The titanic developments in new technology have radically changed the way we watch TV and movies at home. Gone are the days when the TV set was a box in the corner with a 20in screen and a tinny speaker. Now most of us prefer to watch movies and sports casts on a widescreen that's at least 28in from corner to corner.

If you are a fan of home theater then that screen will almost certainly be bigger and be accompanied by a surround sound speaker system to maximise the aural effect and immerse you in the action. As technology improves and equipment becomes cheaper, more and more people will have high definition TVs and home theater set- ups.

However, if Sony has its way, we'll all be enjoying a completely different movie- watching experience, on a 2inch mobile phone screen. Sony wants to see an iTunes- style download service for movies so we can download them and watch them on the move. Would you watch a film on a cellphone? I wouldn't.

Imagine it. Out would go the immersive, involving experience of the movie theater that we've tried so hard to create at home, and in its place would be a screen so small that it would be difficult to make out what was going on. That in itself would create a dilemma for film-makers. If a sizeable number of people who pay to see a movie do so by downloading it on their mobile, will directors have to take this into account when making it? Will studios reject cuts because they contain too many subtle facial expressions or movements that can't be detected on a tiny screen? Will the muted colors of a Saving Private Ryan have to replaced with more lurid and brightly colored scenes to make them more easily visible on a cellphone?

And what of the audio? Is their any point in spending time and money developing a detailed and textured surround-sound experience if a significant portion of your audience will be listening on earbuds while travelling on a train?

These examples may be extreme, but they do highlight the differences between the way a music download service works and the way a movie download service would. Music, by its nature, is immensely portable. OK, the sound quality from an iPod over a pair of earbuds may be nothing compared to that from a decent hi-fi set-up, but for most people most of the time, it's a close enough approximation. That won't be the case with movies. Why did those tiny, battery powered, pocket TVs never really take-off? For the same reason.

Apple CEO, Steve Jobs, who knows a thing or two about the movie industry through his stewardship of Pixar, has often said that he's not interested in producing a video iPod because no-one wants to watch movies on a small screen. Jobs is right about most things, and I think he's right about that. Sony disagrees. It will be interesting to see who's right.

Robert Armsrong is a contributor to The HDTV Tuner - a guide to the kit, the technology and the programming on HDTV.