7-12
Memory Interface
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 rst eld 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 eld 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 ltering,
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 eld repeat are needed. This results in frame store
memory space in the following sizes.
Equation 7.3
Reduced Memory Frame Store Size Calculation
Equation 7.4
With Chroma Line Repeat Display Mode
Equation 7.5
With Chroma Field Repeat Display Mode
The following example shows the space consumed by the display B
frame store in a PAL system. The example uses 44 segments with a
Chroma Line Repeat Display mode.
RMM Luma Frame Store Memory
Num Segments
Pixel Width
×
8
×
=
RMM Chroma Frame Store Memory
Num Segments
Pixel Width
4
×
=
Chroma Frame Store Memory
Pixel Width
Line Height
2
×
=