Hi mate,
I don’t think there is a built in effect that will do this.
However I achieved an automated result by doing the following.
Creating another layer of black text behind the white subtitle text.
Make the black text slightly bigger than the white in the character palette and make sure the text baseline is set in the middle.
Pickwhip the source of the black bg text to the original white text.
Apply a fast blur to the black bg text set at 250 and turning on repeated edge pixels.
Apply a mosaic effect turned up to 1000 horizontal and 500 vertical.
Then finally apply a black light sweep set to maximum intensity and composite mode. You will need to duplicate this effect about 4 times.
That’s it… sorry if that is a bit confusing it was a bit of trial and error. There maybe a really simple way but I have not heard of one.
I’ve uploaded my project for you anyway mate.
https://f1.creativecow.net/4154/subtitle-bg
Good luck