Re: Video overlay generator

On a sunny day (Wed, 15 Aug 2007 18:16:27 -0700) it happened MooseFET
<kensmith@xxxxxxxxx> wrote in
<1187226987.338227.323640@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>:

On Aug 15, 8:46 am, pantel...@xxxxxxxxx wrote:
On 15 aug, 14:34, Frank Raffaeli <SNIPrf_man_frT...@xxxxxxxxx> wrote:

[....Video overlay...]

Yes, this is very possible, I have done it with some simple analog
switches (HC4053).

You don't want to use a HC4053. They switch very quickly. This makes
frequencies up into the chroma carrier frequency. You want a softer
switching so that you stop short of that.

Well, this is not the case in _my_ case.
I used it to add text (and colored text).
When you use a slow chip, like the old CD4053, there is a _MUCH_ to long
switching delay, causing visible artefacs eben in BW video.

I _had_ to use the HC4053.
Used the circuit to add subtitles to VHS recordings before the digital
video times.

Some company in Amsterdam asked me if I could add colored subs that way,
so I made some experimental setup, it worked.
But they did not order it...

As to frequencies in the chroma range, sure, but there is a bit more to adding
for example text, like lead delay and tail delay (character outline),
I created those delays with some counters (say shift registers) and fast clock.
Would have to look it up for the details.
But if chroma bothered you, then for example you could perhaps decide to swith on zero
crossing of the inserted chroma.
You _MUST_ switch within a fraction of chroma period, say well below 10 nS.

Personally I am glad all that analog is gone, and all is digital now.
The quality is much better, and procesing is much easier, just programing.

If the guy just wanted _simple_ BW graphics and text he should use something like
a SAA5246A, its does 25 lines of 40 characters in genlock on a composite signal
PAL or NTSC.
If you wanted to add outline and color you would need more stuff,
the SAA has an RGB output you could use though.

In most cases SAA5246A will be sufficient, if you can still get one.







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