Rainbow artifacts in composite video

Question

Background: I am trying to reverse engineer a small endoscopic camera for a class project. The camera is connected to its own screen, and I do not have the specs for either. Thus far I have learned that the signal that it provides is a composite video signal (because the circuit board for the screen has a TI chip capable of decoding PAL/NTSC/SECAM signals), but I do not know which of the 3 types of composite signals it is.

Problem: I am trying to bypass the screen and attach the camera to my own hardware. However, when I do this the image is black-and-white with rainbow overlay:

The screen that comes with the camera provides the correct color image. I am using a composite to USB dongle I bought on Amazon. I tested the dongle on other composite signals and it works just fine. But when I connect it to this camera it shows the rainbow artifacts you saw above.

The camera provides nice colored picture when used with its own screen.

The colorburst frequency is the one for NTSC.

Here is a trace:

What could be the reason for the rainbow artifacts? Any ideas how to get the video signal from the camera without relying on the screen it comes with?

Answer 1

Beginners read here and picture examples here

Connect Camera ( If NTSC or VGA compatible ) composite video out with RCA cable to monitor or TV, where Yellow is designated for Composite Video.

Since your scope indicates approx. 63 us between sync tips , this is the same as 15.734 kHz Hsync, which is NTSC interlaced video or almost same as VGA interlaced video 640x480 pixels.

If your video decoder is not set up properly for this or is too noisy you will get chroma errors as you indicate. You also have a lot of luminance noise. Normally a TV blanks the chroma decoder if it is not in sync to just show BW TV.

Yours is not normal. Clean it up to the standards or SNR=40dB or better.

• check for supply ripple, cable noise, ground noise, laptop charger noise ( common mode ) etc.. then check at termination with short coax.

Try to compare your decoder specs with std NTSC and get your video field signal to be 1Vpp for 100% white and clamp back porch after sync tip to set black level =0% IRE while sync tip is approx -300mV or approx 1.3Vpp into 75 Ohms.

Chroma burst gating is well documented on web. The chroma burst should be smooth sinX/x envelope of the chroma carrier sinewave without spurious noise.

I count about 22 vertical "Herringbone" chroma cycles which for a ~60Hz field rate tells me your Chroma Oscillator PLL is way out of sync, by the harmonically related beat frequency. The video is also not sharp.

If your signal is not compatible then determine what is different in signal level and get a video amp to achieve 1 ~ 1.3Vpp into 75 Ohms with >4Mhz bandwidth. High res composite video exceeds > 250MHz BW, due to number of pixels * frame rate ( if on very high frame rate CRT) while VGA 640x480 < 5MHz.