Again about internal key

Hi,
I'm trying to use some new generation Decklink Cards, such as Quad 2 and SDI 4K and the internal keyer via sdk.
I'm working on Windows 7, Visual Studio .Net (vb2013).
I'm able to obtain a reference to the IDeckLinkOutput interface as well as the IDeckLinkKeyer one.
Then I enable audio and video output, and all the thinks I need to use the keyer.
All this work is done to overlay an image to the incoming video signal.
The overlaying image is shown, but it has a very poor quality, it seems to be "squared". The live key software has the same bad result.
In particular, the overlayed image is shown as a still interlaced image.
Actually, I call the IDeckLinkOutput.DisplayVideoFrameSync method once a time, and then I turn on or off the overlay by calling the IDeckLinkKeyer.SetLevel with the appropriate values.
Is this right?
Again, the live key software has the same bad result.
I'm working on a PAL system, when I work on an HD 1080i50 signal, the image is better.
Any idea?

Solved.
The solution here:
viewtopic.php?f=12&t=47556