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Cluso99
10-29-2011, 11:27 PM
Phil's work on decoding NTSC http://forums.parallax.com/showthread.php?135244-Propeller-Backpack-COLOR-NTSC-Capture/page2 provoked thoughts of work I did some time ago with the help of others including Eric Ball.

I made a minimal footprint grey-scale 1pin TV Text driver. I used the VGA section to create a duty output to get the varying line levels (IIRC derived from Eric and Potatohead's work).

Then I did some tests with TV to see that some colors were possible with 1 and 2 pins. Using the standard P12, P13 & P14 pins, normal resistors, and a standard TV color driver, it was possible to get some displays as follows:

All test required P13 to be present (so it does some form of modulation?). Without P13, no video could be displayed.
P13 only did lose some sync and had a grey border. So it maybe possible with resistor changes.
P12 & P13 gave a black background and colored text. I marked this as a definate possibility
P13 & P14 gave a background and blue became white. Maybe a possibility with resistor changes
So, what I am wondering... Is there a smart way to generate color text NTSC (or PAL) using 1 or 2 pins ?

AntoineDoinel
10-30-2011, 12:02 AM
Phil's work on decoding NTSC http://forums.parallax.com/showthread.php?135244-Propeller-Backpack-COLOR-NTSC-Capture/page2 provoked thoughts of work I did some time ago with the help of others including Eric Ball.

I made a minimal footprint grey-scale 1pin TV Text driver. I used the VGA section to create a duty output to get the varying line levels (IIRC derived from Eric and Potatohead's work).

Then I did some tests with TV to see that some colors were possible with 1 and 2 pins. Using the standard P12, P13 & P14 pins, normal resistors, and a standard TV color driver, it was possible to get some displays as follows:

All test required P13 to be present (so it does some form of modulation?). Without P13, no video could be displayed.
P13 only did lose some sync and had a grey border. So it maybe possible with resistor changes.
P12 & P13 gave a black background and colored text. I marked this as a definate possibility
P13 & P14 gave a background and blue became white. Maybe a possibility with resistor changes
So, what I am wondering... Is there a smart way to generate color text NTSC (or PAL) using 1 or 2 pins ?

AFAIK P13 alone gives the black level, so without it the TV gets sync level and bright signal (eventually superimposed with LSB and chroma), but it's not able to get a reference level.

Probably P12 high pass filtered (to extract chroma) and P14 PWM modulated (with the current size resistor, using Eric's gray driver) would do, but the two cogs should be synchronized.

---
Edit: thinking about it, maybe this could work with a modified standard driver? (masking P13 out and changing the black level to have the LSB set).



1n 1K
P12 ───┳─────────────┐
│ │
│ 270R 270R │
└─────┳──────╋─── out
1n │
270R  │
P14 ──────────────────┘


(P13)
P14─┐│┌─P12
│││
0x0 = Sync
0x1 = Black
1x0 = Gray
1x1 = White