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SPRAA56
16 DSP/BIOS Real-Time Analysis (RTA) and Debugging Applied to a Video Application
In video applications that handle the full resolution of 720x480, each from contains about 675 KB
of data. Such applications must constantly move video frames from internal working memory
buffers to external frame buffers and back. This often results in several MB of memory transfers
through the external bus for each frame. At 30 frames per second, the memory transfer
bandwidth requirement can be a significant CPU resource requirement. As resolutions increase
to high-definition sizes of 1440x720 or even 1920x1080, and frame rates may be 60 frames per
second, the memory bandwidth requirement can be even more of a limitation than CPU cycles.
The architecture of the video software framework often determines the amount of memory
bandwidth required. Frameworks that repeatedly move video frames from external memory to
internal working buffers and back introduce unnecessary memory bandwidth overhead that may
limit the frame rate. Therefore, it is important to understand the memory bus utilization of the
whole system and its components.
Data structures for measuring the memory bus utilization of the input, processing, and display
tasks are included in the modified example. The actual values logged into the data structures
are estimated, based on the defined size of the frames being moved to internal buffers for
processing.
For the case of YUV4:2:0 to YUV4:2:2 color conversion, the external memory bus utilization and
data flow is shown in Figure 5. A D1 frame (345,600 bytes of luminance data) and 2 chroma
buffers of ¼ that size are copied to internal memory sections for processing, totaling 1.5 times a
frame worth of data. The data copied back out to external memory after conversion has twice as
many chroma samples, for a total of 2 times a D1 frame size in pixels. The estimated total bus
utilization is therefore 1.5N + 2N bytes, where N is the frame size in pixels.
Figure 5. External
↔
Internal Memory Transfers, YUV4:2:0 to 4:2:2 Conversion Function
external memory
external memory
internal L2
memory
Y Y Y Y
Y
Y
Y Y
Y
Y
Y Y
Y Y Y Y
Cb
Cb
Cb Cb
Cr Cr
Cr
Cr
Y Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y Y
Y
Y
Cb
Cb
Cb Cb
Cr Cr
Cr
Cr
Cb Cb
Cb Cb
Cr Cr
Cr
Cr
720*480 = 345,600 B
86,400 B
86,400 B
172,800 B
172,800 B
345,600 B
14,400 B
scratch