described conventional system of multimedia terminals, however, if a line disorder or a disorder of an image receiving unit of a terminal takes place during image data communication between terminals, or if image data is changed at the time of connection/disconnection with a partner's terminal or at the time of changing the transmission capability, omission or disorder of image data occurs and this condition directly influences the displayed picture so that the screen is blacked out or an image disturbance occurs. Moreover, during the period of time for reestablishing the synchronized state, no video and audio signals are supplied and there are no outputs on the screen, which condition gives the operator a feeling of uneasiness, and the operator cannot discriminate whether such a state follows occurrence of an error or completion of communication. There is also the problem of a discontinuity between pictures received before occurrence of a frame synchronization error and after recovery of frame synchronization.