Yes, absolutely, so long as the text is black and white. It will help to use an external sync circuit, such as an LM1881, to provide sync pulses from the external video. And some sort of analog multiplexer will help to switch back and forth between the incoming video and the generated characters. But text overlay is a function that's been implemented with lesser processors than the Propeller, so the Propeller should have no problem.
To provide a color overlay is much harder, since you have to genlock to the incoming chroma signal as well. I'm almost positive the Propeller can be coerced to do it, but the details are pretty evasive so far.
Here's an overlay circuit I found via a Google search:
It contains the LM1881 sync separator and a 74HCT4051 multiplexer. With the Propeller, neither the 74HCT4538A one-shot nor the '4046 PLL would be necessary. You should be able to separate the horizontal sync from the composite sync programmatically using the CSYNC signal directly. This would be even easier if the VSYNC signal were brought into the chip as a reference. (The '1881 is 5V only, so you would need series resistors with these signals.) With its 80MHz clock, the Propeller would not introduce any noticeable jitter in the horizontal dot positions.
Missing from this circuit is a DC restorer for the incoming video. This may be necessary for some signals if they're floating at odd voltages or have a strong 60Hz hum component. I would also add a transistor (2N4401) driver to the output. Driving a 75-ohm load with the multiplexer is a bit of a stretch.
codepro
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Yes, absolutely, so long as the text is black and white. It will help to use an external sync circuit, such as an LM1881, to provide sync pulses from the external video. And some sort of analog multiplexer will help to switch back and forth between the incoming video and the generated characters. But text overlay is a function that's been implemented with lesser processors than the Propeller, so the Propeller should have no problem.
To provide a color overlay is much harder, since you have to genlock to the incoming chroma signal as well. I'm almost positive the Propeller can be coerced to do it, but the details are pretty evasive so far.
-Phil
That said, I'm hoping to be able to do this with the PropCAM also - can't wait for that
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Cheers,
Simon
It contains the LM1881 sync separator and a 74HCT4051 multiplexer. With the Propeller, neither the 74HCT4538A one-shot nor the '4046 PLL would be necessary. You should be able to separate the horizontal sync from the composite sync programmatically using the CSYNC signal directly. This would be even easier if the VSYNC signal were brought into the chip as a reference. (The '1881 is 5V only, so you would need series resistors with these signals.) With its 80MHz clock, the Propeller would not introduce any noticeable jitter in the horizontal dot positions.
Missing from this circuit is a DC restorer for the incoming video. This may be necessary for some signals if they're floating at odd voltages or have a strong 60Hz hum component. I would also add a transistor (2N4401) driver to the output. Driving a 75-ohm load with the multiplexer is a bit of a stretch.
'Hope this helps,
Phil