Texas Instruments SPRAA56 appendix Splitting the Encode and Decode CELLs

SPRAA56

Device		YAfter420 720x576		bitBuf
		414 KB
Driver
				512 KB
Buffer
	Y uv
3 frames	422to		H .263
	420	C bAfter420
			enc
				Shared
		C rAfter420		Scratch
		207 KB	6 KB	92 KB
	scratch1		Instance
	14 KB = 20 lines		m em ory
Key			Internal Memory
DMA Read/W rite (background)			External Memory
CPU Read/W rite			DSP CPU Function

	y
	414 KB
H .263	Cr
dec	CbArrau
	Cb
1.5 KB	207 KB
Instance
m em ory

Y uv 422to 420

scratch2 14 KB

Device

Driver

Buffer

3 frames

Figure 2. Detailed Application Data Flow Showing Memory Buffers

Note: The dotted lines in Figure 2 indicate EDMA moves, and the solid lines indicate CPU reads/writes. The application performs only CPU reads/writes from mapped internal memory, relying on the EDMA to copy working data into internal scratch buffers.

3.1Splitting the Encode and Decode CELLs

In the base example, the H.263 encoder and decoder are wrapped in sequential CELLs in a single channel. This is suitable for an example application, but in actual video systems the input to the decoder would be an encoded bitstream from an external source, and the output from the encoder would be sent to an external source such as a network stream or a hard disk drive. Splitting the encoder and decoder into separate channels better supports external sourcing or transport of the encoded bitstream. Additionally, splitting the encoder and decoder allows them to be benchmarked separately for execution time.

A separate CHAN was created and initialized for the H.263 encoder and the H.263 decoder. At run-time, a separate CHAN_execute command can be executed for each channel.

3.2Adding the Control TSK and MBX Communication

The second change to the base example was the addition of a control TSK to send control commands to the process TSK using the MBX module from DSP/BIOS. A MBX object, mbxProcess, was added in the DSP/BIOS text-based configuration file appThread.tci. That MBX object transmits control commands to the tskVideoProcess TSK to change run-time parameters such as the video frame rate and the encoder bitrate.

8 DSP/BIOS Real-Time Analysis (RTA) and Debugging Applied to a Video Application