ITU-R BT.656


ITU-R Recommendation BT.656, sometimes also called ITU656, describes a simple digital video protocol for streaming uncompressed PAL or NTSC Standard Definition TV signals. The protocol builds upon the digital video encoding parameters defined in ITU-R Recommendation BT.601, which provides interlaced video data, streaming each field separately, and uses the YCbCr color space and a 13.5 MHz sampling frequency for pixels.
The standard can be implemented to transmit either 8-bit values or 10-bit values. Both a parallel and a serial transmission format are defined. For the parallel format, a 25-pin D-Sub connector pinout and ECL logic levels are defined. The serial format can be transmitted over 75-ohm coaxial cable with BNC 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 using CMOS logic levels. Typical applications include the interface between a PAL/NTSC decoder chip and a DAC integrated circuit for driving a CRT in a TV set.

Data format

A BT.656 data stream is a sequence of 8-bit or 10-bit words, transmitted at a rate of 27 Mword/s. Horizontal scan lines of video pixel data are delimited in the stream by 4-byte long SAV and EAV 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 in YCbCr format. After an SAV code is sent, the first 8 bits of Cb data are sent then 8 bits of Y , followed by 8 bits of Cr for the next pixel and then 8 bits of Y. To reconstruct full resolution Y, Cb, Cr pixel values, chroma upsampling must be used.