Selective erase addressing
For high definition television with 1000 or more lines, even dual scanning is insufficient to provide the necessary sustain period for good quality picture. Given a write pulse of 2 µs, then for an XSGA panel (1280 X 1024),
This gives a total sustain period = 20 – 16.38 = 3.62 ms with the equiva- lent value for NTSC of 17.6 – 16.38 = 0.32 ms. The percentage sustain period = 3.62/20 = 3.68% for PAL and 0.32/17.6 = 1.9% for NTSC. Neither value is any where adequate for good quality picture reproduction.
One way of increasing the sustain time is to reduce the duration of the address phase. For HD panels, selective write addressing requires the address cycle time per line to be reduced to 1.0 fls or less. That means addressing frequencies approaching 1 MHz. Such high frequencies would increase the power requirements and with it the cost of the address driv- ers. Furthermore, regardless of how fast the line addressing cycle is, the address pulse itself cannot be very narrow because time is needed to form the wall charge if a cell is to be selected. Another way to reduce the pulse width is to raise the addressing voltage, but this often results in higher power consumption. For this reason, a different technique known as ‘selective erase’ is used.
The ‘selective erase’ (also known as ‘erase wall-charge’) addressing reverses the process of creating the wall charge. In the ‘selective write’ the cells are cleared in the initialisation set phase and a wall is formed during the address phase in those cells selected for emission. With ‘selective erase’, a wall charge is formed in all the cells during the initialisation phase. In the following address phase, those cells that are not required to ignite are ‘de-selected’ by erasing the wall charge, hence the name ‘selective erase’. The three phases are thus:
Initialisation and wall charge formation, addressing (erasing wall charge), sustaining.
The advantage of this is that it takes a relatively short time to discharge a cell and remove the wall charge compared with charging a cell to form the wall charge thus making it possible to use narrower line select pulses. With a typical address (or select) pulse width of 1.2 fls, a SXGA (1280 X 1024) panel with 256 greyscale levels (8 SFs), and the address period is
1.2 flsX8 SFX1024 = 9.83 ms
The resulting total sustain period per TV frame = 20 – 9.83 = 10.17 ms for PAL and 16.7 – 9.83 = 6.84 ms for NTSC. This gives a percentage sus- tain period of 10.17/20 = 50.8% for PAL and 6.84/17.6 = 40.9% for NTSC.