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Noise Problem with ADV 7391 & 7180

Question asked by mstephan on Jan 19, 2012
Latest reply on Jan 24, 2012 by GuenterL

Hello,

 

we currently develop a videoboard with the 7180 and the 7391.

The 7180 is used to convert a analog video signal into a digital signal.

Then the digital Videostream is manipulated by a FPGA.

The ADV 7391 is used to convert the manipulated digital videosignal back to an analog pal signal.

 

Our PCB-Designer used the specifications in the datasheets an took care to keep noise away from the

Videochips.

 

We used table 83. on Page 95 rev.C of the datasheet to configure the ADV7391 via I2C-Bus.

The configuration works but the outputvideo we get still has noise in it. We used the Pixelclock from

the ADV 7180 for the ADV 7391.

testbild_0.png

 

Here you can see a screenshot. We used a videograbber to get this picture.

pal_testbild_oszi.png

Here is a screenshot we took with the scope. This is the pal outputsignal un DAC-1

Generally it looks all right but there is still too much noise.

 

The signal gets better if we use an analog video inputsignal at the adv7180. If we do this the videograbber shows a picture as follows.

testbild_1.png

That is strange because the only signal that the 7391 uses that comes from the 7180 is the pixelclock with 27MHZ and

it should be constant.

 

So my questions are:

 

1. Do I have to configure the 7180 in a special way to get a special pixelclock?

2. Hoe can I reduce the noise?

3.What can I do to get a good output test signal?

 

We hope you can help us,

 

Regards

 

Manuel Stephan

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