Sony RDR-GX7 Technical Notes - Page 23

RDR-GX7 Technical Notes Chapter 4-4 High quality image & sound VBR encoding using Sony's original algorithms DVD uses a form of video compression called MPEG-2 to enable the efficient recording of video signals onto a disc with limited capacity. With MPEG-2 technology, if the same scene continues with very little change in the image, previous data is used to portray the image. During SP mode recording, information is compressed to about 1/40th of the original data. There are two major forms of MPEG-2 encoding, Constant Bit Rate (CBR) and Variable Bit Rate (VBR). CBR uses a fixed bit rate and regardless of the content of the image, encoding is performed at the same bit rate. On the other hand, VBR uses a variable bit rate, employing a high bit rate for complex images, which have a lot of movement, or changes. For images with little movement, or changes, a lower bit rate is automatically selected. Therefore, VBR responds to the changes in the video image and adjusts the bit rate to maintain a fixed level of image quality. When comparing the two systems, VBR is obviously superior in obtaining higher efficiency bit allocation, achieving high image quality without wasting precious bits. The RDR-GX7 uses VBR, operating via special algorithms developed by Sony. Depending upon the degree of complexity or movement in the source image, VBR compression changes the transmission rate accordingly to maintain optimum recording time in the set recording mode, thus maintaining a consistent level of image quality throughout the recording. The VBR algorithms used in the RDR-GX7 work to maintain a continuously even level of image quality, without any variations by performing VBR encoding in short fixed time units. The amount of bits used for previous time frames are considered during operation to keep track of present bit allocation using high level processing technology. Moreover, high picture quality is assured in all DVD+RW, -RW, and -R formats and recording modes. Comparison of constant bit rate (CBR) compression and variable bit rate (VBR) compression Data volume Variable compression bit rate Bit rate The rate of compression is varied depending upon data volume to enable all of the data to be completely compressed without wasting bits, thus increasing efficiency. Time axis Constant compression bit rate Impossible to compress all data Average bit rate Wasted bits When the bit rate is fixed, signal coding is inefficient. In portions where all of the data cannot be completely compressed, playback quality deteriorates. Recordable DVD discs can be recorded not only using constant bit rate compression, but also using variable bit rate compression, which enables high image quality recording of complex video images at the same average rate as used in constant bit rate compression. VBR compression using Sony's original algorithms Clear Fuzzy Bit Sony's original algorithms maintain a consistently high level of image quality. Time Even with the same VBR compression, if too many bits are used at the beginning, the image becomes less clear toward the end of the recording. 23