Original article is located here: http://www.tools4movies.com/2012/05/tip-how-to-make-your-video-files-smaller/
On a regular basis, people ask me why video files end up being so large, and how they can make them smaller so they can store more videos on devices such as the Amazon Kindle Fire, Apple iPad and the Galaxy Tab 10, all of which have limited storage space available and do not have a way to expand the memory by means of memory cards.
In DVD Catalyst Newsletter 52, http://www.tools4movies.com/2012/04/...newsletter-52/ I wrote something small about this topic, but I am hoping that this article will be of a bit more help to you. Some parts of the newsletter will be merged into this article, so there is no need to read back through that.
Before I start with the different techniques you can use to make your movies smaller, let me explain a bit more about what video and conversion actually is.
As a child, you might have played with the corner of a notepad, drawing an image on each page, one slightly different from the next, and afterwards, flipping the pages fast made it look like a small cartoon.
Video, like movies and TV shows, is nothing more than a collection of pictures shown in rapid succession. Making use of the limitations of the human eye, these images are changed at such a speed that we see this as motion.
Similar as taking pictures on a digital camera, each individual picture of a movie, called a frame, takes up space. A picture consists of a lot of individual dots, and in order to show the picture in its full glory, the color for each of these little dots needs to be stored in the file. The larger the picture (resolution) the more pixels, and the larger the actual file.
In order to make the pictures smaller, there are different picture formats, such as BMP, JPEG, GIF, PNG, TGA etc. Each of these formats uses different techniques to read (and store) the pixel information in a file. These formats offer different compression levels and even amount of colors, in order to store the data. Some formats store the color information for pixels in larger blocks, others use formula’s to store the data, and of course this results in smaller files than the formats that store each pixel individually, but it also means that some pixels do not contain the exact color information of the original, but something close to it.
Similar as with images, there are different compression formats for video. DVDs use MPEG2, AVI files often use DIVX or XVID, and most MP4 files, such as the ones you can get from online stores like iTunes, contain AVC video.
Each of these formats have their advantages and disadvantages. MPEG2 (DVDs), was created when the technology was not yet at the high performance parts we have now, and as a result it offers a low compression technique that works well on older hardware. AVC, the newest compression format, was created only recently, and as of such, it relies on a lot more “power” than MPEG2, but in return, it offers a lot better compression.
Video compression in general works by storing differences between frames. So-called key-frames are images stored in their full glory, and then for a number of successive images, only the differences between the key-frame are stored. The bigger the difference between the key-frame and the following image (like fast-action scenes) the more data is required to store those differences, resulting in a larger file.