Demodulation of NTSC YIQ Signal

[Fischer777]

Hello everyone,
I'm working on a project that requires me to demodulate an NTSC composite video signal into an RGB form. Despite having read a book and visited numerous websites detailing how this is performed, I am still confused. First, it seems that the colorburst signal is not in phase (or separated by some multiple of 90 degrees) with either the I or Q signal, so I'm not sure how either the I or Q signal can be demodulated without crosstalk. Second, I have heard of a method of demodulation called R-Y/B-Y, which I understand is appealing because somehow the B-Y signal is in phase with the colorburst, yet I still don't understand how this relationship is derived in a mathematical sense, or even how this particular method works. Thanks for any help with this.

 

[Baluncore]

First, it seems that the colorburst signal is not in phase (or separated by some multiple of 90 degrees) with either the I or Q signal, so I'm not sure how either the I or Q signal can be demodulated without crosstalk.

In NTSC the colour burst phase is locked at transmit but by the time the signal reaches the colour decoder the original phase has been rotated. That is overcome by the PAL correction. NTSC is “Never The Same Color” due to phase rotation causing cross-talk.

Look at page 5 of this document.
http://www.earlytelevision.org/pdf/NTSC_Signal_Specifications.pdf

D. Equation of Complete Color Signal

"The color picture signal has the following composition:"

"The phase reference in the above equation is the phase of the (color burst +180°), as shown in Fig. 3. The burst corresponds to amplitude modulation of a continuous sine wave."

 

[berkeman]

NTSC is “Never The Same Color”

:D

 

[nsaspook]

Here is a old NTSC station calibration video that might help to understand the chroma signal and color burst phase relationship using a vectorscope.

 

[Fischer777]

Alright, I think I've got it now. Thanks guys!