- In such cases, the most appropriate low bit rate version of the bit stream could be chosen at the expense of smaller resolution and lower perceptual quality. - Homogeneous video transcoding algorithms aim to reduce the bit rate, frame rate and/or resolution of the pre-encoded video stream. - On the other hand, the latter method, namely transcoding, partially decodes each of the incoming video streams, combines them in the pixel domain and re-encodes the video data in the form of a single video stream. - This gives rise to a lower bit rate at the output of the transcoder.. - One of the bit rate reduction methods is the re-quantisation of the transform coefficients, as already discussed in Chapter 3. - However, the drawback of the algorithm is that LVQ transcoding leads to MPEG-incompatible bit streams. - The output can then be directly fed into an MPEG video decoder at the very end of the telecommunication system.. - The cascaded method of fully decoding and then re-encoding of the incoming compressed video stream is the conventional tandem operation of two video networks, as seen in Figure 6.4. - Then the decoded transform coefficients are inverse zigzag-scanned and inverse quantised with the quantisation parameter of the video encoder. - This mismatch is an eventual result of the quantisation level differen- ces between the originally encoded and transcoded video frames. - As depicted in Figure 6.5, the rate reduction algorithm within the video transcoder starts with the de-quantisation of the DCT coefficients using the original quantiser levels. - This causes an eventual increase in the bit rate which defeats the objective of bit rate reduction and hence, the functionality of the video transcoder.. - A description of this kind of transcoder architecture is presented in the next section, following the mathematical analysis of the drift phenomenon.. - The analysis of the drift error has been given by Assuncao and Ghanbari (1997).. - Rate reduction with an open-loop transcoding algorithm naturally modifies the above equations due to the addition of the transcoding distortion. - From the first two lines of Equation 6.4, it is clearly seen that the transcoding distortion t BGQRMPR is the difference between the current pictures of the decoder and the encoder. - B L\ stand for the locally decoded current and previous pictures of the input and output streams, respectively. - The last line of Equation 6.10 is particularly significant as it hints at the direct use of the incoming original rate without the need for fully decoding it. - However, the discrepancy arises in the presence of an additional feedback loop as part of the overall. - Thus, the feedback loop corrects the accumulated mismatch errors between the reconstructed images of the source coder and those of the transcoder. - However, this implies a sub-optimal motion prediction that leads to some quality degradation in the transcoded video despite the existence of the drift correction loop. - However, the motion data re-estimation scheme, as the name implies, comprises a full-scale re-estimation of the new MVs. - Moreover, as illustrated in Figure 6.9, due to the existence of the motion re-estimation block within the drift correction loop, it is possible to reduce the effects of non-optimal MVs on the transcoding quality. - The need for the MV refinement procedure was presented by Youn, Sun and Lin (1998), who also elaborated on the mathematical aspects of the problem. - As observed in Figure 6.11, the reconstructed picture within the transcoder R C is also fed into the re-encoding part of the transcoder block, and thus it is similar to the current picture of the transcoder P. - In the above equation, AC (h, v) represents the quantisation error of the current frame in the encoder, and NR (h, v) represents the quantisation error of the previous frame in the transcoder. - where SDQE represents the sum of the differential quantisation error, previously referred to as functions in Equation 6.14. - Equation 6.15 shows that the direct re-use of the incoming original MVs (I. - The MV refinement window has conventionally been chosen to be small to avoid the high complexity operations of the motion re-estimation process in the video transcoder. - Figure 6.13 Average PSNR values of the transcoded Foreman, originally encoded at an average of 116 kbit/s and 25 f/s, versus various refinement window sizes. - Figure 6.14 Average bit rates of the transcoded Foreman sequence of Figure 6.13 versus various refinement window sizes. - Figure 6.13 demonstrates the average PSNR levels obtained at different refinement window sizes, while Figure 6.14 shows the effect of the window size on the average bit rate of the transcoder.. - In this section, the performance of the presented homogeneous transcoding algorithms is evaluated. - In both figures, the effect of the picture drift is clearly represented by the significant reduction in video quality. - Therefore, the choice of a particular transcoding scheme is a compromise between complexity of the algorithm and its performance.. - Similarly, a video transcoder can also be used to reduce the frame rate of the video stream before it reaches a network of lower supportable rates. - In certain situations, reducing the bit rate may not be enough and the video transcoder has to reduce the frame rate of the incoming bit stream for useful transcoding results. - Figure 6.15 Performance evaluation of the homogeneous video transcoding algorithm using the Suzie sequence: (a) PSNR, (b) number of output bits per frame. - Figure 6.16 Subjective results of the 150th frame of the Suzie sequence: (a) direct enc/dec (64 kbit/s), (b) direct enc/dec (32 kbit/s), (c) re-quantisation scheme (64 . - The damage mainly occurs due to the loss of frames that are required for the successful decoding of the sequence. - Table 6.2 Frame rate transcoding of the 200-frame Foreman sequence, MV refinement win- dow : <. - Type of the scheme (kbit/s) (f/s) parameter (dB). - The predictive encoding increases the importance of these dropped frames for the accurate reconstruction of the sequence. - To mitigate the damaging effects of frame dropping, the transcoder has to re-estimate the motion of the transcoded video in reference to preserved frames. - The transcoder makes use of the incoming MVs in order to estimate their new values after frame reduction (Youn, Sun and Lin . - The new MV value is the sum of the corresponding MVs in the skipped and the reference frames. - The size of the refinement window is determined by the number of skipped frames and the magnitudes of their MVs (Hwang and Wu, 1998).. - The down-sampling filter is situated between the decoder and the re-encoder stages of the transcoder (Bjork and Christopoulos, 1998). - In this case, reducing the resolution of the video frame size allows for the successful delivery and display of the requested video material. - For instance, a CIF to QCIF conversion process requires a resolution reduction by a factor of two in each spatial dimension of the video frame. - This requires the selection of the most suitable MV out of four existing MVs in the CIF frame. - Either one of the following three solutions could be adopted:. - Figure 6.18 DCT-domain down-scaling of the Harry sequence from 4CIF (704 . - Therefore, the conversion algorithm consists of the following steps, illustrated in Figure 6.20:. - Video data translation is the major process of the entire transcoding scheme. - However, when heterogeneous transcoding is combined with the homogeneous transcoding operation, the syntax conversion algorithm must be accompanied by one of the drift-free bit rate reduction schemes discussed in earlier sections of this chapter.. - 150 frames of the Suzie sequence are initially encoded at an average bit rate 56 kbit/s and a frame rate of 25 f/s. - Conversely, the cascaded decoding/re-encoding PSNR values are on average 1—1.5 dB less than those of the transcoder. - This is because transcoding uses the DCT coefficients and the MVs of the incoming bit stream (without fully decoding them), while the cascaded decoding and re-encoding scheme re-estimates the MVs and re-calculates the DCT coefficients based on the lossy reconstructed pictures (not the originals). - A 0.5 dB improvement is noticed in the MPEG-4 to H.263 conversion due to the use of the advanced prediction mode that helps improve the picture quality, particularly in high-motion areas.. - Error concealment can be applied on the partially decoded video data using one of the techniques discussed in Chapter 4. - Depending on the status of the output buffer, the output rate can adaptively be. - Figure 6.22 Subjective performance evaluation of heterogeneous video transcoding using 150 frames of the Suzie sequence at 56 kbit/s: (top row) MPEG-4 to H.263, (bottom row) H.263 to MPEG-4, (first column) direct encoding/decoding, (second column) transcoding, (third column) cascaded decoding/re-encoding. - The increase in bit rate caused by AIR is controlled by the rate management feature of the transcoding algorithm. - Figure 6.25 shows the PSNR values achieved by transcoding 200 frames of the Mother and Daughter sequence encoded at 70.5 kbit/s and a frame rate of 25 f/s.. - The PSNR values reflect the performance of the transcoder for different BER values. - For fair comparison, both ver- sions of the transcoder (resilient and non-resilient) shown in Figure 6.25 are made to generate the same output rate. - The subjective quality improvement of Figure 6.26(c) compared to (b) verifies the efficiency of the error-resilient transcoder.. - 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