Pal Ntsc mix and match possible with dahua dvr?

[tangcoral 0]

When I connected my NTSC camera to PAL Dahua dvr I get the flickering/black line.

But when I connect my cheap Pal camera to Ntsc Avtech dvr it works. Whats going on?

From what I heard you usually cannot mix and match two different signals.

 

I am assuming my Avtech dvr is auto-sensing the camera signal or , the cheap PAL camera auto senses (which I doubt)

 

I have a bunch of PAL and NTSC cameras that I would love to utilize on a single dvr but I am afraid to replace my old NTSC avtech for a new NTSC Dahua dvr if it wont accept both system signals.

 

What am I not understanding here, hopefully someone can chime in with some info.