04-07-2013, 04:09 PM
COMPOSITE VIDEO SIGNAL
COMPOSITE VIDEO.ppt (Size: 546.5 KB / Downloads: 19)
Consist of :
Camera signal - corresponding to the desired picture information
Blanking pulses – to make the retrace invisible
Synchronizing pulses – to synchronize the transmitter and receiver scanning
-horizontal sync pulse
-vertical sync pulse
-their amplitudes are kept same
-but their duration are different
-needed consecutively and not simultaneously with the picture signal – so sent on a time division basis
DC component of the video signal
Average value or dc component corresponding to the average brightness of the scene
Average brightness can change only from frame to frame and not from line to line
Low pedestal height – scene darker
Larger pedestal height – higher average brightness
Vertical Sync details
Added after each fields
Complex in nature
Vertical sync period = 2.5 to 3 times the horizontal line period
In 625 line system: 2.5 × 64 = 160µS
Commence at the end of first half of 313th line (end of first field) and terminates at the end of 315th line
Similarly after an exact interval of 20mS (one field period), the next sync pulse occupies the line numbers 1st, 2nd and first half of 3rd .
Shortcomings and its solution
Each horizontal pulse cause a slight rise in voltage across the capacitor, but this is reduced to zero by the time the next pulse arrives (charging period=4.7µS and discharging period = 59.3µS)
But during broad serrated region, capacitor has more time to charge and only 4.7µS to discharge
Situation is different for the beginning of the 2nd field-here the last horizontal pulse corresponding to the beginning of the 313th line is separated from the first vertical pulse by only half-a-line.
Therefore the voltage developed a/c the vertical filter will not have enough time to reach zero before the arrival of the 1st vertical pulse
Hence the voltage developed a/c the o/p filter is some what higher at each instant as compared to the voltage developed at the beginning of the 1st field (shown as dotted chain)
i.e. Oscillator get triggered a fraction of a second early as compared to the first field - upset the desired interlacing sequence