Error Concealment Techniques for MPEG-2 Video Sequences
Presentation of the Concealment Techniques
Signal loss occuring in physical communication channels is unavoidable. In the case of transmission of highly compressed video sequences by the MPEG-2 codec, this leads to significant errors observed to the reconstructed frames at the decoder side. These errors fall into two categories:
- Bitstream Errors.
They are caused by direct signal loss of some or the whole compressed packet of a coded MB, and result in the loss of the whole respective slice information. - Propagation Errors.
They are caused in P- and B-frames uniquely by the additional use of motion compensated time information for their reconstruction at the decoder side. Errors in previously decoded reference frames propagate to the next in the decoding order.
The detection of errors in the decoded frames of the MPEG-2 coded and decoded sequence is initially considered. Bitstream errors are indicated by error tokens, sent to the decoder at the time of signal loss occuring. Consequently, the necessary information about the locations of errors (slice and MB) in a frame is already known by the decoder. The problem arises with the detection of the propagation errors. This can be achieved by the use of available space and time information from the current and reference frames at the decoder.
The information about the locations of errors being available, their concealment is successively considered. A different technique is implemented for I-frames, since the latter are coded independently from the other frames of the video sequence. This technique exploits spatial information only from available neighbooring MBs of the current frame.
Error Concealment in P- and B-frames is performed through the use of both space and time information. The space information is obtained by the available neighbooring MBs of the current frame, while time information is acquired by the previously decoded frames.
Performance Results and Conclusions
In order to observe the performance of the concealment techniques, the Flower Garden image sequence has been used and the PSNR of the processed frame has been evaluated. Considering frame 2 (a B-frame) of the mentioned sequence corrupted by both kinds of errors, the error concealment technique, which uses the errorfree frame 0 (I-frame) to obtain the additional time information, gives the following results in respect with PSNR for each chroma component of the frame:
The observation of the performance results leads to the conclusion that the concealment technique performs better, if the previously decoded frames have been very well concealed, in case they were corrupted. The comparison between the errorfree frame 2 and the concealed respective frame proves the validity of this remark.
