Paper An MPEG-2 to H.264 Intra Transcoding Method using Adaptive Macroblock Pair Type Selection Takeshi YOSHITOME (Member), Kazuto KAMIKURA NTT Cyber Space Laboratories, NTT Corporation 〈Summary〉 We propose an MPEG-2 to H.264 intra transcoding for interlace stream of frame and field macroblock intermingled to improve our previous work. This method uses an adaptive MB pair type selection method that keeps as many discrete cosine transform (DCT) coefficients of the original MPEG-2 bitstream as possible to avoid mixture of re- quantization noise. Experimental results show that the proposed method improves the peak signal-to-noise ratio (PSNR) values by about 0.33–1.55 dB over those of the conventional methods. Keywords: transcoding, MPEG-2, H.264/AVC, adaptive MB pair type selection ing the number of bits in I-pictures is important for 1. Introduction obtaining a smaller bitstream size because I-pictures High quality video transcoding will be one of the have a larger number of bits than P- or B-pictures. most important technologies of visual applications in Our proposed method is effective for both I-pictures the future. For example, a video service using ter- and for the intra MBs included in P- and B-pictures. restrial digital broadcating for Internet protocol (IP) This method will also improve P- and B-picture im- network users has started in Japan 1). In this service, age quality because these pictures refer to the high the MPEG-2 2) compressed digital broadcating sig- quality I-pictures that the method generates. nals sent to a network center, transcoded to H.264 3), 2. Noise Reduction Mechanism of Using and transfered to network users. The most impor- First Encoding Information tant factor in such transcoding services is reducing re-quantization noise during the transcoding proce- A data flow example of transcoding with first dure. It is well known that all decisions of the first MPEG-2 encoding information is shown in Fig. 1. encoding procedure are repeated during the transcod- For simplicity, the macro-block (MB) size is 2 × ing procedure to reduce re-quantization noise 4).This 2. A quantization parameter is represented using noise suppression mechanism is effective when the quantizer-scale-code (QSC) in MPEG-2, or using QP compression standards of the first and second encod- in H.264. Because this paper deals with MPEG-2 ing are identical, such as for MPEG-2/MPEG-2 or and H.264, we use the quantization step Δ which H.264/H.264 conversion. In our previous work 5),6), means the divisor of the DCT coefficient. For ex- we expanded this approach to apply it to MPEG- ample, Δ = 16 represents QSC = 12 in MPEG-2 2/H.264 transcoding for progressive bit-streams. In when q scale type = 1, and represents QP =28in this paper, we show how we used an adaptive mac- H.264. roblock (MB) pair type selection method to apply this In the first MPEG-2 encoding, an input MB that noise suppression mechanism to interlace intra bit- includes the pixel values (A, B, C, and D) is processed streams that consist of frame and field MBs. Reduc- by orthogonal transformation. Then, the transformed 33 The Journal of the IIEEJ vol. 40 no. 1(2011) Table 1 Major differences between MPEG-2 and H.264 MPEG-2 H.264 H.264 re-enact 4 × 4, 4 × 8, Block size 16 × 16 8 × 4, 8 × 8 OK 8 × 16, 16 × 8 16 × 16 Vector resolution 0.5pix 0.25pix OK Intra prediction no yes ? Orthogonal DCT integer ? Transformation DCT Size of orthogonal 8 × 8 8 × 8 (*1) OK Transformation 4 × 4 entropy coding VLC CAVLC OK CABAC Fig. 1 Second encoding with first MPEG-2 encod- Specification of yes yes (*1) OK ing information Quantization matrix Interval of linear log. ? Quantization step discrete cosine transform (DCT) coefficients (44, 33, Specifications of each pair ? 21, and 12) are quantized by a quantization step Δ = Field/FrameMB MB MBs 10. This first quantization process adds quantization (*1): only High-Profile noise to the input MB. In the second encoding, every type of encoding in- act the effect of this noise reduction mechanism, we formation, such as MB type, motion vector, DCT must control H.264 behavior to imitate MPEG-2 be- mode, and quantization step, is re-used. This re- havior as much as possible when the first encoder is use of encoding information does not produce any MPEG-2 and the second encoder is H.264. The two significant difference between motion compensation encoders are similar but are not upper compatible. errors of the first encoding and those of the second The differences between them are listed in Table 1. encoding, nor does it produce any notable difference These differences should be absorbed to recreate between the DCT coefficients of the two encodings. this noise reduction mechanism. The “ok” means This means that the re-quantization noise added in that H.264 can recreate the same MPEG-2 function the second encoding phase is almost zero. Unfortu- in the table. The “?” means that H.264 can process nately, there is no bit reduction because the output the function only in a similar but not identical way bitstream size of the first and second encodings is al- that MPEG-2 does. For example, certain MPEG-2 most the same when these encodings make use of the quantization steps cannot be expressed exactly using same compression tool. But transcoding using dif- H.264 steps because the former are defined by a linear ferent compression tools, such as those for MPEG-2 interval and the latter are defined by a logarithmic in- and H.264, may reduce the bitstream size because the terval. The following four functions are not exactly performance of H.264’s entropy coding (CABAC) is the same between MPEG-2 and H.264: (1) Internal higher than that of MPEG-2’s entropy coding (VLC). picture motion compensation, (2) Orthogonal trans- formation, (3) Available quantization step, and (4) 3. Differences between MPEG-2 and Frame/Field MB specification. The differences from H.264 (1) to (3) were analyzed in our previous work 5),6).In As depicted in Fig. 1, there is a close resemblance the second half of this section, we describe the dif- between the decoded images of the first and second ferences of (4) in detail and show a way to overcome decoders when the compression standards of the first them. and second encodings are exactly the same. To reen- The DCT type specifications for MPEG-2 and 34 Paper : An MPEG-2 to H.264 Intra Transcoding using Adaptive Macroblock Pair Type Selection H.264 MBs are shown in Fig. 2. Although a DCT between upper and lower MBs is about 86%. type can be specified for each MB independently in For the remaining 14%, single agreement is ob- MPEG-2, H.264 specifies the DCT type for each pair tained between MPEG-2 and H.264 for upper or lower of MBs that adjoin each other above and below. MBs, so the overall MB agreement obtained is 93%. When an upper MB is specified as the frame DCT In other words, the quantization noise reduction is anditslowerMBisspecifiedasthefieldDCTin not effective for only 7% of the MBs. MPEG-2, the DCT type of one MB is different from 4. Simple Transcoding Example its DCT type specified in H.264, either type frame or field DCT in H.264. In this section, we show an actual transcoding ex- The noise reduction in the second encoder, men- ample that indicates the re-quantization noise is re- tioned in Section 2, decreases when many disagree- duced when the MPEG-2 DCT type is equal to the ments of DCT type exist in an MB pair. We therefore H.264 DCT type. The MPEG-2 compressed image measure the disagreement quantity using the MPEG- using the quantization step Δ1 =20isshownatthe 2 bit-streams generated by five commercial encoders. right side of Fig. 3. We show the transcoding process These encoders are NEC’s VC-5310, Mitsubishi’s for a certain MB pair, shown at the left side of the 7) 9) MH2200E, and three NTT’s ～ . The percentage figure. The upper MB of this pair was transformed by for which the upper MB has the same DCT type as MPEG-2’s frame DCT, and the lower MB was trans- the lower MB is shown in Table 2. The input images formed by field DCT. Let us assume this MB pair is are 12 sequences included in the standard HDTV se- transcoded using H.264’s frame DCT. Then, Eq. (1) quences. The average of the DCT type agreement shows the 8 × 8pixelsvalue,Up, located at the top- left corner of the upper MB. The DCT coefficients of the upper MB, Ut, are shown in Eq. (2). This coeffi- cient are quantized by re-quantization step Δ2 = 20, which is equals to MPEG-2’s quantization step Δ1. The re-quantized DCT coefficients, Uq,areshownin Eq. (3). All the Uq coefficients are multiples of 20 because DCT coefficients of Ut are quantized by 20. This process adds re-quantization noise. The mean square error of the 8 × 8 coefficients, U!MSE ,isshown Fig. 2 DCT type specification of macroblock in in Eq. (4). The sum total of the coefficients is 127.0. MPEG-2 and H.264 The 8 × 8 pixel values Lp, located at the top-left Table 2 Percentage of the DCT type agreement be- corner of the lower MB, are also transcoded in the tween upper and lower MBs in MPEG-2 same manner.