Movie format conversion can be a tricky business and MPEG streams come in all shapes, sizes and flavors depending on what kind of device they have been created on. A lot of camcorders for example will have their own take on the MPEG recording format and sometimes the conversion or capture software supplied with the hardware isn't all that great, or is Windows-only. In other cases you need to decode a problematic file that won't play back or be read by FCP or Premiere, or actually encode something into an odd format for a specific reason. MPEG Streamclip is an excellent and lightweight video conversion app that specializes in hard-to-deal-with MPEG files as well as more regular ones and, best of all, it's completely free!
Download the MPEG Streamclip application from http://www.squared5.com/ and fire it up. Drop your movie into its window and it should appear and play back using the transport controls. By scrubbing in the timeline while holding the Shift key, you can select a section of the video and then use the Copy and Paste Edit menu commands to perform basic edits in the file.
Click on File > Show Stream Info and you will reveal a window giving you information about the characteristics of the video and audio streams contained within the file as well as its size, data rate and location. These can be useful when trying to understand more about the files.
You can also use the selection feature followed by the Trim command to trim a clip down. To export a file to a new format, choose File > Export. Let's say we want to export it to an Apple device. Choose File > Export to MPEG-4.
Now you see the export options window: To save some time, and if you know the device you are exporting for, you can press the iTunes button to reveal a list of Apple devices. Choosing one of these will enter settings appropriate to that device. You'll see that there are 4:3 and widescreen options for each device as well as SD and HD settings for the Apple TV.
We have chosen a preset and returned to the settings window. You can manually set a data rate if you like and also chose multi-pass encoding for a longer encode but better quality. If you click on the list of codecs you will see all codecs installed on your Mac, though you should only switch if you know why you need to. The list of codecs changes depending on your container format. An MPEG-4 will only have a couple of options, whereas a QuickTime file will have lots.
You can choose to de-interlace the video on formatting if your video is suffering from annoying lines on the screen. Using the controls at the botom you can manually crop and rotate the video during re-crunching too if you like.
Click on Adjustments and you get to alter brightness, contrast, saturation and audio volume during encoding, as well as adding a watermark.
Finally, hit 'Make MP4' or whatever format you have chosen, and the file will be encoded!