Category: PAL

I was happily applying my “fake” VHS or fake PAL effects to a range of pictures thinking how clever I’d been when something started to bother me.

Most images looked fine, but some looked completely wrong. For instance, the “Colour Production” text on Yorkshire Television production captions looked awful. I needed to find out what was wrong.

I went back to Alan Pemberton’s website and after reading this page again I realised what was wrong. A PAL colour picture is composed of a high definition black and white image (which contains the luminance or Y) and a low definition colour image (or chrominance). The luminance image and chrominance images are added together to make a colour image.

However, with the colour turned down, the luminance image looks like a normal black and white picture – complete with all the detail viewers in black and white would expect. Whereas my technique for faking PAL or VHS was chucking away far too much detail, and if I turned down the colour I would certainly not see an acceptable black and white image.

In particular, I was binning far to much information for red and blue, which was the reason that things like the blueish red “Colour Production” on the Yorkshire Television caption looked far to low resolution.

I could quite easily create a PAL luminance image, like this:

And I tried to create a chrominance image, like this one:

However, when I tried to combine them, something was wrong.

The answer was that for my chrominance image to work, the PNG format would need to hold negative colour values. The chrominance image is what’s called a difference image and therefore contains negative as well as positive values. Alan Pemberton explains why this is much better than I could on his site.

So, I couldn’t be completely accurate in the way I created PAL images, but I thought I could do a better job than I had been. So I fired up The GIMP and came up with this.

I load my source image and create two layers. I call the bottom one Luminance, the other Chrominance. I make the Chrominance layer’s blend mode Addition.

For my Luminance layer I adjust the Red channel output level to 76, the Green channel output level to 150 and the Blue channel output level to 29.

For the chrominance layer I adjust the Red channel output level to 179, the Green channel output level to 105 and the Blue channel output level to 226.

You may notice that each pair of numbers adds up to 255. The Luminance (Y) layer values were worked out using:

Then depending on what I am doing (PAL VHS or just PAL), I scale the Luminance and Chrominance layers horizontally and then scale them back to full size. For a nice PAL effect I found that scaling the Luminance layer by two thirds width and then back again and the Chrominance layer to a third width and back again gave a good effect.

For VHS I found scaling the Luminance layer by 32% and the Chrominance layer by 16% looked OK.

As a comparison, here’s a Yorkshire television caption done using the revised technique.

The next job is to turn my method into plug-in filters for The GIMP. Time to dig out my Python book!

Recently I’ve been trying to work out how to recreate the PAL/VHS look in The GIMP. One article I read said that the colour (or chrominance, as they put it) signal on VHS recordings was very noisy. You can clearly see this on screen grabs taken from real VHS tapes.

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.

My unsuccessful attempt to simulate it using HSV noise

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 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!

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:

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.

The noise causes the slight blotchiness of the background

Here is an example of a finished “fake VHS” image with the real chrominance noise added (click to enlarge):

Very happy with the way this turned out

And here’s the Granada International example from earlier, but this time with real chrominance noise applied as a comparison (click to enlarge):

I think we just missed something dull and northern

That wraps it up for PAL and VHS – sorry I went on about it so much!

This is the list of scale sizes I use for scaling images to make “PAL VHS” screens in The GIMP:

            Broadcast    VHS (46.25% worse)
Y   0.299R  215 x 576        100 x 240
    0.587G  423 x 576        195 x 240
    0.114B   82 x 576         38 x 240

I tend to use non-square pixel 720 x 576 images as source material, or 788 x 576 source material that I scale back to 720 x 576.

Incidentally, the type of scaling you use is important. I nearly gave up on creating VHS style images at first because I was using Cubic interpolation when scaling in The GIMP. It gives very disappointing, blocky results (click to enlarge):

However I soon worked out that, thanks to a bit of help from a clever Hungarian chap called Lanczos Kornél, I could use Sinc (Lanczos 3) interpolation when scaling.

That makes all the difference to the end results. Here is the same image created with Sinc Lanczos scaling:

The sad thing is I probably spent more time on Border Television’s graphics than Border Television ever did when I was writing this post…