Thursday, January 24, 2013

NTSC even->odd field transition explained in detail

I've done some more oscilloscope captures this morning to try to better learn how NTSC works.  And this has proved incredibly helpful.

First I captured two lines from the LM1881, the vsync and the even/odd field lines.

Here is how they turned out:

Odd field ending, even field starting:

Even field ending, odd field starting:

Once I got this far, I noticed that the LM1881's vsync pulse is a slightly different width depending on whether the field is even or odd.  This gave me the confidence to start capturing the actual NTSC video and making notes because it allowed me to establish quickly whether I was looking at an even or an odd field transition.  In retrospect, I could've just captured the even/odd line instead of the vsync line.  Oh well!

As you can see, I basically took notes on everything interesting about this transition.  It turned out to be the even->odd field transition which is line 1 of the picture, so it is probably a good place for me to start.  (yes, line 1 is the first line, not line 0!  I got confused about this due to my computer science background)

A few handy things to note that I did not notice earlier when reading other NTSC materials:

The vsync pulse period aligns perfectly with horizontal line numbers.  So it is very easy to find the boundaries.  If you just looked at it without knowing this fact, you might get very confused (I sure did!).

Also, the non-visible lines start on line 10.  Line 11 is used by laserdisc for the "white flag" line.  Neat, eh?

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