How do I stop my letterbox movies from being squished?

[AndrewFryett]

I have a number of DVDs that I've ripped into my iTunes library so that I can watch them around the house using my Apple TVs, iPads etc. Now I've upgraded my phone to a Lumia 920, I was hoping that the movies would play perfectly on the phone as they do on the other devices. However, my "letterbox" format movies get squished so that they fill the Lumia's screen. How can I get the movies to play in the correct picture format?