A NMR Optimized Bitrate Transcoder for MPEG-2/4 LC-AAC

This paper presents a noise-to-masking-ratio (NMR) optimized MPEG-2/4 AAC low-complexity (LC) transcoder, called as fast rate-distortion optimized transcoder (FRDOT). The FRDOT searches for the optimal scale factors under the NMR criterion according to the magnitudes of quantized input coefficients and the target bitrate. The computation of NMR difference is replaced by the derivation of signal-to-noise-ratio (SNR) difference since the audible masking thresholds of the original and transcoded bitstreams are identical. The SNR value is further converted to a noise-to-signal-ratio (NSR) to represent distortion energy of audio signals. With the NSR values, a table lookup technique is used to speed up the search of the optimal scalefactor increment. To further reduce the execution time, a bandwidth limiter is used to remove the iterative rate-distortion optimization of a frame. In addition, a bitrate control module is proposed to make the averaged bitrate of output bitstream close to the target bitrate. The results show that the FRDOT has smaller NMR values than the cascaded transcoder (CT) by 0.5-3.0 dB at different bitrates with 6-7 times speedup on the average.