16
SERIAL DIGITAL BASICS
625 and 525 digital component video is produced by applying a 4:2:2 sampling structure to
the analog signal. This process is defined by a sub-set of international standards ITU-R
BT.601 and BT.656. (these were formerly known as CCIR-601 and CCIR-656. The label
'CCIR601' is commonly applied to digital video coded in this manner.)
The luminance (Y) component is sampled at 13.5 MHz, and the colour difference
components (U and V) are both sampled at 6.75 MHz. With 10 bit quantisation, this results in
a data stream of 10 bit words at a clock frequency of 27 MHz. If the signal source uses 8 bit
quantisation, 10 bit data is used with the two least significant bits of each sample code set to
binary zero. This is to maintain the same data rate.
The quantizing levels employed in the analog to digital conversion are set to give 66.4mV
headroom above peak white and 51.1mV below black. Coded U and V signals have 50mV
above and below their normal maximum and minimum excursions.
The synchronisation pulses are discarded in the coding process, and are replaced by Timing
Reference Signals (TRS), which are inserted into the data stream to serve the same purpose.
Two TRS's are used to synchronise the data stream, EAV (End of Active Video) and SAV
(Start of Active Video). These are placed at the beginning and end of the horizontal video
blanking period. See fig 3.
Each TRS consists of 4 words:
1) 3ff hex i.e. all '1's
2) 000 hex i.e. all '0's
3) 000 hex i.e. all '0's
4) XYZ, which determines the type of TRS pulse:
XYZ:
Bit 9: always '1'
Bit 8: 0 = frame 1
1 = frame 2
Bit 7: 0 = normal
1 = field blanking
Bit 6: 0 = SAV
1 = EAV
Bit 5: Bits used for Hamming correction.
Bit 4: Bits used for Hamming correction.
Bit 3: Bits used for Hamming correction.
Bit 2: Bits used for Hamming correction.
Bit 1: Always '0'
Bit 0: Always '0'