In this paper, two systems for low-complexity MPEG-2 to H.264 transcoding are presented. Both approaches reuse the
MPEG-2 motion information in order to avoid computationally expensive H.264 motion estimation. In the first approach,
inter- and intra-coded macroblocks are treated separately. Since H.264 applies intra-prediction, while MPEG-2 does not,
intra-blocks are completely decoded and re-encoded. For inter-coded macroblocks, the MPEG-2 macroblock types and
motion vectors are first converted to their H.264 equivalents. Thereafter, the quantized DCT coefficients of the
prediction residuals are dequantized and translated to equivalent H.264 IT coefficients using a single-step DCT-to-IT
transform. The H.264 quantization of the IT coefficients is steered by a rate-control algorithm enforcing a constant bit-rate.
While this system is computationally very efficient, it suffers from encoder-decoder drift due to its open-loop
The second transcoding solution eliminates encoder-decoder drift by performing full MPEG-2 decoding followed by
rate-controlled H.264 encoding using the motion information present in the MPEG-2 source material. This closed-loop
solution additionally allows dyadic resolution scaling by performing downscaling after the MPEG-2 decoding and
appropriate MPEG-2 to H.264 macroblock type and motion vector conversion.
Experimental results show that, in terms of PSNR, the closed-loop transcoder significantly outperforms the open-loop
solution. The latter introduces drift, mainly as a result of the difference in sub-pixel interpolation between H.264 and
MPEG-2. Complexity-wise, the closed-loop transcoder requires on average 30 % more processing time than the openloop
system. The closed-loop transcoder is shown to deliver compression performance comparable to standard MPEG-2