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The following article outlines the process of downloading the SD version of your purchase movie, if available.
Syncing video to iPhone 3G or earlier, iPod touch (1st or 2nd generation), iPod classic, iPod 5th generation, or iPod nano (3rd, 4th, or 5th generation)
1. Make sure you have the latest version of iTunes installed on your computer. You can download the latest version for free here.
2. Open iTunes on your computer.
3. From the Store menu, choose Store > Sign In, and then enter your Apple ID and password.
4. Navigate to your Purchased page. Find it by clicking Movies or TV Shows from the top navigation bar in the iTunes Store, and then click Purchased from "QUICK LINKS" in the iTunes Store.
5. Choose the appropriate video type: Movies or TV Shows.
6. In the lower-right corner of the screen, deselect the box next to "Download HD When Available."
7. Search for the desired video and click the download button.
8. This should download the standard definition version to your computer so you can sync to your device.