CAVLC Research Articles

A 4-way parallel CAVLC design for H.264/AVC 4Kx2K 60fps encoder

This paper presents a high performance design for Context-Based Adaptive Variable Length Coding (CAVLC) used in the H.264/AVC standard. A two-stage encoder is proposed to make the scan and encode stage work simultaneously. The scan engine scans four coefficients at each cycle. Parallel encoder for four “levels” and parallel encoder for four “Run_before” are adopted to accelerate the encode engine. Only 120 cycles at most are needed to process one MB. The proposed CAVLC encoder can support 4Kx2K@60fps (frame per second) real-time encoding at 250MHz and the gate count is about 32k.

Implementation of a H.264/AVC SVC decoder with multi-symbol prediction CAVLC for advanced T-DMB receiver

This paper presents a low-cost high-performance H.264/AVC SVC decoder with multi-symbol prediction context-based adaptive variable length coding (CAVLC) for advanced T-DMB receiver. Proposed multi-symbol prediction CAVLC mainly consists of a simple arithmetic operation unit with reduced look-up tables and a leading zero detector. We also propose a multi-symbol run_before decoder and it decodes more than 2.5 symbols in a cycle. Gate count of a H.264/AVC SVC decoder is about 1,300K gates when synthesized with 0.18 um CMOS process and it can be operated at 120 MHz clock frequency. For the verification of a H.264/AVC SVC decoder chip, prototype mobile movie player systems have been implemented using a 7 inch LCD and USB controller.

A design of high-performance pipelined architecture for H.264/AVC CAVLC decoder and low-power implementation

In this paper, we propose a highly efficient VLSI architecture for context-based adaptive variable-length coding (CAVLC) decoder. In multimedia data processing systems, the real-time processing requirement is the most critical problem and the only requirement that must be satisfied. Thus, an architecture which has a short processing time though a high throughput, can meet the requirement at low operating frequencies. Consequently, the architecture can have an advantage of low power consumption. We propose two methods to improve the throughput of CAVLC decoders. The first method eliminates the pipeline hazard in a pipelined architecture for CAVLC decoder. The second method expands the capacity of the barrel shifter. Experimental results show that the proposed architecture can improve throughput by about 45%. As a result, the proposed architecture greatly reduces the operating frequency for real-time processing, which is the key factor of reducing power consumption. The synthesis result shows that the design achieves the maximum operating frequency at 125 MHz, and the hardware cost is about 12.6 K under a 0.18 um CMOS process.

H.264/AVC entropy decoder complexity analysis and its applications

Context-based adaptive variable length coding (CAVLC) and universal variable length coding (UVLC) are two entropy coding tools that are supported in all profiles of H.264/AVC coders. In this paper, we investigate the relationship between the bit rate and the CAVLC/UVLC decoding complexity. This relationship can help the encoder choose the best coding parameter to yield the best tradeoff between the rate, distortion, and the decoding complexity performance. A practical application of CAVLC/UVLC decoding complexity reduction is also discussed.

Reconfigurable architecture for entropy decoding and inverse transform in H.264

Reconfigurable hardware is an effective design option for dealing with the increasing demands of flexibility and computation power in system design. This paper explores techniques to combine the two entropy decoding methods defined in the H.264 standard, context-based adaptive binary arithmetic coding (CABAC) and context-based adaptive variable length coding (CAVLC), using a coarse-grain reconfigurable architecture. An analyzing of the similarities and differences between these two decoding processes shows that CAVLC can be effectively merged into a CABAC decoder. Experimental results show that about 1.5K gates can be saved using the proposed reconfigurable cell (RC) architecture, which corresponds to a 25.4% area savings in the implementation of the CAVLC decoder. Using the idle time in RC arrays, the base cell can be extended to carry out the inverse transform with very limited overhead. The proposed entropy decoder design, which operates at 66 MHz, can decode video sequences at Baseline and Main profiles at Level 3.0 under the real-time constraint.

New Lookup Tables and Searching Algorithms for Fast H.264/AVC CAVLC Decoding

In this paper, new codeword structures, tables, and searching methods for fast and efficient <emphasis emphasistype="italic" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">coeff_token</emphasis> , <emphasis emphasistype="italic" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">total_zeros</emphasis> , and <emphasis emphasistype="italic" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">run_before</emphasis> decoding are developed. This new achievement is mainly based on the fact that the context-adaptive variable length coding (CAVLC) decoding can be modeled as a finite state machine. In order to quantitatively evaluate the proposed method in terms of decoding speed and complexity, we define the iteration bound <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\left({{1}\over {\mathtilde{\tau}}}\right)$</tex></formula> and the <emphasis emphasistype="italic" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">complexity ratio</emphasis> <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(CR)$</tex></formula> . Using these gauge variables, we show that the new algorithms reduce <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\mathtilde {\tau}}$</tex></formula> to about one third and <emphasis emphasistype="italic" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">complexity ratio</emphasis> to 0.95. This means that the proposed techniques reduce the decoding time to about one third and memory access count by 90% compared to those of the conventional methods without implementation overheads. Multiple-symbol parallel decoding method for <emphasis emphasistype="italic" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">run_before</emphasis> syntax element is proposed based on a <emphasis emphasistype="italic" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bit-positioning</emphasis> with the critical path latency of only one multiplexer for the post-combination process. The proposed methods make it possible to implement a fast and efficient CAVLC decoding without losing video quality on any environments.

Efficient information hiding in H.264/AVC video coding

This paper proposes a new real-time information hiding algorithm on latest H.264/AVC video coding standard. The information is embedded into the Trailing Ones of 4×4 blocks during the Context-based Adaptive Variable Length Coding (CAVLC) process. This algorithm is efficient with low computational complexity. The simulation results show that the degradation of video quality is negligible, and the same overall bit-stream length is maintained. Based on this information hiding method, a video subtitle transmission scheme is proposed. Under the simulation of different RTP packet loss channels, the embedded information can be well recovered. The comparison with other algorithms shows the superiority of our proposed method.

Efficient entropy coding scheme for H.264/AVC lossless video coding

Context-based adaptive variable length coding (CAVLC) and context-based adaptive binary arithmetic coding (CABAC) are entropy coding methods employed in the H.264/AVC standard. Since these entropy coders are originally designed for encoding residual data, which are zigzag scanned and quantized transform coefficients, they cannot provide adequate coding performance for lossless video coding where residual data are not quantized transform coefficients, but the differential pixel values between the original and predicted pixel values. Therefore, considering the statistical characteristics of residual data in lossless video coding, we newly design each entropy coding method based on the conventional entropy coders in H.264/AVC. From the experimental result, we have verified that the proposed method provides not only positive bit-saving of 8% but also reduced computational complexity compared to the current H.264/AVC lossless coding mode.

Improved CAVLC for H.264/AVC Lossless Intra-Coding

Context-based adaptive variable length coding (CAVLC) for the H.264/advanced video coding (AVC) standard was originally designed for lossy video coding, and as such does not yield adequate performance for lossless video coding. In this paper, we propose an improved CAVLC for lossless intra-coding by considering the statistical differences in residual data between lossy and lossless coding. From experimental results, we confirm that the proposed method provides approximately 9% bit saving in terms of a compression ratio compared with the current H.264/AVC fidelity range extensions high profile.

An Improved Runbefore Coding for H.264 CAVLC

Context-based adaptive variable length coding (CAVLC) is an entropy coding scheme employed in H.264/AVC for transform coefficient compression. The CAVLC encodes levels of nonzero-valued coefficients. Then indicates their positions with run_before which is number of zeros preceding each nonzero coefficient in scan order. In H.264, the run_before is coded using lookup tables depending on number of zero-valued coefficients that have not been coded. This paper presents an improved run_before coding method which encodes run_before using tables taking both zero-valued and nonzero-valued coefficients into consideration. Simulation results report that the proposed method yields an average of 4.40% bit rate reduction for run_before coding over H.264 baseline profile with intra-only coding structure. It corresponds to 0.52% bit rate saving over total bit rate on average.

High-throughput low-cost VLSI architecture for AVC/H.264 CAVLC decoding

This study develops a low-cost very-large-scale-integration (VLSI) hardware architecture for entropy coding with increased throughput using the statistical properties of context-based adaptive variable-length coding (CAVLC) in AVC/H.264. Statistical analyses show that better symbol length prediction was achieved by breaking the recursive dependency among codewords for the multi-symbol decoder implementation. The proposed CAVLC decoder easily meets the real-time requirements for high definition (HD) (1920 × 1088) applications. The clock speed is only 13 × MHz under the best case scenario.

Efficient Level and Zero Coding Methods for H.264/AVC Lossless Intra Coding

Since H.264/AVC was designed mainly for lossy video coding, the entropy coding methods in H.264/AVC are not appropriate for lossless video coding. Based on statistical differences of residual data in lossy and lossless coding, we develop efficient level and zero coding methods. Therefore, we design an improved context-based adaptive variable length coding (CAVLC) scheme for lossless intra coding by modifying the relative entropy coding parts in H.264/AVC. Experimental results show that the proposed method provides approximately 6.8% bit saving, compared with the H.264/AVC FRExt high profile.

Adaptive Zero-Coefficient Distribution Scan for Inter Block Mode Coding of H.264/AVC

Scanning quantized transform coefficients is an important tool for video coding. For example, the MPEG-4 video coder adopts three different scans to get better coding efficiency. This paper proposes an adaptive zero-coefficient distribution scan in inter block coding. The proposed method attempts to improve H.264/AVC zero coefficient coding by modifying the scan operation. Since the zero-coefficient distribution is changed by the proposed scan method, new VLC tables for syntax elements used in context-adaptive variable length coding (CAVLC) are also provided. The savings in bit-rate range from 2.2% to 5.1% in the high bit-rate cases, depending on different test sequences.

Stream header decoder and context-adaptive variable-length decoder hardware module for H.264/AVC codec

This paper describes the implementation of stream header decoder and context-adaptive variable-length decoder in conformity with the H.264/AVC standard. This module is able to decode headers of syntax elements and to decode blocks of transform coefficients coded using context-adaptive variable-length coder. The designed module is synthesized based for FPGA and ASIC technologies and verified with the reference model JM in version 16. The implementation results shows that the architecture can work at 100 MHz for FPGA Stratix 2 devices and can support HDTV in real time.

Fast coeff_token decoding method and new memory architecture design for an efficient H.264/AVC context-based adaptive variable length coding decoder

A fast coeff_token decoding method based on new memory architecture is proposed to implement an efficient context-based adaptive variable length-coding (CAVLC) decoder. The heavy memory access needed in CAVLC decoding is a significant issue in designing a real system, such as digital multimedia broadcasting players, portable media players, and mobile phones with video, because it results in high power consumption and delay in operations. Recently, a new coeff_token variable-length decoding method has been suggested to achieve memory access reduction. However, it still requires a large portion of the total memory access in CAVLC decoding. In this work, an effective memory architecture is designed through careful examination of codewords in variable-length code tables. In addition, a novel fast decoding method is proposed to further reduce the memory accesses required for reconstructing the coeff_token element. Only one memory access is used for reconstructing each coeff_token element in the proposed method.

High-speed CAVLC encoder for H.264/AVC using parallel zig-zag scanning

A high-speed context-based adaptive variable length coding (CAVLC) encoder for H.264/AVC is presented. Since CAVLC scans residues in a block one by one to obtain the statistics of the data to be encoded, it requires a large number of cycles. In the proposed scheme, two residues are scanned in one cycle to reduce scanning time by half.

Design of a High-Throughput CABAC Encoder

Context-based Adaptive Binary Arithmetic Coding (CABAC) is one of the algorithmic improvements that the H.264/AVC standard provides to enhance the compression ratio of video sequences. Compared with the context-based adaptive variable length coding (CAVLC), CABAC can obtain a better compression ratio at the price of higher computation complexity. In particular, the inherent data dependency and various types of syntax elements in CABAC results in a dramatically increased complexity if two bins obtained from binarized syntax elements are handled at a time. By analyzing the distribution of binarized bins in different video sequences, this work shows how to effectively improve the encoding rate with limited hardware overhead by allowing only a certain type of syntax element to be processed two bins at a time. Together with the proposed context memory management scheme and range renovation method, experimental results reveal that an encoding rate of up to 410 M-bin/s can be obtained with a limited increase in hardware requirement. Compared with related works that do not support multi-symbol encoding, our development can achieve nearly twice their throughput rates with less than 25 % hardware overhead.

Context-based entropy coding in AVS video coding standard

In this paper, two context-based entropy coding schemes for AVS Part-2 video coding standard are presented. One is Context-based 2D Variable Length Coding (C2DVLC) as a low complexity entropy coding scheme for AVS Part-2 Jizhun profile. C2DVLC uses multiple 2D-VLC tables to exploit the statistical features of DCT coefficients for higher coding efficiency. Exponential–Golomb codes are applied in C2DVLC to code the pairs of the run-length of zero coefficients and the non-zero coefficients for lower storage requirement. The other is Context-based Binary Arithmetic Coding (CBAC) as an enhanced entropy coding scheme for AVS Part-2 Jiaqiang profile. CBAC utilizes all previously coded coefficient magnitudes in a DCT block for context modeling. This enables adaptive arithmetic coding to exploit the redundancy of the high-order Markov process in DCT domain with a few contexts. In addition, a context weighting technique is used to further improve CBAC's coding efficiency. Moreover, CBAC is designed to be compatible to C2DVLC in coding elements which simplifies the implementations. The experimental results demonstrate that both C2DVLC and CBAC can achieve comparable or even slightly higher coding performance when compared to Context-Adaptive Variable Length Coding (CAVLC) in H.264/AVC baseline profile and Context-Based Adaptive Binary Arithmetic Coding (CABAC) in H.264/AVC main profile respectively.

다차원 DCT를 이용한 비디오 부호화기 설계

본 논문은 H.264/AVC가 이전의 비디오 코덱에서 사용하는 8<TEX>$\times$</TEX>8 변환이 아닌 4<TEX>$\times$</TEX>4 변환을 도입하면서 인트라 및 인터 예측 성능을 높인 반면 공간적 압축도가 낮은 점을 개선하기 위한 다차원 변환 방법을 제안한다. 다차원 변환 방법은 H.264/AVC가 갖는 시간적 예측의 장점과 공간적 압축도를 동시에 충족시킬 수 있는 방법이다. 먼저 실험을 통해 다차원 DCT가 H.264/AVC의 2차원 정수 변환(Integer Transform)보다 에너지 압축율이 높다는 것을 보였다. 다차원 DCT를 위한 정수형 변환과 양자화기를 설계하였으며, H.264에서 사용하는 컨텍스트 기반 적응 가변 길이 코딩 (CAVLC)을 엔트로피 코더로 사용하여 다차원 부호화기를 설계하였다. 다차원 부호화기에는 다차원 변환에 따른 블록 주사 방식과 파라미터 갱신, 다차원 변환 모드 선택 등의 도구가 적용되었다. 실험 결과, 다차원 부호화기는 낮은 비트율에서 H.264/AVC와 유사한 압축 효율을 보였지만, 엔트로피와 0이 아닌 계수를 계산하여 비교한 통계적 성능 비교에서는 높은 성능을 보였다. 따라서, 다차원 부호화에 대한 추가적인 연구가 진행된다면 기존의 H.264/AVC의 성능을 보완할 수 있는 부호화 알고리즘으로서 발전할 수 있을 것이다. In H.264/AVC, 4<TEX>$\times$</TEX>4 block transform is used for intra and inter prediction instead of 8<TEX>$\times$</TEX>8 block transform. Using small block size coding, H.264/AVC obtains high temporal prediction efficiency, however, it has limitation in utilizing spatial redundancy. Motivated on these points, we propose a multi-dimensional transform which achieves both the accuracy of temporal prediction as well as effective use of spatial redundancy. From preliminary experiments, the proposed multi-dimensional transform achieves higher energy compaction than 2-D DCT used in H.264. We designed an integer-based transform and quantization coder for multi-dimensional coder. Moreover, several additional methods for multi-dimensional coder are proposed, which are cube forming, scan order, mode decision and updating parameters. The Context-based Adaptive Variable-Length Coding (CAVLC) used in H.264 was employed for the entropy coder. Simulation results show that the performance of the multi-dimensional codec appears similar to that of H.264 in lower bit rates although the rate-distortion curves of the multi-dimensional DCT measured by entropy and the number of non-zero coefficients show remarkably higher performance than those of H.264/AVC. This implies that more efficient entropy coder optimized to the statistics of multi-dimensional DCT coefficients and rate-distortion operation are needed to take full advantage of the multi-dimensional DCT. There remains many issues and future works about multi-dimensional coder to improve coding efficiency over H.264/AVC.

An Advanced Total_Zeros Decoding Method Based on New Memory Architecture in H.264/AVC CAVLC

A new total_zeros decoding method based on efficient memory architecture for variable-length code tables (VLCTs) is proposed in this paper. In general, a large amount of memory accesses are required for context-based adaptive variable-length coding (CAVLC) decoding in the H.264/AVC standard. It is an important issue for low-power implementation of mobile video applications because the memory access function results in considerable power consumption. Some distinctive features in the VLCTs of the total_ zeros have been found through a careful analysis of the codewords. Efficient memory architecture and an advanced decoding are developed in order to remove redundant memory access. The simulation results show that the proposed method achieves an approximately 80%-90% memory access saving, compared with conventional decoding.