H. 264/Avr Analysis

In the techniques discussed thus far, the encoder operates independent of the decoder in order to combat the effects of transmission error in the video bitstream. In a situation where both the encoder and decoder are required to interactively combat transmission losses, then a feedback channel can be designed between the decoder and the encoder. The decoder can relay information about which part of the transmitted video data is corrupted by errors, and the encoder can strategize its operation to mitigate the effect of such errors [186]. A simple approach is to retransmit the lost packet especially in an underlying network that supports ARQ or to intra code the data. However, as previously explained, this will incur delays and losses that can
Usually, positive acknowledgement message consists of information received by the encoder about the correct reception of a particular frame from the decoder while, the negative acknowledgement message is a feedback information from the decoder that indicates the presence of error in a particular frame. H.264/AVC allows the application of RFS to a particular portion of a picture rather than the complete picture, the decoder must be informed about the reference picture that was selected to predict a particular segment of the frame. This requires that the temporal reference (TR) of the reference picture to be utilised is sent along with the picture header information to indicate which of the several reference pictures the decoder can use to decode a particular frame [187]. Furthermore, this concept can allow frames to be kept in short term or long term memory buffers for future reference; also this concept can be exploited by the encoder for different purposes like achieving efficient compression, bit rate adaptation, and for error resilience
Instead, it can select the reference frame prior to the corrupted picture whenever feedback information is received. One form of reference picture selection can be demonstrated in Fig. 4.11, it shows the transmission of frames 1 through 9. The encoder prediction is indicated by the use of curved arrows in the figure. In this concept, the most recent frame is usually utilized as the reference frame for motion-compensation prediction. However, if frame 4 gets corrupted by packet loss, the decoder immediately signals a negative acknowledgement (NACK) message to the encoder about the error. This informs the encoder that frame number 4 is damaged, and that all subsequent frames are prone to error propagation. In this regard, the encoder then selects the most recent frame known to be correctly received (frame 3 in this figure) as a reference for the encoding of the next frame, which is frame 6. Round Trip Time (RTT) determines the required amount of time for the prediction structure to be altered, and consequently determines the number of frames that will be affected by the error propagation