Sony RDR-GX7 - Manual (Page 23)

Chapter 4-4
High quality
image & sound
RDR-GX7 Technical Notes
23
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.
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.
Variable
compression bit rate
Constant
compression 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.
When the bit rate is fixed, signal coding is inefficient.
In portions where all of the data cannot be completely
compressed, playback quality deteriorates.
Data volume
Time axis
Impossible to compress all data
Bit rate
Wasted bits
Average bit rate
Comparison of constant bit rate (CBR) compression and variable bit rate (VBR) compression
Bit
Time
Clear Fuzzy
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.
Sony’s original algorithms
maintain a consistently
high level of image quality.
VBR compression using Sony's original algorithms