NTSC 4.43

[sebastien.durand]

Hello all,

I try to generate NTSC 4.43 signal with a NI - 5421 signal generator. For this I use the video generation library and more exactly the low level image generation VI. In this VI I just edit one parameter which is called "Subcarrier period per line" and I force its value to 281.79 that, for me is coresponding to the chroma subcarrier at 4.43 MHz.

When i launch my VI, I generate a signal but the image obtained is black and white. The chroma doesn't seem to be transmitted properly.

My question is : do I forget something (another parameter should be changed??) or the video library isn't able to generate NTSC 4.43 ??

Thanks for help.

Regards

[kalyanramuv]

Hello,

NTSC 4.43 is a pseudo color system that transmits NTSC encoding (525/29.97) in a color subcarrier of 4.43 MHz instead of 3.58MHz. The resulting output is only viewable by TVs that support the resulting pseudo-system (usually multi-standard TVs). Using a native NTSC TV to decode the signal yields no color.

Can you please post some more details as how you are viewing the video or does your TV support NTSC 4.43 specifically?

[sebastien.durand]

Well in fact I use a multi-standard monitor to view the image and it does support NTSC 4.43.

The fact is that if i use a TV signal generator with a NTSC signal with a color subcarrier of 4.43 MHz, the image is properly displayed (colours etc ...).

Moreover I had a quick look to the signals with an oscillocope and there is no fundamental difference between the signal emitted by the NI-5421 and by my TV signal generator.

So I don't really see what it can be (phase?? timings?? ...) but the oscilloscope is not quite appropriate to perform all those measurements.

Hope this helps.

Regards

[kalyanramuv]

Hi Sebastian,

In regular NTSC, the number of subcarrier periods per line is 227.5. The number of periods of subcarrier in 2 frames is 227.5* 525*2, an exact integer. In NTSC 4.43, the number of subcarrier periods per line is 281.79. This means that the number of periods of subcarrier in 2 frames is 281.79* 525*2 = 295879.5 periods which is not an exact integer. The .5 period corresponds to a phase of 180 degrees. This means that the subcarrier phase after 2 frames is 180 degrees. Also, we can notice that in NTSC 4.43 it takes 4 frames to finish integer number of
subcarrier periods.

By default, NI-Video driver generates 2 frames per image for NTSC so that the phase of subcarrier starts at zero for next image. But for NTSC 4.43, we have to create 4 frames for the image so that the phase of subcarrier starts at zero for next image. However, we can use an alternative way (i.e., playing with the parameter "subcarrier start phase"). We can alternate the subcarrier start phase between 0 and 180 for series of images. NI-Video has an attribute "subcarrier start phase" which you can modify.

A good way to test this solution is by taking a BMP file and converting it into a Video Waveform with subcarrier period per line as 281.79 and subcarrier phase as 0 degrees. This will give us waveform1. Then the same BMP file is converted into video waveform with subcarrier period per line as 281.79 and subcarrier phase as 180 degrees.This will give us waveform2. Then concatenate these two waveforms and download them onto memory. This should generate a color image.