a question on the AD725 from the Uzebox schematic Rev E1

[unebonnevie]

Hi,

New with TV interfacing via the AD725 and for learning purposes, I have the below questions while reading the Uzebox schematic Rev. E1.

The optiomal luma trap part. What's for? And what the benefits are?

Thanks!

[jaak]

The luma trap is explained in the AD725 data sheet.

Put simply, when color signals (chroma) were added to black and white (luma) television standards, the designers did it in such as a way that the new color signal would be backwards compatible with older black and white TVs. The signals are mixed together to form composite video. The problem is some of the signals can interfere with each other and the TV (which has to separate the combined signal) can't tell some times which part is chroma or luma. This causes an effect commonly known as dot crawl.

The AD725 solution to this problem is to use a notch filter that "traps" the part of the luma signal that interferes with the chroma signal. If you use S-Video the luma and chroma are always kept separate and do not have this problem.

Another point is that all (?) modern TVs have filters on their composite inputs today that do the above (and more), so you might not notice much difference with the luma trap in place (but you might notice it on an older TV).

[uze6666]

Can't give a better explanation! I've added the question & answer to the Uzebox FAQ.

-Uze

[unebonnevie]

Thanks for the answers, both! One more question:

The 3.3V voltage regulator in the BOM has a 1.5A rating. Would a 3.3V regulator of 100mA good enough to use?

Thanks

[uze6666]

Indeed the power is way to high, perhaps that was the model I had on hand at he time! I'm not sure about the power rating required for the SD card though (and at least another peripheral like an Ethernet module). Would need to look at the SD spec.

-Uze