L64005 MPEG-2 Audio/Video Decoder Technical Manual
6-7
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
6.3
Reduced
Memory Mode
The L64005 offers two memory modes: regular and reduced. In regular
memory mode, the L64005 requires that three frame stores are allocated
in memory. In reduced memory mode, the L64005 requires that only 2.55
frame stores be allocated in memory. Setting the RMM bit in Group 7
vating the reduced memory mode.
Reduced memory mode releases memory space for other functions,
such as the OSD, audio channel buffer, and video channel buffer. For
example, in reduced memory mode, the frame stores require 1,589,760
bytes for a PAL resolution (720 x 576) picture with interlace chroma line
repeat and lter chroma (DMODE[2:0] bits are set to 1112. For more
ory mode, this example PAL picture would require 1,866,240 bytes. By
using the reduced memory mode, the L64005 can save 276,480 bytes
for use with other functions.
Reduced memory mode works by reducing the amount of memory that
the display frame (B frame) requires. The forward and backward
anchored frames cannot be reduced and are preserved as complete
frame stores. The display frame shrinks from a full frame store in regular
memory mode to 0.55 of a full frame store in reduced memory mode.
The user controls the reduced memory mode by selecting the number of
segments used in the display frame. Each segment contains eight lines
of luma, and four lines of chroma. Selecting chroma line repeat
(DMODE[2:0] bits are set to either 1102 or 1112) reduces both luma and
chroma.
Equation 6.1 shows how to calculate the number of segments
required for a frame.
Equation 6.1
For example, a full PAL eld has 288 lines/eld. Regular memory mode
would use the equivalent of 72 segments per frame. Using
Equation 6.1,reduced memory mode requires only 40 segments per frame. Write this
value into Reduced Memory Mode Segments (SEG) Group 7, Register
27, bits [7:2] to dene the number of segments for reduced memory
mode. The L64005 uses this value only in reduced memory mode. In
normal memory mode, the L64005 ignores this value.
lines
() field
()
8
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