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Compression
Now that DV video has become mainsteam, sharing your video has become just as important. However, if you want people to be happy when watching your video you are going to have to learn how to make the file size of your video smaller so that it is sharable. That's where video compression comes in and DVvideo.com shows you how to do it.

By Ben Miskie
Jun 21, 2004

Why Compression is Important

DV video has empowered the average person to be a filmmaker. Yet, unless your uncle is the president of Fox Searchlight pictures, your best chance of seeing your movie played to a large audience is on public access. Of course, the Internet has changed that. With the increase of broadband popularity, the web has become a serious means of getting people to see your work. Although the speed at which people access the Internet is increasing, it is still limited. As a result, the limitations of the Internet require making compromises. Understanding the limitations of web video and the compromises that must be made, is the key to publishing successfully over the web.

Overview of Video Compression: The Big deal.

Have you ever said to your friend, "I love waiting in lines?" Of course not! You would be an idiot if you said that and you would have no friends. Nobody looks forward to waiting in line or waiting for a movie to download, it pisses people off. Unless you are the late Andy Kaufman, you don't want to have your film performed in front of a pissed off audience. Video compression takes a raw video file and reduces it to a smaller size. However, as a result of compressing the size of the video, the quality of the video will be reduced. Your job, then, is to decide how much video quality can be sacrificed, without losing the meaning of your film.

Compression is a Tradeoff

As a general rule, the smaller the movie's file size is, the worse off tthe quality will be. The trick is to decide how much shittiness is acceptable to the viewer versus how big you want the file to be. There are a few basic things you can choose from when compressing your video: frames per second, color depth, resolution, video format, video compression codec. Pick the right format, select the right color depth, reduce the amount of frames per seconds, resolution and compression, and you'll have happy Internet viewers downloading your videos in no time. Don't forget to make sure people like your story.

The Nitty Gritty of Adjusting Compression

Frames Per Second
Film is 24 frames per second. NTSC DV video (what you see on a normal TV) is roughly 30 frames per second. Web video on the other hand can be anything. Obviously, the more frames per second, the larger the file size. For the web, it is generally a good idea to stick between 10-15 frames per second. Go any lower, and the video may be difficult to watch.

Color Depth
Color depth is the number of colors shown in a video. The number of colors can range from millions of colors to black and white. For color video, it is generally best to not go below 256 colors. Another trick to reducing file size is removing the color completely and making the video black and white. Not only does it make the file size smaller, but it also gives the movie a backwoods artsy fartsy unique feel.

Common web resolution settings:

  • 120 x 90

  • 160 x 120
  • 180 x 135
  • 240 x 180

  • 320 x 240

Resolution and Aspect Ratio
Normal TV resolution is usually 640 pixels wide by 480 pixels deep. Web video on the other hand can be any resolution or aspect ratio you desire. However, while you will most likely want to reduce the resolution of your film before putting it online, you probably won't want to stretch your film to a different aspect ratio. Doing so can make everybody look really fat, or look really skinny.

Different Formats: The Big Playz

MOV
Created by Apple, QuickTime is a very versatile format that works on both the Macintosh and Windows Platform. The initial QuickTime version was based on the CinePak compression.

AVI
Created by Microsoft, AVI is the file of choice for windows computers. Most AVI files are capable of being played in a QuickTime player.

RMA
Real Media is a file format created by Real Media Inc. A plug-in is available for both the Mac and PC. Generally this file format is associated with streaming..

ASF
ASF was created by Microsoft and is generally associated with windows machines. Unfortunately, ASF files are very difficult to play on the Macintosh operating system.

MPEG
The MPEG format was created by a public consortium. It is a very effective compression scheme that can be played on both Mac and Windows machines.

Streaming Vs Download: To stream or not to stream.

There are two basic methods of delivering video over the Internet: Streaming and Downloading. The easiest and most popular method to deliver video content over the Internet is through downloading. A user will simply download a video much like they would download a picture. Once the download has started, the browser (i.e. Netscape) will examine the file extension (i.e. .mov or .avi) and then match that file extension with a program or plug-in based on what is set in the program's preferences. Most browsers already have the preferences set, so you need not worry.

On the other hand, streaming is a little more complicated requiring the server to be set up with special streaming software. Here's a basic overview of how the process works.

First a user will request a stream from the server. Depending on how busy the server is, the server will decide to either accept or deny the request. If the request is accepted, the server and the user's computer will begin to negotiate to see how fast the user's computer connection is with the server. Once the speed of the user's connection has been determined, the server will decide how big of a file it can stream reliably. The slower the connection, the smaller the streamed file will be. Once the server has selected the size of the file, streaming will commence. The stream of video will be sent to the user's computer. However, before it begins to play it will feed into a temporary storage area called a buffer. Because an Internet connection is never at one consistent speed, the information will feed into the buffer so that the user has less of a chance of video being interrupted. The player then takes the video out of the buffer and the video is played.

What the Hell is a Codec?

A Codec is a compression algorithm designed to make video or sound files smaller. To compress video, you need a codec. If you don't compress video, you are working with raw video. A codec uses a special algorithm that compares groups of video frames and then decides which pixels need to be changed from frame to frame. Theoretically, by not changing pixels from frame to frame, file size is reduced. In contrast, Raw video ensures that every pixel is refreshed during each frame change, making it necessary to store the complete information on each individual frame. Raw uncompressed video ensures the highest quality video, but also comes with the largest file size. We recommed using the Sorenson 3 codec for QuickTime.

Don't neglect sound, it can be a size stinker.

While images can create large files, sound ain't no stranger to size either. Just like video, you can make your sound shittier by reducing the sampling rate, bit dept, converting from stereo to mono, or adding a compression codec.

Sampling
CD sound is 44.1 kHz. Most DV camera's record at 32 kHz as the default. If you decide to have you video have one these two sampling rates, you are going to be very very mad. Try reducing the sample rate to somewhere between 11.025 kHz and 22.05 kHz. Go any lower and your movie will sound like someone talking on a cell phone while flying on a jet.

Bits
Bits are the amount of divisions in a sound sample cycle. The higher the bit rate, the smoother the sound will be and consequently the larger the file size. It is generally a good idea to keep your sound at 16 bit.

Stereo
Stereo (HiFi for you old school homies), has two tracks: right and left. Mono sound on the other hand only has one track. Obviously, you can reduce your sound size simply by making your sound mono.

Compression Codecs
Sound, like video, has algorithms to help in compression. Some work better on music (MP3) and some work better for voice. The trick is finding the best codec that works for you and is compatible with your viewers.

Special Programs

Finding the right compression scheme sweet spot takes a lot of trial and error. Luckily there are a few products on the market with preprogrammed compression settings that help reduce the time it takes to find the sweet spot. One example of these programs is Cleaner. Cleaner is set up like a compression wizard. The program asks a series of questions about your video and about your targeted audience. Based on your answers, it comes up with a compression scheme that they hope will work best for your audience.

Another great option for compressing video is Apple's QuickTime Pro available for both Mac and PC. For about $30 the free QuickTime player can be used to save clips in a variety of different for formats with complete control over most compression elements.

-BM



Category: Learn

Updated: Dec 22, 2004 5:42:49 PM

 
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