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ITU-R Recommendation BT.656, sometimes also calledITU656, is a simpledigital video protocol for streaming uncompressedPAL orNTSCstandard-definition television (625 or525 lines) signals. The protocol builds upon the4:2:2 digital video encoding parameters defined in ITU-R RecommendationBT.601, which providesinterlaced video data, streaming each field separately, and uses theYCbCr color space and a 13.5 MHz sampling frequency for pixels.[1]
The standard can be implemented to transmit either 8-bit values (the standard in consumer electronics) or 10-bit values (sometimes used in studio environments). Both a parallel and a serial transmission format are defined. For the parallel format, a 25-pinD-Sub connector pinout andECL logic levels are defined. The serial format can betransmitted over 75-ohm coaxial cable withBNC connectors, but there is also a fibre-optical version defined.
The parallel version of the ITU-R BT.656 protocol is also used in many TV sets between chips usingCMOS logic levels. Typical applications include the interface between a PAL/NTSC decoder chip and aDACintegrated circuit for driving aCRT in a TV set.
A BT.656 data stream is a sequence of 8-bit or 10-bitwords, transmitted at a rate of 27 Mword/s. Horizontal scan lines of videopixel data are delimited in the stream by 4-byte long SAV (Start of Active Video) and EAV (End of Active Video) code sequences. 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.
Individual pixels in a line are coded inYCbCr format. After an SAV code (4 bytes) is sent, the first 8 bits of Cb (chroma U) data are sent then 8 bits of Y (luma), followed by 8 bits of Cr (chroma V) for the next pixel and then 8 bits of Y. To reconstruct full resolution Y, Cb, Cr pixel values,chroma upsampling must be used.
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