from a mailing list I subscribe to:
"I found about this recently too. Regarding video quality the
difference can vary greatly - it depends how the uploader uploaded
the video. If he/she uploaded it as 320x240 there will be little/no
improvement in the video (all old-standard videos on YouTube are
320x240). Also, anything uploaded before June last year (introduction
date was possibly 2007/06/24) will not have a high quality version
available. HQ videos generally seem to be 472x360 or around there -
still not good enough for fullscreen IMO but a damn sight better than
before.
The main benefit you'll experience is the sound. Normal YouTube
videos only have 64kbps mono MP3 audio (some even only 22kHz!), but
HQ videos have 128kbps 44.1kHz AAC-LC. Great for music
More info here (including how to download HQ videos): http://
forum.doom9.org/showthread.php?t=135897
>
> Hey kids!
>
> Here is a kewl trick you can do at YouTube. I got this via a
recent
> YouTube video by Charles Trippy (who is hilarious and insane and
has
> two cute puppies):
>
> 1) Go the the page for the video you want.
> 2) Add the following at the END of the URL:
>
> &fmt=18
>
> 3) Hit RETURN to go to this revised page, which is in 'HQ' (high
> quality) video.
> 4) Hit the small-box-in-larger-box button on the far RIGHT below
the
> video screen. This puts the video in full screen mode.
>
> RESULT: You get a higher quality video that actually looks good in
> full screen mode.
>
> If you don't do the '&fmt=18' trick and you use full screen mode,
> what you get is the lousy old rez, which looks crummy.
> ---------------
>
> Also, I am sad to say, YouTube have started inflicting video
banners
> on top of recent video posts. Very annoying. But I suppose this is
> how they are going to start making a profit for a change. (They
never
> ever have). Thankfully they are kind enough to put a big 'X'
circle
> in the upper right corner of the ad that you can click to kill the
> thing."