The blotches here are due to genuine VHS/PAL chrominance noise
I tried to simulate this by adding RGB or HSV noise to the red, green and blue images I was creating. The results looked totally wrong. The trouble was that the noise was too even, with all the little blobs of noise being the same size. Click on the image below to enlarge it.
What I needed was some more realistic VHS noise, in which the blobs of noise were different sizes. Chrominance noise is rather like film grain in this respect, so after Googling "film grain gimp" I came up trumps with a very good article by Eric Kidd about film grain in The GIMP documentation which gave me the solution.
So, here is how I made my nice clean chrominance channels dirty:
- I opened a real screen grab taken from a PAL VHS tape in The GIMP.
- I duplicated the image onto a new layer so I had two layers in the same file containing the same image.
- I blurred it the upper layer with Gaussian Blur until there was no noise visible.
- I set the blend mode of the blurred layer to Grain Extract.
- I flattened the image.
What you should end up with is an image that looks like this (click on it to enlarge it):
The grey areas are pure PAL/VHS chrominance noise
The grey areas contain the noise - which is what we are after. The next thing you need to do is choose a portion of the noise that looks best, and make it into a new image. After that you should make you noise image into a seemless tile the Make Seamless Map filter. This allows you to fill large areas with noise.
Here is the sample of real VHS noise I ended up with - it doesn't look like much, but it works a treat!
This small PNG image was all I found that I needed
Now we can use the noise image to help us make a fake "PAL VHS" image. You apply it when you have scaled your red, green and blue images down to their "small" size - that's before you restore them to 720 x 576 and composite them together.
First, I copied my noise image in The GIMP so it was on the clipboard. Then I used a Fill with the "Pattern Fill" fill type with "Clipboard" as the fill source. I also set the affected area to "Whole Selection" and the mode to "Grain Merge". The bottom of your toolbox panel should look like this:
Here's the recipe for those playing along at home
So all I am doing is blending my real VHS chrominance noise image over each of my colour channel images. Here's my scaled green channel just after I applied the real chrominance noise.
Here is an example of a finished "fake VHS" image with the real chrominance noise added (click to enlarge):
And here's the Granada International example from earlier, but this time with real chrominance noise applied as a comparison (click to enlarge):
That wraps it up for PAL and VHS - sorry I went on about it so much!