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View Full Version : Hack the 2.5" LCD A/V Display ??



Rayman
09-20-2007, 08:42 PM
I just got around to trying out the 2.5" LCD TV from Parallax. I'm not terribly impressed by the image quality... I can read the text good in 16x12 text mode as in the "Graphics_Palette" example. But, text is quite blurry in the 40x13 text mode of the "TV_Text_Demo"...

I saw an earlier post that described hacking a larger LCD TV to make it into sort of an LCD RGB monitor that then used a modified form of a VGA driver to do the display...

Anyone think this could be done with this little display? Think this would improve the clarity of small text?

I guess the main benefit would be that it would be a progressive rather than interlaced update. Maybe this will reduce the flicker I see in this monitor also...

Rayman
09-21-2007, 01:43 AM
Well, I opened it up to see what I'd be facing and it looks a bit more complex that I though. Plus, they scratched off the labels on the main ICs...

But, I did play with the four pots on the board and was able to improve the display a hair. At least its centered now (The pot labelled H.P. is horizontal position.)

I've attached a photo of the insides. There's another big IC and other stuff on the back side of the circuit board too.

deSilva
09-21-2007, 01:52 AM
You should NOT operate it in interlaced mode (which the normal setting of TEXT.SPIN does) or PAL with 5 or 10 MHz crystals.

Rayman
09-21-2007, 02:00 AM
What? This is a NTSC/PAL monitor... I would think it only works in interlaced mode.

deSilva
09-21-2007, 02:21 AM
It has a 320x240 matrix; this generally means interlaced data will lead to a noticeable deterioration of quality! 5 or 10 MHz crystals do not match the required colour clock for PAL very well. Most low cost monitors are very touchy with a deviation from 4.433.... Mhz

I changed the TEXT.SPIN to grayscale non-interlaced. This gives a very sharp image on those kind of monitors!

Rayman
09-21-2007, 02:51 AM
I just tried changing the mode value in "TV_Text.spin" from %10010 to %10000, which makes it non-interlaced. But, the screen goes black... Did you post that code somewhere?

Andreas Jakob
09-23-2007, 07:26 PM
Dont forget that if you disable Interlace, you must half the number of rows.
The driver wont work else.

deSilva
09-23-2007, 10:32 PM
It is simple, but not easy http://forums.parallax.com/images/smilies/smile.gif The ROM font can only be output from TV_TEXT in interlaced mode, as the "32-bit mode" (Bit 5) seems to be coupled...

I hezitatingly post a modified _TEXT which is in an undefined state (not worked on it for weeks)...

Rayman
09-24-2007, 02:45 AM
Thanks Andreas, deSilva.

deSilva: Thanks for the example!

I just got your file working with this change:
_clkmode = xtal1 + pll16x'pll8x
_xinfreq = 5_000_000 '10_000_000
videoPin = 12'24

This "non-interlaced" mode is much better for the tiny (and inexpensive) Parallax TV monitor. The display doesn't flicker and the text is much more readable.
I don't know why you go with B&W though... It looks better in color.

Is this a true "non-interlaced" mode? Or, is it just sending the same field twice? I'm just thinking that my big TVs accept 480p signals. But, I don't think this is really that, is it?

potatohead
09-24-2007, 05:26 AM
Nope.

Instead of a full frame taking 1/30 sec, it takes 1/60 sec. Vertical resolution is divided by two, and the half line that normally offsets the vertical scanlines is not sent. Most older computers and video game consoles sent video this way to prevent flicker.

I'm sure overall resolution capability / RAM has as much to do with that as flicker does.

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potatohead
09-24-2007, 05:27 AM
Nope.

Instead of a full frame taking 1/30 sec, it takes 1/60 sec. Vertical resolution is divided by two, and the half line that normally offsets the vertical scanlines is not sent. Most older computers and video game consoles sent video this way to prevent flicker.

I'm sure overall resolution capability / RAM has as much to do with that as flicker does.

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deSilva
09-24-2007, 06:47 AM
Interlacing is a most ingenious way to display 480 rather than 240 lines within 1/30 second without the observer noticing a top-to-down roller-blind effect (because one "field" is displayed every 1/60 second, being unnoticeable for most people)

VGA does not have this need, as it displays a full frame within 1/60 second, (and thus needs twice the bandwidth).

For the Propeller Video Logic it makes not much difference whether sending twice the same field or even/odd lines.

However when the real number of lines on a monitor is 240 rather than 480+ different even/odd lines it will result in worse quality due to the necessary (and incontrolled) averaging.

For a "tiled" driver there is not so much difference wrt memory usage (of course the number of tiles is doubled)