Cap Hack May Be Incorrect!

[Forklift]

As part of a discussion I've been having with Uze on the Uzebox forums (which can be found here: http://uzebox.org/forums/viewtopic.php?f=5&t=184&p=1723#p1723), I've discovered that the cap hack as described by Ladyada here does nothing to improve image quality for my box. After some live testing, I realized that using the S-video output jack ground instead of the ICSP header ground for the cap doesn't just improved quality slightly, its night and day. All flicker and artifacting through composite out are gone. And I mean GONE. It's been a real "so this is what its supposed to look like" experience.

So what's going on here? I'm not entirely sure. See the Uzebox forum post for more info. Suffice to say, from what I can tell on the Uzebox schematic, this shouldn't be correct. But it certainly works better. ALOT better.

Here's a pic of my current cap hack:



I'm sure there is a better way to make this connection, this is just what I did temporarily.

[Amberwolf]

I don't know the reasons for it, as I'm not familiar with the Fusebox/usebox, but a good place to hook the cap up to for easy mounting and reliable connection would be from the point on the lower right you already have it from, to the ground plane itself (I'm assuming that the point you've soldered it to at the top is indeed the same ground plane as that surrounding that point).

This could be done by either soldering to any of the three thru-holes near the edge of the ground plane closest to the bottom, or by scraping a bit of conformal coat off the ground plane in the open area away from other leads, and soldering to that point.

Both methods, if done with the cap body bridging the gap between the two planes, would ensure the leads of the cap cannot touch any other component leads even without insulating tubing. A drop of glue under the body of the cap should ensure it does not move from vibration/thermal/etc.

[uze6666]

Like for Forklift, the existing cap patch does nothing perceptible to my eyes (or my scope ). However, the 100nf cap going to Analog GND greatly improves picture quality. That said, 100nf is not enough to eliminate some other artifacts like "ripple banding". See these comparison pics:

With a 100nf capacitor:


With a 10uf tantalum capacitor (as per the AD725 spec):


I've done some testing with non-tantalum caps and the results were pretty much identical (to my eye and scope again). So if you don't have some 10uf tantalum lying around, you can put any electrolytic cap from 4.7uf to 10uF (for both types, take care of the polarity!)

Here's my version of the fix with a 10uf tantalum cap. If you don't have tubing to protect the long pins from shorting your board, use some gauge 22 wire piece and just keep the shrouding (what I've done).


So, in conclusion, with this fix applied, I can confirm you have what is closest to the "Uzebox experience" (Forklift coined the term ). And I noticed no additional or significant noise on the digital lines. I realized my schematic mentioned the caps going to the digital ground, but in my wire-wrap prototype both AGND and DGND are the same. Hence that 'perfect' picture is what is expected. Note the best picture possible could be attained by implementing the luma trap on the AD725. That gives a sharper picture with less color bleeding when using the composite output. Perhaps there could be a way to hack that too on the Fuzebox for the ultimate experience!

Cheers,

Uze

[uze6666]

So, did anybody tried the fix? Any comments?

Uze

[Flecko]

I tried a 10uF tantalum cap, as you have pictured Uze, and I get the correct video output.

So, if you want the true Uzebox experience, an extra cap between analog and digital ground does the trick. Cleaned the picture right up on my 42" LCD.

Thanks for the tip!

[mos6502]

I can second that improvement in video quality.

After I fired up my fuzebox for the first time I immediately left for radio shack to get a 10uf tantalum capacitor - I couldn't handle the distortion.

Looks great now though!

Thanks for the tip Uze!

[adafruit]

Do you guys mind telling me what sort of TVs you were using that had NTSC problems?
thx!

[mos6502]

I'm using a 20" LG LCD tv.
Model 20LS7D.

[uze6666]

Great to hear it helped guys!

@Ladyada:
Problems happened on all my TVs:
-13" Insigna CRT
-32" Toshiba LCD HDTV
-28" Sony WEGA CRT
-9" Sony Potable banned (This one only locks the video signal on Clay's AVCore!)

Did you finally have a chance to get a CRT TV to check it out?

Uze

[adafruit]

i found one on the street! but it doesnt have RCA in (just RF, argh!)

[Dan_K]

I applied this today and I can confirm that it cleans up the signal greatly. Much cleaner now

[Nender]

I might have to go out and try that... looks interesting.