L64105 LSI Logic Corporation, L64105 Datasheet - Page 239

• Manufacturer Part Number: L64105
• Description: Mpeg-2 Audio/video Decoder
• Manufacturer: LSI Logic Corporation

7.6.4 Reduced Memory Mode (RMM)
This mode of operation is used in PAL systems and allows a partial frame
store for decoding and displaying B frames. A complete description of
this mode is given in
has some restrictions defined in MPEG. Primarily, it uses the Repeat
Last Field instead of Repeat First Field in 3:2 pulldowns and freeze
modes. This is necessary because the RMM decoding operation makes
use of less than one frame store for B frames. Part of the frame store is
reused during the decode operation.
The frame store is broken into segments. Each segment represents eight
lines of the decoded image and is used as the basis for decoding and
displaying the B frames. As the first field of the B frame is being
displayed, some of the segments are no longer needed for display and
they are reused during the decode of the remainder of the B frame. This
saves memory but only one field of the B frame is stored at any one time.
SDRAM space allocated for the frame store is similar to the anchor frame
stores in the normal mode. However, space must be allocated for the B
frame store based on the number of segments allowed for the B frame
reconstruction. Segments must be allocated in pairs. The minimum
number of segments is half a frame size. For a PAL image with 576 lines,
the picture is divided into 576/8 = 72 segments. The minimum number
of used segments is 72/2 = 36 segments. For most applications, 40 to
44 segments are recommended. More segments allow the decoder to
decode ahead of the display and improve the bandwidth constraints on
the decoder. This is important in a display mode with letterbox filtering,
since the decoder is constrained in decode time. In letterbox display
modes, 44 segments are recommend for PAL systems. Note that the
chroma frame store size may be the full normal size if display modes
using chroma field repeat are needed. This results in frame store
memory space in the following sizes.
Equation 7.3
Equation 7.4
Memory Frame Store Allocation
RMM Chroma Frame Store Memory
RMM Luma Frame Store Memory
Reduced Memory Frame Store Size Calculation
With Chroma Line Repeat Display Mode
Section 9.7, “Reduced Memory Mode.”
=
=
Num Segments Pixel Width 8
Num Segments Pixel Width 4
This mode
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