A friend of mine has a Philips 47PFL7422 LCD TV and it includes some kind of dynamic backlight feature which automatically dims the backlight when the image onscreen is dark, probably to save power.
Unfortunately, it seems too aggressive and can do it during dark scenes in films which creates a horribly jarring effect when the backlight level changes suddenly.The problem is also described here: http://www.geekzone.co.nz/forums.asp?topicid=22737
In any case, for this model there is no service menu adjustment to turn the feature off, nor were Philips ever forthcoming with a firmware update, apparently. Which is stupid, since the brightness control in the main user menu doesn't do a thing to adjust the backlights either - it simply adjusts what seems to be the picture gamma. That's two things they should have fixed. Anyway....
The TV has two inverter boards, a CMO I420H1-20B-MASTER and a CMO I420H1-20B-SLAVE. Since these aren't made by Philips, they don't appear in the schematic, of course. They're based on the OZ9928SN.
From what I measured, however, the Master inverter board receives at its "E-PWM" pin, a PWM signal from the mainboard which is about 3.3v p-p and has a duty cycle of I guess about 99% (see first scope shot, DSC02108.JPG). The duty cycle of this signal does NOT change when lowering the "Brightness" control in the main menu, although you can see the image get duller as it appears the gamma or such is changed.
At ~3 divs and 0.5ms per div, this seems to be about 666Hz.
The inverter also receives 3.3v from the T-CON board, to a pin labelled "DIM-IN", and this appears on the pin "DIM-OUT" as well, which seems to go back to the T-CON board. This signal does not appear to be a PWM signal - if it is, it's at 100% duty cycle or so close to it that I cannot see where it goes to 0v at any point. This signal also does not change when the brightness value in the menu is changed.
When the backlight dimming feature kicks in, both the "E-PWM" and "DIM-IN" lines change to a square wave with much lower duty cycle, around 30%, and the frequency seems to drop to 166Hz (3 divs at 2ms per div). The "DIM-OUT" line also changes, with a slower falling edge.
See:
DSC02108 for E-PWM normal brightness
DSC02110 E-PWM dimmed
DSC02111 DIM-IN dimmed
DSC02112 DIM-OUT dimmed
My original idea was to disconnect the "E-PWM" line from the mainboard and attach my own PWM generator using a microcontroller to give a constant PWM signal that would not be affected by the TV software. However, this was before I found that the T-CON is also involved. Now I am not so sure what to do.
I could of course disconnect both E-PWM and DIM-IN, and feed them the same signal (probably from separate pins on the microcontroller or using an optoisolator so they are not tied together directly), but I wonder if the T-CON would not be impressed that the DIM-OUT signal would no longer match up with what DIM-IN was.
It doesn't help that there seem to be no good datasheets for the OZ9928, and the inverter schematic seems nonexistent (as usual). I did find http://wenku.baidu.com/view/d58277c4...6bcebbc4c.html and a few similar websites which claim to have the schematic but you have to pay to download it and I don't speak Chinese!
Does anyone have any ideas or experience with this kind of thing?
Unfortunately, it seems too aggressive and can do it during dark scenes in films which creates a horribly jarring effect when the backlight level changes suddenly.The problem is also described here: http://www.geekzone.co.nz/forums.asp?topicid=22737
In any case, for this model there is no service menu adjustment to turn the feature off, nor were Philips ever forthcoming with a firmware update, apparently. Which is stupid, since the brightness control in the main user menu doesn't do a thing to adjust the backlights either - it simply adjusts what seems to be the picture gamma. That's two things they should have fixed. Anyway....
The TV has two inverter boards, a CMO I420H1-20B-MASTER and a CMO I420H1-20B-SLAVE. Since these aren't made by Philips, they don't appear in the schematic, of course. They're based on the OZ9928SN.
From what I measured, however, the Master inverter board receives at its "E-PWM" pin, a PWM signal from the mainboard which is about 3.3v p-p and has a duty cycle of I guess about 99% (see first scope shot, DSC02108.JPG). The duty cycle of this signal does NOT change when lowering the "Brightness" control in the main menu, although you can see the image get duller as it appears the gamma or such is changed.
At ~3 divs and 0.5ms per div, this seems to be about 666Hz.
The inverter also receives 3.3v from the T-CON board, to a pin labelled "DIM-IN", and this appears on the pin "DIM-OUT" as well, which seems to go back to the T-CON board. This signal does not appear to be a PWM signal - if it is, it's at 100% duty cycle or so close to it that I cannot see where it goes to 0v at any point. This signal also does not change when the brightness value in the menu is changed.
When the backlight dimming feature kicks in, both the "E-PWM" and "DIM-IN" lines change to a square wave with much lower duty cycle, around 30%, and the frequency seems to drop to 166Hz (3 divs at 2ms per div). The "DIM-OUT" line also changes, with a slower falling edge.
See:
DSC02108 for E-PWM normal brightness
DSC02110 E-PWM dimmed
DSC02111 DIM-IN dimmed
DSC02112 DIM-OUT dimmed
My original idea was to disconnect the "E-PWM" line from the mainboard and attach my own PWM generator using a microcontroller to give a constant PWM signal that would not be affected by the TV software. However, this was before I found that the T-CON is also involved. Now I am not so sure what to do.
I could of course disconnect both E-PWM and DIM-IN, and feed them the same signal (probably from separate pins on the microcontroller or using an optoisolator so they are not tied together directly), but I wonder if the T-CON would not be impressed that the DIM-OUT signal would no longer match up with what DIM-IN was.
It doesn't help that there seem to be no good datasheets for the OZ9928, and the inverter schematic seems nonexistent (as usual). I did find http://wenku.baidu.com/view/d58277c4...6bcebbc4c.html and a few similar websites which claim to have the schematic but you have to pay to download it and I don't speak Chinese!
Does anyone have any ideas or experience with this kind of thing?
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