High-Quality AVCHD Recording, Plus the Professional PH Mode Boosts the Quality of HD Images
AVCHD Format for High- Quality, Efficient HD Recording
Panasonic AVCCAM camcorders use the AVCHD format for tapeless recording with high image quality and low bit rates. This format complies with the latest H.264 motion image compression standard, and employs the High Profile standard to improve compression efficiency. Featuring twice the compression efficiency of HDV (MPEG-2), the AG-HMC150 series achieves extended HD recording. The following four new technologies make this possible.
Comparison of HD Recording Formats
| HDV | AVCHD |
Pixel (H x V) | 1440 x 1080 | 1920 x 1080 |
Compression Method | MPEG-2 | MPEG-4 AVC/H.264 |
Sample comparison: When a flash causes large contrast differences and reduces depth perception, HDV shows considerable block noise, while AVCHD in the PH mode minimize break-up.
MPEG-4 AVC/H.264 Technologies
Intra-Frame Prediction
This process generates predictive pixels based on the adjacent pixels within each frame. It then selects the optimal predictive mode. The generated predictive image is subtracted from the original input image, and the residual data is compressed and recorded at a low bit rate. The entire process is conducted within the frame, so prediction accuracy remains high even with fast-motion images.
Variable Block Size Motion Compensation
In contrast with MPEG-2, in which inter-frame compression based on the correlation between adjacent frames uses fixed blocks of 16 x 16 pixels, AVCHD divides the blocks into multi-sizes as small as 4 x 4 pixels. In this method, it is able to use large blocks to process images that show only slight changes on the screen, and smaller blocks to process images that have considerable change. This raises the
MPEG-2 (fixed block size)
Blocks of the same size are used to process parts both with and without motion.
16×16
MPEG-4 AVC/H.264 (variable block size)
Block size is precisely varied to match the size of the moving part contained in each block.
16×16 | 16×8 | 8×16 | 8×8 |
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8×8 | 8×4 | 4×8 | 4×4 |
accuracy of motion compensation to boost the quality of fast-motion images while increasing compression efficiency.
Loop Filter Prevents the Propagation of Compression Distortion Because MPEG-2 uses a decoding image that contains compression- induced block distortion as a reference image for motion compensation, it exhibits residual distortion — even within the same frame — when a large amount of block distortion is generated. MPEG-4 AVC/H.264 detects block distortion in the decoding image and removes it with a context-adaptive filter that functions according to the degree of distortion. This prevents the propagation of block distortion by keeping the reference image clean at all times.
New CABAC Entropy Encoding
The AVCHD format uses CABAC (Context Adaptive Binary Arithmetic Coding) for its variable-length encoding. Compared with the variable- length encoding of MPEG-2, in which the compression efficiency is greatly affected by subject type, CABAC provides lossless compression with constantly high efficiency and no distortion for virtually all subject types. Because MPEG-2 compresses and converts data according to the standard's fixed conversion rules, the compression efficiency may drop for image types other than those that were considered when the standard was established. In place of fixed conversion rules, CABAC provides the best possible conversion method by constantly optimizing and automatically tracking the image that is being processed, in parallel with the compression process.
