ITU656

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The ITU656 (BT.656) standard published by ITU-R describes a simple digital video protocol for streaming uncompressed PAL or NTSC Standard Definition TV. The protocol supports interlaced video data, streaming each field separately. The ITU656 protocol may be used to send video frames to an output DAC integrated circuit for TV display. Alternately an ITU656 stream may be output from an analog to digital converter video capture integrated circuit (e.g. Philips SAA7113H chip).

[edit] BT.656 data stream format

A BT.656 data stream is sent eight bits at a time in parallel alongside a video pixel clock signal running at 27 MHz. Horizontal scan lines of video pixel data are delimited in the stream by SAV (Start of Active Video) and EAV (End of Active Video) codes. SAV codes also contain status bits indicating line position in a video field or frame. Line position in a full frame can be determined by tracking SAV status bits, allowing receivers to 'synchronize' with an incoming stream.

[edit] Colour space

Individual pixels in a line are coded in YUV format. After a SAV code (4 bytes) is sent, the first 8 bits of Y (luma) data are sent then 8 bits of Cb (chroma U), followed by 8 bits of Y for the next pixel and then 8 bits of Cr (chroma V). To reconstruct complete Y,Cb,Cr pixel values, chroma subsampling must be used.