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Digital Audio/Video File Formats: The Basics
These days, virtually all audio and video entertainment is digital. Even if you're not enjoying MP3s or videos streamed from the Internet, chances are the music you're listening to or movie you're watching comes from a digital source. CDs and DVDs are digital media. Satellite TV and satellite radio broadcast digital signals. Most pictures and home movies are shot with digital cameras and camcorders — even over-the-air television is now digital.
To get the most out of your entertainment sources, it helps to understand some general concepts about digital audio/video file formats, and how they store information. In addition to this article, be sure to check out our glossary of file formats for descriptions of many of today's widely used formats.
What is a file format, anyway?
The way a computer or digital A/V device stores information — any kind of information — is through a series of 0's and 1's. Each 0 or 1 is known as a bit; a string of 8 bits makes up a unit of memory known as a byte. A file format is simply a specified way of arranging a given type of information into bytes.
How compression helps
The number of 0's and 1's required to represent CD-quality music or a full-color picture can be difficult to grasp. (For instance, the average audio CD holds roughly 650 megabytes of data — that works out to over 5,000,000,000 bits!). The sheer volume of data contained in many types of audio and video files can make them cumbersome, particularly when it comes to storing them or downloading them over an Internet connection.
In order to help lighten the load, many audio and video file formats employ what's known as a codec (short for compressor/decompressor). A codec is a system for compressing a relatively large amount of data into a smaller, more manageable file, which can then later be opened or decompressed to deliver the original contents.
|Codecs let you store your digital music, photos, and movies in the form of compact, manageable files.|
If a codec causes a degradation of sound or picture quality in the process of squeezing and then unpacking a file, that codec said to be "lossy." That is, certain portions of the data which are judged to be less essential are discarded, and can't be restored upon playback. Alternately, if a codec can reproduce the original data with no loss of quality or nuance, it's called "lossless."
Using the right compression
There are different kinds of compression for different jobs. There are audio codecs (like MP3) that allow you to quickly download music via the Internet and store lots of songs on your portable player. There are also video codecs that let you watch short clips and TV shows online, or create DVD movie discs from your own home video footage.
Most audio/video digital formats allow you to select different degrees of compression when you're creating, or encoding, a file. For instance, MP3 files can be encoded at different rates of compression for varying file size and sound quality. File resolution is measured in kilobytes per second (kbps) — that is, how many thousands of bytes it takes to store one second of music. This figure is known as the bitrate.
The higher the resolution, the more information from the original source is retained. A 256kbps file, for example, holds twice as much data as a 128kbps file. Generally, the smaller the bitrate, the more a file is compressed and the more it will be subject to a loss of quality. However, smaller files are easier to store and quicker to download or transfer. When you're downloading files or encoding your own, consider how you'll be using them, and choose from your compression options accordingly.
|This chart shows the relative file sizes of the same three-minute song saved in different formats, starting with the original CD track on the left. The smaller the file, the more sonic information is lost.|
Several kinds of digital audio and video recording devices use compression for efficient storage. Encoders in the software of these devices compress the recorded content during the recording process. Here are a few examples:
- Both iTunes® and Windows Media® Player software by default compress ripped CD tracks down to 128 kbps for their respective file formats (AAC for iTunes and WMA for Windows Media Player). This significantly reduces the file size, letting about 130 songs be stored in the same amount of memory that the original uncompressed file would require. Both iTunes and Windows Media let you adjust the bitrate for either higher or lower compression.
- DVD recorders use MPEG2 compression to store movies and shows on blank DVD discs. The user can almost always choose from a range of recording time or picture quality settings — what you're actually selecting is how heavily your recordings will be compressed.
- Digital cameras use JPEG image compression to allow many pictures to be stored in a limited amount of memory. Some cameras let you capture uncompressed photos (usually in TIFF or RAW files), which take up several times more memory than JPEG images.
Compatibility and software
When working with audio or video on your PC, you may find that some types of files can be opened by more than one software application. Some types of files are even usable on different operating systems (like Windows and Macintosh). Other file formats may only be compatible with a single specific application. When you're downloading audio/video files, keep in mind that you'll need to have compatible software to open them and translate them back into an audible or viewable form.
When it comes to encoding your own files, you may find that some applications produce better-sounding or better-looking results than others. Likewise, if you're faced with a choice between two or more formats for the same job — for instance, choosing between MP3 files or Windows Media Audio files for your portable player — you may find that you have a personal preference for one format or another.
When choosing between formats or software, try out the available options whenever you can to see what works best for you. If you'll be sharing files with others, stick with common codecs to help ensure playback compatibility.
Though compressed audio and video files are usually much smaller than raw, uncompressed ones, they can still sometimes take a long time to download — even if you have a broadband or DSL Internet connection. One way web designers can make digital audio and video content more readily available to the average 'Net surfer is through streaming media.
Streaming is a technique that allows data to be translated into viewable images or audible sound "on the fly" — that is, the file plays as it's being downloaded. Streaming is commonly used by Internet radio sites to offer continuous music and news broadcasts. Many online video sites — such as YouTube.com — use streaming as well.
Data can be streamed on the Web at different speeds. Broadband Internet connections allow you to receive streaming content at higher bitrates than dial-up connections, and higher bitrates usually give you better picture and/or sound quality.
There are a few file formats, like RealMedia and Adobe Flash, which are used almost exclusively in streaming applications. Other formats, like MP3 for audio and MPEG4 for video, can deliver both streaming and downloadable content.