Texas Instruments SPRAA56 - Manual (Page 18)

SPRAA56
18 DSP/BIOS Real-Time Analysis (RTA) and Debugging Applied to a Video Application
Most current encoders use three primary frame types: Intracoded frames, Predicted frames, and
Bidirectional predicted frames. These are referred to as I, P, and B frames. The H.263 encoder
supplied with the example application encodes I and P frames only, but you can configure the
ratio of I to P frames. Often this ratio is used in the quality vs. bitrate tradeoff. The H.263
encoder has hooks to allow for monitoring or selecting the frame type. This example application
only monitors the frame type, and can be configured to display benchmark information on every I
frame, for example.
Hooks for manipulating the Q (quantization) factor are provided with the H.263 algorithm in this
example, but they are not modified after startup in this example. The encoder does not provide
hooks for viewing statistics on the actual Q factor, so although this benchmark may be desirable
in many applications, its measurement is not possible unless the algorithm provider provides API
access to its status.
For many video applications, the encoders and decoders are purchased from third parties. The
level of visibility and control accessible via APIs should be a factor considered when choosing
algorithms, depending on the system's needs for control and benchmarking. Some applications
require minimal control of the encoder (for example, to set the target bitrate), while other
applications require more advanced control.
The percentage of macroblocks that are intracoded is another benchmark that could potentially
be useful. Some encoders can report this benchmark, but the H.263 encoder algorithm used in
this application does not. This number is the percentage of blocks for which no suitable motion
vector could be found to describe the motion of that block from its location in a previous frame.
When a macroblock is intracoded, it is encoded independently of any other frame, as opposed to
being encoded as a difference from a block in a previous frame. The larger the percentage of
macroblocks intracoded, the higher the CPU performance required to encode that frame. A high
percentage often occurs for video with rapid movement or scene changes.
4.9 Methods for Transmitting Measured Performance Data
In the modified example, variables to enable benchmarking are contained in a single data
structure for each stage: benchVideoCapRta, benchVideoProcRta, and benchVideoDisRta. The
benchmarking structure for processing is shown below:
typedef struct BenchVideoProcRta {
BenchTime
timeProcess;
BenchTime timeLastIframe;
BenchTime busUtilization;
BenchTime bitBucketSize;
BenchVal frameProcessCount;
Int frameType;
BenchVal controlledFrameSkip;
BenchVal framesDropped;
BenchVal cpuLoad;
} BenchVideoProcRta;
Similar structures are defined for run-time control of processing, and for benchmarking the
capture and display tasks. An instance of the BenchVideoProcRta structure type is declared
globally in tskProcess.c for use by the benchmarking routines.