 D2 digital formats, conversion from analogue composite, and be- Sync. tween various combinations of digital interface has been developed. For D2, sampling frequencies, phased to the colour sub carrier reference burst and at four times the subcarrier frequency, are Figure 57.15 RGB to digital YUV interface approximately 17.73MHz for 625/PAL and 14.32MHz for 525/NTSC. Digital composite recording is not practical for SECAM tions can be differential, and relatively small defects can signifi- working, and in SECAM environments the signal must first be cantly affect the colour balance of a picture. Differential delays decoded back into Y, R' DB and then re-encoded into PAL or else within or without analogue component equipment can produce processed in component form. significantly more visible impairments than with a single composite In converting an RGB signal to digital YUV, (see Figure 57.15) signal. The visibility of some effects depends on the actual lumin- the signal is matrixed, filtered, clamped and digitized. ance and colour values of the picture. It has been noted that this may introduce a number of possible CCIR Recommendation 656 (1986): 'Interfaces for digital com- signal impairments including: ponent video signals in 525 line and 625 line television systems', Part I covers common signal format of the interfaces and Part II 1. Lift errors (clamp, low frequency or digitizing problems). covers bit parallel interfaces, including a table of contact assign- 2. Delay inequalities (filter or path length differences). ments, as in Table 57.5. 3. Phase response differences (filter differences). Any spare pairs connected to contacts 11,24 or 12,25 are reserved 4. Non linearities (digitizing problems). for bits of lower significance than those carried on contacts 10,23. 5. Quantization noise (digitizing effect). Parallel interfaces comprise 8 X 27Mbit/s pairs. Part III covers bit serial interface, including the recommendation Such impairments can affect component more than composite that the 8 bit data words are enc