Find direct video.google.com url of a youtube video

Some youtube videos server by google cache servers and others by youtube cache servers. I want to find the direct url of a youtube video which server by google servers. For example the direct url for youtube video(id=Q_6qRyYn-68) is http://r7.fra07s04.googlevideo.com/videoplayback?ip=78.0.0.0&sparams=id%2Cexpire%2Cip%2Cipbits%2Citag%2Calgorithm%2Cburst%2Cfactor&algorithm=throttle-factor&itag=5&ipbits=8&burst=40&sver=3&expire=1287626400&key=yt1&signature=35C71F45A33FB158D6454EE904A66D087CC6C2AF.AB5EC691E682D5671611E5012DB28B9FB64B0239&factor=1.25&id=43feaa472627fbaf&st=lc

I don't know if i,t is possible 开发者_开发知识库because signature and expire params are dynamic. If not possible then another question appears. Is there a way to force the youtube video pull from google servers instead of youtube servers?

There are several YouTube downloaders around the internet, but they tend to stop working after a while. This would indicate that Google from time to time changes how links to videos are generated.

For me the only way to reliably download the YouTube video is to use Safari and then open Activity window (alt-cmd-A). There I can see all http downloads and the one constantly running (since YouTube uses progressive http download) is the video http url.