[FranceBB]

Uh... I don't get it.
Your source is 8bit, your target is 8bit.
You originally said

I tried to launch avs dither tools for debanding

Perhaps you mean that your 8bit sources have banding problems and you wanna apply debanding?

I mean, dithering is used whenever you have to go from a bit depth to another: truncation creates banding and, in order to avoid that, dithering is applied. For instance, 10bit sources that have to be brought to 8bit can be dithered down to avoid banding: instead of seeing "bands" of colours, you see many little "points" of different colours on the edges to make the transition from one color to the other smoother.

https://upload.wikimedia.org/wikiped..._example01.png

As you can see in the example, this is done in a meaningful way.
If your source is not high bit depth and you wanna get rid of banding, just use a debanding filter like f3kdb:

f3kdb(range=15, Y=45, Cb=30, Cr=30, grainY=64, grainC=64, sample_mode=2, blur_first=true, dynamic_grain=true,  opt=3, mt=true, keep_tv_range=true, input_depth=8, output_depth=8)

With these settings, it will apply debanding and then insert grain in both Luma and Chroma with a different patter at each frame. If you want the pattern to be consistent, just change dynamic grain to "dynamic_grain=false"; you can also adjust how much grain you want by lowering or raising "grainY" and "grainC" (you can even set them to 0 to avoid to add grain).

If you don't wanna apply debanding and you just wanna get grain, just use AddGrainC:

AddGrainC(var=5.0, uvar=0.0, constant=true)

You can set "constant=false" to add dynamic grain.

Anyway, I'm not sure you'll be able to get results in real time, especially with debanding.