Electronics > Projects, Designs, and Technical Stuff
Progressive to Interlaced Converter
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paul_g_787:

--- Quote from: Benta on October 16, 2019, 07:25:30 pm ---The correct way of going from progressive to interlaced scan is to shift the Vsync half a line back and forth. This has to done in sync with the Hsync.

Unfortunately, VGA outputs do not provide serration or equalization pulses, so you'll have to create a synchronous clock at double the Hsync rate to precisely move the Vsync pulse.

A PLL does the trick, together with a bit of logic: FFs + gating.

The first and last scan line will look crappy, but they're normally outside the scan area.

BTW: it's an open question whether your TV will accept interlaced video without serration/equalization pulses.

--- End quote ---

The input is RGBS input with 4 lines. So the first step is to make a circuit to get a clean H sync and a clean V sync. What would be the best way to achieve this?

By PLL do you mean a Phase Locked Loop? I only touched briefly on these about 10 years ago so I do not know much about them.
From memory I think we used a CD4046. Would this be suitable for this purpose?
I think I remember someone using the NE555 as a PLL too.
paul_g_787:
Just found this.

https://circuitswiring.com/vga-to-tv-scart/

Can anyone shed some light on this schematic?
helius:
To get readable text or computer graphics out of this type of converter, it must perform spatial and temporal convolution.
Of course, it is possible to discard every other line, but that simply throws away information and causes visual artefacts.
ejeffrey:

--- Quote from: paul_g_787 on October 17, 2019, 03:54:33 pm ---
--- Quote from: ejeffrey on October 16, 2019, 06:29:16 pm ---It sounds like he wants to go progressive to interlaced at the same refresh rate so it is actually much easier than that.  It is just dropping every other scan line and playing back the remaining ones at half the speed.  In that case you only need a single scanline of buffer.

--- End quote ---

Yes exactly. Ideally I would not have to buffer the signal at all if possible

Instead of storing the scanlines then playing them back would it be possible to just 'turn off' the even scanlines on the first frame then on the second frame 'turn off' the odd scanlines? If that makes sense?

--- End quote ---

I don't know what typical CRT TVs will accept but you need storage to generate a correct signal.  The duration of a single scanline for 578i vs 578p at 50 Hz is different by a factor of two.
paul_g_787:

--- Quote from: ejeffrey on October 17, 2019, 04:47:36 pm ---
--- Quote from: paul_g_787 on October 17, 2019, 03:54:33 pm ---
--- Quote from: ejeffrey on October 16, 2019, 06:29:16 pm ---It sounds like he wants to go progressive to interlaced at the same refresh rate so it is actually much easier than that.  It is just dropping every other scan line and playing back the remaining ones at half the speed.  In that case you only need a single scanline of buffer.

--- End quote ---

Yes exactly. Ideally I would not have to buffer the signal at all if possible

Instead of storing the scanlines then playing them back would it be possible to just 'turn off' the even scanlines on the first frame then on the second frame 'turn off' the odd scanlines? If that makes sense?

--- End quote ---

I don't know what typical CRT TVs will accept but you need storage to generate a correct signal.  The duration of a single scanline for 578i vs 578p at 50 Hz is different by a factor of two.

--- End quote ---

So essentially stretching the scanline out so it take twice the time to draw it on the screen?
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