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ADV7171 fuzzy output issue

Question asked by Dr.Lightning on Mar 6, 2011
Latest reply on Mar 7, 2011 by Dr.Lightning

My custom DM6467T board, generally derived from the EVM reference design, has instead an ADV7171 chip for video output, using the ADV7170 driver than came with my git download.  I had to modify the driver's default values for the ADV7171 mode registers, but now I have video output.  See "ADV7171 video output SYNC issues"  http://ez.analog.com/thread/6306?tstart=0 for more information, but I believe my fuzzy issue is independent of the sync issue.

 

This is NTSC.

 

The top half of my synthesized image is green, YUV=(95,82,82), but the bottom half is a bright whitesh magenta or something, YUV=(100,131,131).  The green looks good on my display.  The other color randomly moving dots of noise plus a barely visible slight diagonal pattern of more-blue vs more-white, slowing moving across the screen.

 

While I'm an expert at digital design and programming, including much mixed signal, I'm new to video.  In NTSC, is the color due to RELATIVE PHASE of the chroma modulation that's on top of the luma?  Is my green color in a "safe area" where small errors [in phase?] don't cause much difference, while my bright whitesh magenta or something is in a different area [of the gamut] where small errors [in phase?] cause significant problems?

 

Please let me know what your thoughts are.  I need to be sure of what the problem is before I spend too much time trying to fix that problem!


Also, I note that I may have a significant amount of noise running around the board, coming from my 27MHz clock(s).  I realize some of this may just be coming through the air and is a problem with my scope (Tektronix TDS 210) or leads or setup, as opposed to noise actually running around on my board.  Nevertheless, I found that when my board is running idle (command prompt on console), the 27MHz clock that comes out of the core is off.  Therefore, no 27Mhz clock is going over the 2" FFC (flat flexible cable) that connects my Core board to my SD Composite board.  In this situation, I have perhaps a 7mV p-p noise signal on my scope, with random spikes up to rarely narrow 100mV p-p.  However, when I run my application, the clock turns on and reaches my SD Composite board.  Then when triggered on the video signal and looking at the minimum flat of the frame sync, I see a solid 30mV p-p noise signal at 27MHz.  This seems noise persists through the complete frame, although I can't directly see it during the chroma oscillations.  (Note, even though this SD Composite board is on the end of a cable, supplying power and data and clock, I should have plenty of cap on the power near the ADV7171 as well as my video input chip, THS5147.  I could add some more cap right at the cable connector.  It's a six-layer board: Top,Gnd,Inner1,Inner2,Vcc,Bottom. Except for a single 3/4" long trace on top, coming out of a buffer, all the clock is on Inner1. )

 

I realize I should address this noise regardless, but looming demo deadlines and other business strategy suggest I should NOT work on the noise until later, if it's NOT the cause of my fuzzy output.

 

Tremendous thanks in advance for your help,

Helmut

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