How do I get rid of the terrible banding on IPS (PremierColor) M4600?

I would have these questions:
Which GPU drives the HDMI?
Which GPU drives the internal display?
How is the internal display connected? (What is the cabling standard used in your M4600?)

 

With the IPS display, the Intel GPU is not used. The discrete GPU drives all displays.

 

Exactly. I don't know what you mean by cabling standard, but the IPS is connected via an LVDS and a separate - I assume eDP - connector. The GPU should 'sense' the DP standard and output in 10 bit.

 

And this corresponds to what the Nvidia driver shows in system topology (and display resolution too)?
Is there any confirmation that 10 bits are being sent?

 

I don't think there's a way to confirm 10bit from the system topology. It confirms DisplayPort connection though.

BUT...

I incidentally discovered the problem - a faulty cable or connector indeed. I downloaded some gray gradient photos. I accidentally moved the display forcefully backward - and the gradient suddenly displayed nicely. I double checked with other photos and the severe banding disappeared. I then fiddled some more with the display and it's back to the awful state. Then back to good, then bad again. And now, no matter how much effort I put into it, it won't revert to good. But at least I know the cause.

Thanks for taking an interest in this.

 

Great then, that’s the least expensive part.

In my old laptop, the display cable was the only part that eventually broke. As it has to go through the lid hinge, it will accumulate abuse over the years.

 

Yup, it's some 30 bucks on aliexpress. The display was another 110 (got it cheap for an RGB IPS). Still, that's already 25% over the price of the M4600. This being said, it's still a major relief.

Thanks again for the interest.