Texas Instruments TMS320DM648 manual Video Capture Mode Selection, BT.656 Video Capture Mode

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Video Capture Mode Selection

3.1Video Capture Mode Selection

The video capture module operates in one of five modes as listed in Table 3-1. The transport channel interface (TCI) selection is made using the TCI bit in the video port control register (VPCTL). The CMODE bits are in the video capture channel x control register (VCxCTL). The Y/C and 16-bit raw capture modes may only be selected for channel A and only if the DCDIS bit in VPCTL is cleared to 0.

When operating as a raw video capture channel, no data selection or data interpretation is performed. The 16-bit raw capture mode is designed to accept data from A/D converters with resolution higher than eight bits (used, for example, in medical imaging).

Table 3-1. Video Capture Mode Selection

TCI Bit	CMODE Bits	Mode	Description
0	000	8-Bit ITU-R BT.656 Capture	Digital video input is in YCbCr 4:2:2 with 8-bit resolution
			multiplexed in ITU-R BT.656 format.
0	010	8-Bit Raw Capture	Raw 8-bit data capture at sampling rates up to 80 MHZ.
0	100	8-Bit Y/C Capture	Digital video input is in YCbCr 4:2:2 with 8-bit resolution on
			parallel Y and Cb/Cr multiplexed channels.
0	110	16-Bit Raw Capture	Raw 16-bit data capture at sampling rates up to 80 MHZ.
1	010	TCI Capture	8-bit parallel TCI capture at rates up to 30 MHZ.

3.2BT.656 Video Capture Mode

The BT.656 capture mode captures 8-bit 4:2:2 luma and chroma data multiplexed into a single data stream. Video data is conveyed in the order Cb, Y, Cr, Y, Cb, Y, Cr, etc. where the sequence Cb, Y, Cr refers to co-sited luma and chroma samples and the following Y value corresponds to the next luminance sample. The data stream is de-multiplexed and each component is written in packed form into separate FIFOs for transfer into Y, Cb, and Cr buffers in DSP memory. (This is commonly called planar format).

In BT.656 video capture mode, data bytes in which the 8 bits are all set to 1 (FFh) or are all set to 0 (00h) are reserved for data identification purposes and consequently, only 254 of the possible 256 8-bit words may be used to express signal value.

3.2.1 BT.656 Capture Channels

In dual channel operation, the video port can support capture of two BT.656 data streams or one BT.656 data stream and one raw data stream. In the latter case, the BT.656 stream may occur on either Channel A or Channel B. In either case, the BT.656 stream(s) must have embedded timing reference codes and the appropriate VCTL input must be used as a CAPEN signal.

If the port is configured for single channel operation, capture will take place on Channel A only. The unused half of the VDATA bus may be used for GPIO or for another peripheral function. For single channel operation, non-standard BT.656 data streams without embedded timing reference codes are supported through the use of the timing control (VCTL) input signals.

3.2.2 BT.656 Timing Reference Codes

For standard digital video, there are two reference signals, one at the beginning of each video data block (start of active video, SAV), and one at the end of each video block (end of active video, EAV). (Technically each line begins with the EAV code and ends just before the subsequent EAV code.) Each timing reference signal consists of a four sample sequence in the following format: FFh, 00h, 00h, XYh. (The FFh and 00h values are reserved for use in these timing reference signals.) The first three bytes are a fixed preamble. The fourth byte contains information defining field identification, the state of field blanking and state of line blanking. The assignment of these bits within the timing reference signal is listed in Table 3-2.

46 Video Capture Port SPRUEM1 –May 2007

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