I think it’s a wonderful idea. In fact, I propose that someone invent a device to do just this. I’ve already played my part by coming up with the idea. Surely there’s some brainiac out there with the necessary means to make personal subtitling happen.
On the down side, personal subtitling could land you in a lot of hot water. Like if you accidentally left your subtitling on while whispering to a colleague about what a tool your boss is. Or if you were playing Chinese whispers. And it’s probably not very helpful for people who can’t read. Or are dyslexic. Or for when you’re on the phone to someone you can’t understand.
Still, it would be useful a lot of the time. So I’m offering this invention idea for free. Tossing it out into the world wide web so that some techno boffin (I love that word!) can stumble across it and invent a truly useful piece of technology. I'd be particularly pleased if Apple were to come up with this little knick knack - not because I particularly like Apple products, but because they're the only company who could get away with calling it the 'iUnderstand'.
