ITU-R BT.656
TheITU recommendationBT.656 specifies the structure of the data stream for digital video according to BT.601. BT.601 specifies the sampling rates, color spaces and color depths, but not the transmission of the luminance and chrominance values, which can be parallel or multiplexed. In addition, the blanking signals and synchronous pulses can be different, since they are not digitized.
In BT.601, the visible video area is instead marked by sync signals. This starts with Start of Active Video( SAV) and is located at the 132nd sample after the analog sync pulse in the PAL television standard. The end of the visible video area of a line is End of Active Video( EAV) and is 12 samples before the following horizontal sync pulse. The digitized video signal is transmitted in the area between Start of Active Video and End of Active Video. The SAV and EAV characters each consist of four data words. In the area between End of Active Video and Start of Active Video audio, test or other signals can be transmitted.
BT.656 defines the transmission frame for the data stream and specifies the clock signals. In the ITU recommendation, only digitization with 10 bits is specified; with 8 bits, the last two bits are set to zero.
BT.656 specifies a parallel interface with a 25- pin sub-D connector for a distance of 50 m, and the Serial Digital Interface( SDI), a serial interface for the video data transmitted in time-division multiplex. The data stream transmitted via the serial interface has a frequency of 270 MHz. It is scrambled and the data is encoded using Non Return to Zero Inverted( NRZ-I). The connection technology is BNC and the impedance is 75 Ohm.